BRITISH STANDARD
Code of practice for
control of condensation
in buildings
ICS 91.120.99
12&23<,1*:,7+287%6,3(50,66,21(;&(37$63(50,77('%<&23<5,*+7/$:
BS 5250:2002
Incorporating
Amendment No. 1
BS 5250:2002
Committees responsible for this
British Standard
The preparation of this British Standard was entrusted by Technical
Committee B/540, Energy performance of materials, components and
buildings, to Subcommittee B/540/2, Building performance — Energy, upon
which the following bodies were represented:
Association for the Conservation of Energy
Association of Building Component Manufacturers
Association of Manufacturers of Domestic Appliances
Autoclaved Aerated Concrete Products Association
Brick Development Association
ODPM — British Board of Agrément
ODPM — Building Regulations Division
ODPM — Represented by BRE
Chartered Institution of Building Services
Concrete Block Association
Consumer Policy Committee of BSI
Department of the Environment for Northern Ireland
Electricity Association
EURISOL
Flat Glass Manufacturers’ Association
Gypsum Products Development Association
HEVAC Association
Institution of Structural Engineers
MoD — UK Defence Standardization
NHBC
Scottish Executive
Steel Construction Institute
This British Standard was
published under the authority
of the Standards Policy and
Strategy Committee
on 1 November 2002
Timber Research and Development Association
© BSI 23 December 2005
First published October1975
Second edition June 1989
Third edition November 2002
The following BSI references
relate to the work on this
British Standard:
Committee reference B/540/2
Draft for comment 01/102726 DC
ISBN 0 580 40488 9
Amendments issued since publication
Amd. No.
Date
Comments
16119
23 December 2005 See national foreword
BS 5250:2002
Contents
Committees responsible
Foreword
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Page
Inside front cover
iii
Scope
Normative references
Terms and definitions
Behaviour of water vapour in the air
Causes of condensation
The effects of condensation and high humidity
Design principles
Application of design principles: building fabric
Application of design principles: heating
Application of design principles: ventilation
Diagnosis and remedial work
Particular aspects
Precautionary measures during construction
Building user information
Annex A (normative) The interrelationship of moisture contents and
temperatures
Annex B (normative) Moisture generation and ventilation in occupied
buildings
Annex C (normative) Material properties
Annex D (normative) Calculation methods
Annex E (informative) Vapour resistances: Conversion factors for unusual
units
Bibliography
Figure 1 — Relationship between air temperature, vapour pressure and
relative humidity
Figure 2 — Balance of factors
Figure 3 — Masonry cavity wall
Figure 4 — Solid wall: internal insulation
Figure 5 — Solid wall: external insulation
Figure 6 — Framed wall
Figure 7 — Framed wall with tile cladding
Figure 8 — Warm steel frame wall
Figure 9 — Steel frame wall with frame within the insulation
Figure 10 — Site assembled metal wall
Figure 11 — Composite panel wall
Figure 12 — Pitched roof with insulation on a horizontal ceiling —
Ventilated below the underlay
Figure 13 — Pitched roof — Large ventilated void above the insulation
and a type LR underlay unsupported with an air-open roof covering
Figure 14 — Pitched roof — Large ventilated void above the insulation
and a type LR underlay unsupported with a tight roof covering
Figure 15 — Pitched roof — Large ventilated void above the insulation
and a type LR underlay supported on sarking boards
Figure 16 — Pitched roof — Small ventilated void above insulation
and a type HR underlay
Figure 17 — Pitched roof — No void above insulation and a type LR
underlay
© BSI 23 December 2005
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BS 5250:2002
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Figure 18 — Pitched roof — Small void above insulation and a
type LR underlay
Figure 19 — Ventilation positions for room in the roof construction
requiring ventilation
Figure 20 — Ventilation positions for room-in-roof construction including
a flat roofed dormer window
Figure 21 — Ventilation positions for roofs with dormers
Figure 22 — Framed flat roof: cold type
Figure 23 — Framed continuous membrane roof: warm type
Figure 24 — Framed continuous membrane roof: warm type inverted
Figure 25 — Concrete continuous membrane roof: cold type
Figure 26 — Concrete continuous membrane roof: warm type
Figure 27 — Concrete flat roof: warm type inverted
Figure 28 — Site assembled metal roof
Figure 29 — Composite panel roof
Figure 30 — Timber suspended ground floor
Figure 31 — Precast concrete suspended ground floor
Figure 32 — Solid ground floors
Figure 33 — Timber deck with external finish of low vapour resistance
Figure 34 — Solid externally exposed floor
Figure 35 — Standard glazing unit
Figure 36 — Drained glazing unit
Figure A.1 — Example of use of the psychrometric chart
Figure A.2 — Psychrometric chart
Figure B.1 — Variation of internal humidity classes with external
temperature
Table 1 — Effect of condensate on an impermeable surface
Table A.1 — Saturaction vapour pressures for air
temperatures 30.9 °C to –20 °C
Table B.1 — Typical moisture generation rates for household activities
Table B.2 — Typical moisture generation rates from heating fuels
Table B.3 — Daily moisture generation rates for households
Table B.4 — Typical ventilation rates
Table B.5 — Internal humidity classes: building types and limiting relative
humidities at Te = 0 °C
Table C.1 — Thermal conductivities and vapour resistivities
Table C.2 — Vapour resistances
Table C.3 — Thermal resistances for surfaces and air spaces
Table D.1 — Monthly mean temperature and relative humidity for
interstitial condensation calculations (1983–2002)
Table D.2 — Corrections to monthly mean temperatures and relative
humidities from a mean year to achieve condensation risk years with
various return periods
Table E.1 — Factors for converting unusual permeance units to Èg/N·s
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© BSI 23 December 2005
BS 5250:2002
Foreword
This British Standard code of practice has been published under the direction of
the Basic Data and Performance Criteria for Civil Engineering and Building
Structures Standards Policy Committee and supersedes BS 5250:1989,
published as a code of practice for the design of buildings. BS 5250:1989 is now
withdrawn.
The start and finish of text introduced or altered by Amendment No. 1 is
indicated in the text by tags !".
In buildings, condensation can occur when water vapour, usually produced by
the occupants and their activities, condenses on exposed building surfaces
(surface condensation) where it supports mould growth, or within building
elements (interstitial condensation). Condensation and mould problems are
widespread, affecting about 15 % of homes in the United Kingdom to some
degree.
Condensation is not always a problem; for example, it regularly occurs on the
inner surface of the outer leaf of a cavity wall, which receives very much more
water from driving rain. Nevertheless, damage can occur to the building fabric
and contents, and the dampness and associated mould growth can be distressing
to occupants and a major cause of respiratory allergies. The control of
condensation is therefore an important consideration in building design and
construction.
The occurrence of condensation is governed by complex interrelationships
between heating, ventilation, moisture production, building layout and
properties of the materials making up the fabric of the building. Under
reasonable conditions of use, the designer's choice of heating system, ventilation
provision, building plan and component materials will provide an environment
where the risk of condensation is kept to a minimum. Good workmanship and
supervision and the builder's understanding of the designer’s intentions will
result in constructions free from the risk of condensation. It should be recognized
that occupants by choice, lack of understanding or force of circumstances often
do not use buildings in the manner intended or expected by the designer.
Increased awareness of the need for efficient use of energy in the design and
management of buildings, as recommended in BS 8207, has led to greater
insulation levels and reduced ventilation in both new and existing buildings. In
turn, this has caused an increase in condensation problems. The complex
interrelationships between the factors which affect condensation means that
particular care is needed when designing new buildings or considering changes
or attempting to remedy problems in existing buildings.
As a code of practice, this British Standard takes the form of guidance and
recommendations. It should not be quoted as if it were a specification and
particular care should be taken to ensure that claims of compliance are not
misleading.
This publication does not purport to include all the necessary provisions of a
contract. Users are responsible for its correct application.
Compliance with a British Standard does not of itself confer immunity
from legal obligations.
Summary of pages
This document comprises a front cover, an inside front cover, pages i to iii, a
blank page, pages 1 to 82, an inside back cover and a back cover.
The BSI copyright notice displayed in this document indicates when the
document was last issued.
© BSI 23 December 2005
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BS 5250:2002
1 Scope
This British Standard code of practice describes the causes and effects of surface and interstitial
condensation in buildings and gives recommendations for their control.
The principles of control and the recommendations given can be applied generally to both new and existing
buildings. Some constructions, e.g. curtain walling or those around cold stores and those buildings with
unusually high internal humidities, such as swimming pools or buildings with wet industrial processes, are
outside the scope of this standard and need specialized treatment.
This standard provides guidance for building designers, contractors, owners, managers and occupiers and
includes recommendations for heating, ventilation and construction which can control condensation.
Methods of calculation are also given to help assess and quantify risk.
Methods are given to determine the occurrence and assess the effects of:
a) surface condensation, or mould growth, one of its associated effects; and
b) interstitial condensation.
2 Normative references
The following normative documents contain provisions which, through reference in this text, constitute
provisions of this British Standard. For dated references, subsequent amendments to, or revisions of, any
of these publications do not apply. For undated references, the latest edition of the publication referred to
applies.
BS 3533, Glossary of thermal insulation terms.
!BS 5534:2003, Code of practice for slating and tiling (including shingles)."
BS 8215, Code of practice for design and installation of damp-proof courses in masonry construction.
BS EN ISO 6946, Building components and building elements — Thermal resistance and thermal
transmittance — Calculation method.
BS EN ISO 10211-1, Thermal bridges in building construction — Heat flows and surface temperatures —
General calculation methods.
BS EN ISO 13788, Hygrothermal performance of building components and building elements — Internal
surface temperature to avoid critical surface humidity and interstitial condensation — Calculation
methods.
NFRC Technical Bulletin Number 6: Pitched roof underlays. London: NFRC Publications.
3 Terms and definitions
For the purposes of this British Standard the terms and definitions given in BS 3533 and the following
apply.
3.1
airtight layer
layer that prevents the convective movement of air under the normal pressure differences found in
buildings and which may also act as a vapour control layer
NOTE Such a layer is sometimes referred to as “convection tight”.
3.2
!breather membrane
membrane with a vapour resistance of less than or equal to 0.6 MN·s/g
3.3
high water vapour resistance (type HR) underlay
underlay which has a vapour resistance greater than 0.25 MN·s/g"
© BSI 23 December 2005
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BS 5250:2002
!3.4
low water vapour resistance (type LR) underlay
underlay which has a water vapour resistance less than or equal to 0.25 MN·s/g
NOTE 1 This definition is consistent with BS 5534:2003.
NOTE 2 Type LR underlays are sometimes referred to as “vapour-permeable”, or “vapour-open” underlays.
NOTE 3 Some LR underlays may also possess a degree of air permeability, see 8.4.2.1.3.
3.5
condensate
water formed by the process of condensation (3.6)
3.6
condensation
process whereby water is deposited from air containing water vapour when its temperature drops to or
below dewpoint (3.14), or the vapour pressure rises above the saturated vapour pressure at a given
temperature
3.7
interstitial condensation
condensation (3.6) occurring within or between the layers of the building envelope
3.8
surface condensation
condensation (3.6) occurring on visible surfaces within the building
3.9
harmful condensation
interstitial condensation (3.7) or surface condensation (3.8) that is likely to cause damage to the building
fabric, degrade its thermal performance or support mould growth
3.10
inconsequential condensation
condensation (3.6) that is not harmful
3.11
nuisance condensation
surface condensation (3.8) that is not harmful
3.12
reverse condensation
interstitial condensation (3.7) formed by water vapour travelling from outside to inside, i.e. the reverse to
normal condensation
3.13
cooler side
side of a structure with a lower temperature compared to the warmer side (3.37)
NOTE
The cooler side usually has a lower vapour pressure compared to the warmer side.
3.14
dewpoint
temperature at which air becomes saturated with water vapour
3.15
evaporation
process whereby liquid water becomes a vapour when in contact with unsaturated air
3.16
hygroscopic material
material capable of absorbing water vapour from air"
2
© BSI 23 December 2005
BS 5250:2002
!3.17
moisture content by weight
mass of water contained within a kilogram of dry material
3.18
moisture content of air
mass of water vapour present in unit mass of dry air
NOTE This is expressed as kilograms per kilogram or as grams per kilogram of dry air.
3.19
night sky radiation
loss of heat from the outside surface of a building to a clear night sky, which lowers the outside surface
temperature below the external air temperature
3.20
passive stack ventilation
PSV
ventilation system using ducts from the ceiling or walls of rooms to terminals on the roof which operate by
a combination of the natural stack effect, i.e. the movement of air due to the difference in temperature
between inside and outside and the effect of wind passing over the terminal
3.21
pattern staining
discolouration on the internal surfaces of buildings caused by preferential deposition of dust at relatively
warm locations
3.22
relative humidity
ratio of the vapour pressure in air at a given temperature to the saturation vapour pressure at the same
temperature; commonly expressed as a percentage
3.23
sarking boards
sawn softwood boards, typically 150 mm wide, laid across the rafters, with a 2 mm gap between
3.24
saturation vapour pressure
water vapour pressure in air at dewpoint temperature
3.25
sheet sarking
continuous sheets of OSB, plywood, chipboard or similar material laid over the rafters below tiles or
slates
3.26
sponge effect
ability of the fabric of a building and the building contents to absorb and desorb water vapour
3.27
thermal bridge
cold bridge
part of a structure of lower thermal resistance which bridges adjacent parts of higher thermal resistance
and which can result in localized cold surfaces on which condensation, mould growth and/or pattern
staining can occur"
© BSI 23 December 2005
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BS 5250:2002
!3.28
vapour control layer
material of construction that substantially reduces the water vapour transfer through any building
component in which it is incorporated by limiting both vapour diffusion and air movement
NOTE 1
It is usually a membrane.
NOTE 2 The term “vapour control layer” has been adopted throughout this standard in preference to the terms “vapour check” and
“vapour barrier” which usually refer to materials alone. The performance of a vapour control layer is dependent upon the material,
workmanship and buildability, all of which need to be assessed by the designer.
3.29
vapour diffusivity
rate at which water vapour will diffuse through a unit of thickness of material when a difference of unit
water vapour pressure exists on opposite sides of the material
NOTE
Vapour diffusivity is expressed as 4g·m/N·s, which is numerically equivalent to g·m/MN·s.
3.30
vapour pressure
part of atmospheric pressure due to water vapour present in the air
NOTE Vapour pressure is expressed in kPa (1 kPa = 10 mbar = 1 000 N/m2).
3.31
vapour resistance
measure of the resistance to water vapour diffusion of a material or combination of materials of specific
thickness
NOTE 1 Vapour resistance is expressed in MN.s/g.
NOTE 2 For thin membranes, performance is stated as vapour resistance. For other materials, it is obtained by multiplying thickness
by vapour resistivity.
3.32
vapour resistivity
measure of resistance of a unit thickness of material to water vapour diffusion when a difference of unit
water vapour pressure exists between the air on the opposite sides of the material
NOTE Vapour resistivity is expressed as MN.s/g.m.
3.33
vented air space
cavity or void that has openings to the outside air placed so as to allow some limited, but not necessarily
through, movement of air
3.34
ventilated air space
cavity or void that has openings to the outside air placed so as to promote through movement of air
3.35
ventilation rate
rate at which air within a building is replaced by outside air
NOTE The ventilation rate may be expressed as: a) number of times the volume of air within a space is changed in one hour (air
changes per hour (h–1)); b) rate of air change in litres per second (l/s).
3.36
warmer side
side of structure with a higher temperature than the cooler side (3.13)
NOTE
The warmer side usually has a higher vapour pressure compared with the cooler side.
3.37
water vapour
water in its invisible gaseous phase"
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© BSI 23 December 2005
BS 5250:2002
4 Behaviour of water vapour in the air
At any temperature, air is capable of containing a limited amount of moisture as an invisible vapour; the
warmer the air the more water vapour it can contain before it becomes saturated. If moisture-laden air
comes into contact with a cold surface, either inside the building or at an interface within the fabric,
condensation will occur at the temperature at which the air becomes saturated (the dewpoint).
Water vapour in the air exerts a pressure, the vapour pressure, so air containing a large mass of water
vapour has a higher vapour pressure than drier air, which causes vapour to diffuse from high to low
pressure areas. The term usually used to describe whether air is dry or water-laden is relative
humidity (r.h.).
Figure 1 is a psychrometric chart showing the inter-relationship of these factors. The vapour pressure is
plotted on the vertical axis with the temperature on the horizontal axis. The curved lines show the
percentage relative humidity resulting from the combination of temperature and vapour pressure.
Percentage relative humidity is a good indicator of the risk of condensation, mould growth and degradation
of absorbent materials. Where the air remains around or above 70 % r.h. for lengthy periods, there is a high
risk of mould growth on some parts of the external fabric. The arrows on the chart indicate that the risk
can be reduced by increasing the temperature, by decreasing the vapour pressure or by a combination of
these two factors. The inter-relationship of moisture content and temperature is given in greater detail
in Annex A.
Relative humidity (%)
1.50
100 90
80
1.40
70
1.30
Vapour pressure (kPa)
60
1.20
50
1.10
40
1.00
0.90
0.80
0.70
0
4
8
12
Temperature ( ºC)
16
20
24
Figure 1 — Relationship between air temperature, vapour pressure and relative humidity
© BSI 23 December 2005
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BS 5250:2002
5 Causes of condensation
5.1 General
Two categories of condensation should be distinguished:
a) condensation on surfaces within the building; and
b) interstitial condensation within or between the layers of the building envelope.
Most materials will absorb water vapour from the environment: some, subjected to high humidity, can
absorb moisture sufficiently to cause damage even though no actual condensation has taken place. In
considering the risk of condensation, consideration should therefore also be given to the actual levels of
humidity to which materials will be exposed.
Sources of water vapour include atmospheric moisture, construction water, the occupants and their
activities and any wet processes within the building (see Table B.1, Table B.2, Table B.3, Table B.4 and
Table B.5 in Annex B).
5.2 Causes of surface condensation
Surface condensation occurs on surfaces, such as the internal surface of external fabric elements or cold
pipes and cisterns, that are at or below the dewpoint of the air in contact with them, and is controlled by
the temperature of the surface and the vapour pressure of the air.
The temperature of the surface depends on the following factors:
a) the type(s), amount, time and rate of heating of the building;
b) the ventilation rate;
c) the thermal properties and surface finish of the building fabric;
d) the external temperature.
The vapour pressure of the air is determined by:
a) the water vapour production within the building;
b) the ventilation rate;
c) the moisture content of the “replacement” outdoor air;
d) the ability of the building fabric and contents to absorb or desorb water vapour (sponge effect). This
will reduce or increase the vapour pressure depending on whether the building is cooling or heating.
NOTE
Anything that warms surfaces or reduces vapour pressure of the air will reduce the incidence of surface condensation.
5.3 Causes of interstitial condensation
5.3.1 General
In the winter, the interior of buildings will usually be warmer and the air will contain more moisture
(i.e. have a higher vapour pressure) than outside. Heat and water vapour will diffuse out through the
materials of the structure and be carried by bulk air movement through gaps and cracks into and through
the structure.
For diffusion, rates of flow will vary depending on the interior/exterior conditions and the thermal and
vapour resistance properties of each part of the structure. For air leakage, rates of flow will depend on wind
and stack pressures and on the dimensions of the openings, joints and cracks. Unless these gaps are sealed,
it has been found that the dominant internal/external transport mechanism of water vapour is usually by
mass movement of air.
Interstitial condensation occurs within the fabric of a building when the temperature of some part of the
structure equals the dewpoint at that point, which is determined by the balance of flows of moisture to and
from the point. At this temperature, the air is saturated; thus further vapour passing through the structure
will condense rather than increase the vapour pressure. Such condensation is more likely to occur on the
surfaces of materials within a structure, particularly on the warm side of relatively vapour resistant layers,
but it is possible to have condensation occurring within the material when the dewpoint and the structural
temperatures coincide throughout the material. It is also possible to have interstitial condensation on more
than one surface in a structure due to moisture evaporating from one surface and recondensing on a colder
one.
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© BSI 23 December 2005
BS 5250:2002
Although interstitial condensation usually occurs when water vapour is diffusing out from the interior of a
building, there are circumstances, e.g. an air-conditioned building in warm, humid weather, in which the
interior is cooler and drier than outside; water vapour will then enter the structure from outside. In this
document, reference will be made to the warmer sides and cooler sides of the structure; in all but the most
exceptional circumstances, these will correspond to the higher and lower vapour pressure sides
respectively. In spring and autumn as well as summer, even though the external air temperature may be
lower than inside, the external surface of south facing walls, which might have been wetted by driving rain,
can be sufficiently heated by the sun to cause water vapour to diffuse into cooler areas where it can
condense (reverse condensation).
5.3.2 Hygroscopic materials
Most building materials are hygroscopic, i.e. they have a porous structure that absorbs water vapour from
the air, even before interstitial condensation has taken place. Water can therefore be built into a
construction by:
— the water of hydration in cement, concrete or mortars;
— the inclusion of hygroscopic materials which have been stored outside undercover in humid conditions.
For example, 25 mm plywood stored at 90 % r.h. will hold almost 3 kilograms of water in every square
metre;
— rain impact during construction before the weatherproof layer is in place. For example, 10 mm of rain
falling on an absorbent insulation layer of a roof will deposit 10 kg/m2.
This water can then move through a structure under temperature and humidity gradients by a mixture of
vapour diffusion and liquid flow through the pores and accumulate at impermeable layers.
The absorption of water by hygroscopic materials can have a buffering effect, reducing the chance of
interstitial condensation during short periods of cold weather, or on clear frosty nights, when the external
surface can cool by night sky radiation.
Many structural elements are subjected to significant diurnal temperature changes; the external surface
temperature of a flat roof in spring or autumn can rise to 50 °C during a sunny day and fall to –10 °C on a
clear night. This causes movement of water into the structure during the day and outwards overnight. The
water that is initially spread uniformly through the structure at low concentrations can then become
concentrated at interfaces raising the moisture content of vulnerable materials such as timber high enough
to cause local problems of decay. High external surface temperatures due to solar gain can force water in
through gaps in a vapour control layer during the day, giving rise to roofs that apparently leak only in hot
dry weather.
5.3.3 Reverse condensation
An excellent example of moisture movement in hygroscopic materials under temperature gradients is given
by reverse condensation. This phenomenon is most frequently observed when the sun shines on damp
walls. It is caused by the moisture in the wall being vaporized by the heat of the sun; the resulting pressure
difference drives the water vapour towards the inside of the building. If a vapour control layer is included
in the construction, interstitial condensation can occur on the outside face where it can run down to affect
vulnerable materials.
This is most likely to be observed in the thermal improvement of solid walls by the use of internal insulation
systems. Although the severity of the problem is not known, it is more common in thin masonry walls, walls
of an absorbent nature or on walls that remain saturated because of their exposure. A weatherproof
treatment or system can reduce the moisture content of such walls and the consequent risk of reverse
condensation. Weatherproofing should be applied to the outer surface of the wall and should be of low
vapour resistance or be vented.
NOTE This type of reverse condensation should not be confused with the problems of interstitial condensation that can occur in
building elements, e.g. in cold stores or air conditioned buildings, where the internal conditions are colder and drier than outside.
These complex phenomena, such as liquid water movement under temperature gradients, are becoming better understood and a
number of computer models that can give reliable performance predictions are currently under development and in use by
consultants. Work is under way to standardize these and develop a formal protocol for the assessment of structures.
© BSI 23 December 2005
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BS 5250:2002
6 The effects of condensation and high humidity
6.1 General
Condensation can reveal itself in a number of ways, the most common being the presence of condensate,
mould growth, decay of timber and corrosion of metals.
6.2 Condensate on surfaces
Condensate frequently occurs on:
— single glazing in bedrooms overnight or in kitchens and bathrooms at any time;
— double glazing, especially near to the frames, in rooms with relatively high humidities;
— on WC cisterns or cold pipes in bathrooms or kitchens;
— on the walls of hall ways and stairs in buildings of heavy masonry construction after a change
from cold dry weather to mild wet weather;
— on the underside of lightweight single skin roofs of industrial buildings due to night sky
radiation;
— on massive floors in offices or industrial buildings, which remain cold after a change to warmer
more humid weather, or when heating is turned on in the morning;
— on the walls or surrounds of swimming pools.
Condensate is often only a nuisance. However, more serious consequences can result from, for
example:
— condensate from glazing promoting decay in the wooden window frames or condensate running from
sills onto the wall below, damaging the décor;
— condensate dripping from roofs onto food preparation processes or sensitive electronic
equipment;
— condensate on certain floor types, leading to a slip hazard.
It is sometimes possible to deal with the condensate by drainage or by mopping up before it collects and
runs to vulnerable areas. However, persistent severe condensation on glazing, especially double glazing,
in many rooms, suggests that there can be excessive moisture production or inadequate ventilation within
the dwelling, which can lead to the more serious problems described in 6.3 and 6.4.
6.3 Mould growth
Mould growth is often associated with surface condensation and damp houses can provide good conditions
for its development.
Mould spores exist in large numbers in the atmosphere and to germinate need a nutrient, oxygen, a
suitable temperature and moisture. Sources of nutrition are widespread in buildings and the internal
environment provides a suitable temperature for growth. As oxygen is also always present, mould growth
is particularly dependent on moisture conditions at surfaces and the length of time these conditions exist.
Studies have shown that moulds do not require the presence of water, but can germinate and grow if the
relative humidity at a surface rises above 80 %. This is a considerably less severe criterion than the
100 % r.h. required for surface condensation to occur. As the internal surfaces of external walls will be
colder than the air temperature within the building in winter, the relative humidity at the wall will be
about 10 % higher than in the centre of a room. This temperature and relative humidity difference will be
reduced if the walls are well insulated. As a guide, however, it may be assumed that, if the average relative
humidity within a room stays at 70 % for a long period of time, the relative humidity at the external wall
surfaces will be high enough to support the growth of moulds.
Moulds and mildews can occur on furniture, curtains, carpets and clothing, especially leather jackets, shoes
or suitcases, if they are situated in unheated spaces or in parts of rooms sheltered from heating systems.
Unheated bedrooms, cupboards or wardrobes placed against external walls and items stored in roof spaces
are especially vulnerable.
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© BSI 23 December 2005
BS 5250:2002
6.4 Interstitial condensation
Interstitial condensation can increase the moisture content of components in a structure, but this can be
inconsequential, e.g. condensate occurring on the outer leaf of a masonry cavity wall, where the amount of
condensate can be small compared to the effect of wetting by rain.
Sustained condensation can cause decay of timber or corrosion of metal coverings and/or components and
so should be termed harmful. Hygroscopic materials should not be used in locations where a high relative
humidity is maintained as they can cause degradation even though no condensate is deposited upon them.
Persistent timber moisture contents in excess of 20 % (by mass) can lead to decay. Over a winter season,
absorbent and hygroscopic materials are likely to accumulate moisture; during the summer, this moisture
will tend to evaporate. The rate of this evaporation is difficult to calculate, but it should be borne in mind
when assessing whether condensation is harmful.
Accumulation of condensate within thermal insulation will significantly increase the thermal conductivity
of the insulation. Dimensional changes, migration of salts, liberation of chemicals and electrical failure can
also result.
7 Design principles
7.1 General
7.1.1 Introduction
This section is primarily intended to give guidance on the design of new buildings but in many respects
is equally applicable to complete refurbishment of older buildings, although some options are not then
available. More recommendations on upgrading or remedial work in existing buildings is given
in Clause 10.
Design for the control of condensation depends upon obtaining a satisfactory relationship between air
conditions (internal and external air temperatures and humidity) and the properties of the external
elements of construction (thermal and vapour resistance).
The objectives should be as follows:
a) prevention of harmful surface or interstitial condensation;
b) prevention of mould growth;
c) economical reduction of nuisance condensation.
Condensation control should be considered as part of the design process. Successful control will depend on
factors such as prevailing winds, room layout, number of storeys and type of heating system as well as the
more usually accepted aspects such as construction, heating, ventilation and moisture production. All these
aspects, therefore, should be considered carefully and, as they are interdependent to a greater or lesser
degree, they should be considered together.
The fundamental principle in designing to minimize condensation is to maintain a balance of the three
factors shown in Figure 2 in order to achieve either low vapour pressure and/or high structural
temperature.
7.1.2 Controlling surface condensation
To minimize surface condensation, it is necessary to do one or more of the following:
a) obtain low vapour pressures by ventilation and/or reduced moisture input to the building;
b) obtain high surface temperatures by providing more insulation and/or increasing the heat input.
7.1.3 Controlling interstitial condensation
To minimize interstitial condensation, it is necessary to do one or more of the following:
a) obtain low vapour pressures by ventilation, and/or reduced moisture input to the building;
b) use materials of high vapour resistance near to the warmer side of the construction;
c) use material of low vapour resistance, or provide ventilated cavities, near the colder side of the
construction;
d) use materials of low thermal resistance near to the warmer side of the construction;
e) use materials of high thermal resistance near to the colder side of the construction.
NOTE Any one particular procedure taken in isolation might not necessarily minimize the risk of condensation.
© BSI 23 December 2005
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BS 5250:2002
1
Key
2
3
1 Thermal and vapour properties of the structure
2 Heat input
3 Ventilation
Figure 2 — Balance of factors
7.2 Occupant activity and heating and ventilation regime
In considering the effect of occupant requirements and activity on building plan and structure and on
heating and ventilation requirements, designers and other users of this document, should be aware
that:
a) occupants and the activities and processes within buildings, including some domestic appliances,
generate moisture; some industrial processes, canteens, kitchens, laundries, shower rooms or swimming
pools generate very large amounts of moisture;
b) fuel costs can make occupants reluctant to provide adequate heating for buildings or alter the type and
pattern of heating;
c) patterns of use of buildings have changed and can change, e.g. there has been an increase in
intermittent heating of dwellings due to alterations in working patterns;
d) the function of a building could change completely, e.g. a building built as a warehouse could be
changed to a wet process factory.
7.3 Building configuration
Water vapour is often generated locally within buildings in wet process areas such as kitchens, shower
rooms, laundries or a swimming pool in a larger sports centre. The dominant mechanism for transporting
this water vapour to other drier and often colder areas is the airflows through the building which depend
on the environmental conditions and the internal configuration of the building.
In a heated building in winter, warm, moist air will rise and leak from the building at high level often via
the roof and be replaced by colder, drier entering at low level; this is known as the stack effect. At the
same time, the wind will tend to push outdoor air in from one side of a building and stale air out of the
other side. Stack effect will dominate in cold calm weather, wind becomes important in mild windy
conditions. Usually, these effects will combine to dominate the airflows through most buildings, resulting
in air entering at low level on the windward side and leaving at high level on the leeward side. This
pattern can, however, be distorted by mechanical ventilation, ranging from a domestic extract fan or PSV
stack in a kitchen or bathroom up to a fully mechanically ventilated office block.
In addition, water vapour will tend to spread from areas of high vapour pressure to those of low vapour
pressure (irrespective of the relative humidity and temperature), i.e. from areas of high moisture content
to areas of low moisture content.
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Thus, in the absence of other constraints, moisture production areas should be located with regard to these
flows so that air and moisture tend to flow directly out of the building rather than spread within it,
especially to unheated areas such as bedrooms. Great care should be taken in the design of buildings to
ensure that warm moist air cannot move from local major sources of water vapour, such as a swimming
pool, which could have been designed to cope with the high humidity environment, into other areas which
have not been designed to as high a standard. Attention should be given to possible movement through
concealed spaces such as wall cavities, ventilation ducts or spaces above suspended ceilings. Any
mechanical ventilation systems should be designed to draw air from the rest of the building through the
moisture generating areas to the outside.
7.4 Construction
There are many forms of construction available for walls, floors, glazing and roofs. Often the choice will be
made on grounds other than condensation control, e.g. structural requirements or client preferences.
However, the following principles should be considered.
It is important to match the thermal response of the internal layers with the proposed heating and activity
regime. High mass elements will warm and cool slowly (slow thermal response) and they are therefore more
suitable for buildings which are heated for long periods. Low mass elements will warm and cool quickly
(fast thermal response) and are particularly suitable for infrequent or intermittent heating.
Some constructions, such as massive concrete floor slabs, contain a large amount of built-in water, often
known as “construction water”; this will take many months to dissipate and should be considered as a
significant source of water vapour within the building during this period.
Some floor finishes can become very slippery when wet; care should be taken in using these in situations
where they can become wet from direct condensation or from dripping of condensate from above.
The more a part of the structure is insulated, the warmer the internal surface will be for the same room
heat input and, consequently, the risk of surface condensation or mould growth will be lower. However,
layers to the outside of any extra insulation will be colder, and therefore more prone to interstitial
condensation. If that condensation is judged to be harmful, then steps should be taken to limit the amount
of moisture reaching the colder elements by using vapour control layers or inner layers of relatively high
vapour resistance or by the inclusion of a ventilated air space between the insulation and the outer
elements.
Thermal bridging should be minimized by careful design of vulnerable areas such as wall floor junctions,
roof eaves and areas around window and door openings (see 8.6).
7.5 Heating and ventilation
7.5.1 Heating systems
Heating will normally be tailored to personal comfort in the building, taking cost into consideration.
However, in addition, for condensation control, it should match the combined effects of occupancy pattern,
building mass and insulation, the period it is intended to heat the building, and any ventilation system,
natural or induced. The principles are explained by reference to extreme conditions.
If the heating maintains comfort levels in the whole building at all times, condensation problems will be
minimized, but costs will be high. If only one room is heated infrequently, that room could suffer
condensation because the structure will remain cold; other rooms will remain cold and moisture migrating
to them will cause severe condensation problems. These intermittent heating effects will be exacerbated if
the structure has a high thermal mass and if the heating is purely convective. A whole range of conditions
exists between these two extremes.
7.5.2 Ventilation
The building regulations for England and Wales (Approved Document F), Scotland (Technical
!Handbooks Section 3"), and Northern Ireland (Technical Booklet K), contain requirements for
ventilation of specific rooms in domestic and non domestic buildings, expressed in terms of openable areas,
background or trickle vents and the provision of extract fans or passive stack ventilators. There are also
specific requirements for ventilation of:
— areas in which specialist activities are taking place;
— enclosed car parks and garages; and
— rooms with mechanical ventilation or air conditioning plant.
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If stale air is extracted from a room and replaced by external air (and the loss in temperature made up by
heating), condensation risk is again minimized, but costs are high. At the other extreme, if minimal
ventilation exists and the air movement is from the moisture producing areas into the rest of the building,
condensation problems are likely.
The ideal ventilation system would extract air from the moisture producing areas to outside and replace it
with outdoor air flowing in via the other rooms. This would reduce the amount of moisture at source,
prevent its spread and ventilate the whole building with outdoor air.
Adequate ventilation for condensation control exceeds the minimum rate of outdoor air change necessary
for health and comfort and should normally be between 0.5 and 1.5 air changes per hour for the whole
building (see Table B.4 in Annex B and BS 5925).
There are a number of ventilation mechanisms that can be employed, including:
— passive devices such as trickle ventilators;
— passive stack ventilators which extract moist air from kitchens and bathrooms via a duct to the
roof ridge;
— supply ventilation systems, installed in a loft which supply air to the dwelling space.
Mechanical ventilation systems have also been effectively installed, providing a quiet, reliable energy
efficient solution at reasonable operating costs.
The ideal ventilation system is controllable, responds to occupancy and extracts air from the moisture
producing areas to outside during periods of high moisture generation and replaces it with a controllable
amount of outdoor air flowing in via the other rooms. This would regulate the amount of air required to
remove moisture at source, prevent its spread and ventilate the whole building with outdoor air in a
controlled manner.
7.6 Heating and ventilation costs
Control of condensation is always carried out at some cost, which can be minimized by good design, and
having designed the building with its heating and ventilation system, the running costs can be determined.
If these are not acceptable, some alteration to the heating and ventilation system will be required, possibly
in conjunction with improvements to the building fabric. Where achieving these acceptable running costs
involves compromise, this can result in condensation risks.
NOTE The designer should agree with the client the heating programme required to produce the minimum amount of heating
necessary to minimize condensation. The building owners should then decide whether or not to retain sufficient control of the heating
system to ensure that this heating is provided. Where this obligation is transferred to users, they should be provided with clear
operating instructions. To avoid misunderstandings in landlord/tenant situations, the obligations of each party should be defined in
the leasing agreement.
7.7 Risk assessment
At this stage, it is recommended that full checks are made on the likelihood of surface and interstitial
condensation and determine if these would be harmful. BS EN ISO 13788 contains recommended
procedures for the assessment of the risk of:
— surface condensation and mould growth; and
— interstitial condensation.
These calculation procedures are discussed in Annex D. Saturation vapour pressures are given in Table A.1
and thermal and vapour properties are given in Table C.1, Table C.2 and Table C.3 in Annex C.
BS EN ISO 13788 contains three criteria for assessing structures.
a) To avoid mould growth, the thermal design of the structure should be sufficient to keep relative
humidity at internal surfaces below 80 % in the most severe month of the year, given internal conditions
appropriate to the use of the building.
b) Any interstitial condensation that occurs within the structure in the winter should all evaporate
during the next summer to prevent an accumulation from year to year.
c) If interstitial condensation occurs over the winter and evaporates in the summer, the risk of
degradation to the materials present should be considered in terms of the maximum accumulated
condensation.
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At present it is not possible to provide hard information as to the amount of condensate that would cause
a problem in most situations. However, the following guidelines can be followed.
— Extensive work on flat roofs with continuously supported membranes has shown that they will
perform satisfactorily if the winter peak in condensate retained within the roof does not exceed 350 g/m2
provided that there is no accumulation from one year to the next.
— The prediction of any amount of condensate on wood or wood based materials, especially structural
components, should be treated with great caution and steps should be taken to eliminate it.
— An exterior leaf of masonry, which will be wetted by the rain can withstand substantial amounts of
interstitial condensate without adverse consequences.
— Condensate on impermeable surfaces such as a metal roof or plastic will not always cause any damage
where it occurs but can run or drip onto more vulnerable areas. The effect of various amounts of
condensate is summarized in Table 1.
If the checks are considered satisfactory, then construction can proceed as far as condensation control is
concerned; if not, it means the design will have to be reassessed by following the above procedure again.
Table 1 — Effect of condensate on an impermeable surface
Amount of condensate
g/m2
0 – 30
30 – 50
50 – 250
Effect
A fine mist that does not run even on vertical surfaces
Droplets forming that will start to run on vertical surfaces
Large drops forming that will run on sloping surfaces
70 g/m2 will run at a 45° slope
>250
150 g/m2 will run at a 23° slope
Drops forming that are large enough to drip from horizontal surfaces
8 Application of design principles: building fabric
8.1 General
Basic principles for condensation control have been outlined in Clause 7. The following clauses provide
more detailed guidance and recommendations on building according to these principles. It is essential that
designers read the general information on the particular element before referring to specific examples
(e.g. read 8.3.1 before 8.3.2).
Different types of construction are described according to their structure and position of insulation, with
sketches, notes and comments. Figure 3 to Figure 36 are not working drawings but illustrate principles
previously outlined, and are commented on in further detail. It is important to note that in illustrating
these principles, the thickness of some materials might be exaggerated for emphasis, and other components
such as damp-proof courses and fixings are omitted for clarity. Although a range of popular forms of
construction is shown, these will vary and all eventualities have not been described.
Due to the diversity of materials and construction methods, slight variations can be encountered. It is
recommended therefore that the risk of interstitial condensation is assessed with the calculation
procedures discussed in Annex D, for a range of values, in order to determine where it is likely to take place
and whether it is harmful. It should then be possible to decide whether a vapour control layer is needed
and/or if it is necessary to vent or ventilate any air space.
The ventilation openings referred to in the following clauses are geometric free areas; particular attention
should be paid to potential restrictions to air flows, including the effects of mesh. To overcome this difficulty
ventilator performance may by expressed as a particular flow rate in litres per second against a fixed
pressure difference of typically 10 Pascals which can be converted into an equivalent geometric free area.
Due to the air resistance of long path lengths, it is necessary to have a larger space between the layers of
the building element even though at or near the entry or exit points the gap can be considerably less. Care
should be taken to ensure that external outlets from moisture producing appliances (e.g. balanced flue
heaters) are placed away from ventilation air inlets.
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Where the desired thermal performance can be achieved only by the combination of two or more separate
material layers, careful attention should be paid to the relative properties of these layers. For example, in
an existing masonry cavity wall with partial cavity fill, interstitial condensation is likely to occur at the
outer leaf, where it is inconsequential. If this wall were upgraded with an insulated dry lining, the
interstitial condensation can move to the inner leaf where it could be harmful, necessitating the inclusion
of a vapour control layer. Similarly, where an existing roof construction is satisfactory, the provision of an
insulated suspended ceiling can cause a condensation problem unless the thermal balance and its effect on
the thermal response are considered.
Construction, occupancy, heating and ventilation all interrelate and it is essential that they are considered
as a whole.
8.2 Vapour control layers
!The measures required to achieve a functional vapour control layer must be carefully considered at the
design stage. A vapour control layer should extend over the whole of the element into which it is
incorporated and must be integrated with and sealed to adjoining elements, such as masonry, upstands and
glazing systems, and to any VCL in those elements.
Vapour control layers may be formed with a membrane within the structure, a lining board with an integral
membrane, or with a suitable coating applied to the internal surface of an element. A vapour control layer
should be of appropriate vapour resistance and should be situated on the warm side of the insulation.
In practice it is extremely difficult to construct a layer which is totally impermeable to water vapour. The
performance of a vapour control layer depends upon the vapour resistance of the material selected, the
practicability of the design and the standard of workmanship involved in its installation. Any unsealed
holes, fixings, pipes, electrical fittings, etc. which pass through the vapour control layer, will downgrade
performance; methods of avoiding such penetrations should be considered in the design stage.
Side and end joints in a flexible sheet vapour control layer should be kept to a minimum. Joints should be
made over a solid backing member or substrate, lapped not less than 50 mm and sealed with an appropriate
sealant. Similarly, tears and splits should be repaired using the same material, jointed as above. If
polyethylene sheeting is used, it should be protected from heat and sunlight to reduce the risk of
degradation occurring.
Where a vapour control layer is incorporated in a rigid board or on a profiled metal liner sheet, joints
between adjacent boards or sheets should be sealed to avoid mass transfer of water vapour due to air
leakage. These seals should be designed to accommodate thermal or other movements which may occur
during the design life of the building.
A vapour control layer can also act as an air leakage barrier, which, by reducing air movement, has the
added benefit of reducing the heat lost by convection. This is an increasingly important consideration as
the incorporation of greater amounts of insulation into the building fabric reduces heat loss by conduction.
Values of vapour resistance for various materials are given in Annex C, Table C.1 and Table C.2."
8.3 Walls
8.3.1 General
The designer should take account of five sources of dampness: the weather, ground moisture, surface
condensation, interstitial condensation and construction water. Whilst the problems of dampness from the
ground can be dealt with by the use of suitably placed damp-proof courses, the selection and arrangement
of materials to keep out the weather have implications in dealing with the other four sources.
Design guidance on differing types of wall construction is given in 8.3.2, 8.3.3, 8.3.4, 8.3.5, 8.3.6 and 8.3.7.
Other, more unusual wall constructions, such as curtain walling, walls of cold stores, breathing walls or
traditional constructions, such as cob walls, are not covered; specific advice on the performance of these
should be sought. However, the following factors should also be considered.
a) To prevent surface condensation, the thermal resistance of the wall should be sufficient to maintain
the inner surface above the dewpoint temperature for the design conditions. Decisions need to be made
therefore on the type, thickness and position of insulation required to achieve this, taking into account
the relationship between mass and thermal response described in 7.4.
The relationship of the insulation to the detailed structure at openings or junctions of elements, which
could contain dense high thermal transmittance materials, should be considered so that thermal
bridging is minimized.
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b) To minimize harmful interstitial condensation within a wall, designers should aim to specify materials
of decreasing vapour resistance from inside to outside. As an approximation, materials on the warm side
of any insulation should have a total vapour resistance of at least five times the sum of the vapour
resistances on the cold side of the insulation.
This might not always be practicable as some external claddings, such as profiled metal sheeting, are not
only weather-resistant but are also highly resistant to the passage of water vapour. Other constructions
such as plywood sheathed timber-framed panels have intermediate layers with relatively high vapour
resistance.
If condensation at these layers is considered harmful then:
1) a vapour control layer of adequate resistance should be located on the warm side of the insulation;
possibly in conjunction with
2) a vented or ventilated airspace provided to the immediate inside of any highly resistant material or
layer.
Service openings through a vapour control layer should be avoided. Where this is not possible, they
should be kept to a minimum and any openings taped and/or sealed.
Vented air spaces should have openings to the outside air of not less than 500 square millimetres per
metre length of wall. Where cavities are not continuous but occur between studs, frames or cavity
barriers, each individual cavity should have at least one vent. Care should be taken to prevent the
ingress of large insects, small mammals or birds and to avoid rainwater penetration. A nominal
mesh/grill size of 4 mm is recommended, to avoid excessive airflow resistance.
Where external claddings require the use of a membrane to avoid rainwater penetration, or where such
a membrane protects the insulation, this should be a breather type meeting the requirements of BS 4016.
c) During the drying out of the building, the risk of surface and interstitial condensation (including
reverse condensation) will be higher than when the construction has dried out.
d) Consideration should be given to the vulnerability of certain internal and external insulants or
insulation systems to mechanical damage and fire performance.
If the above recommendations are followed, the risk of decay to timber components will be minimized.
Nevertheless, for practical reasons, it might be advisable to increase the durability of structural timber
components by preservative treatment.
NOTE Timbers rated moderately durable or better (see BRE Digest 296 [1]) and which contain no sapwood do not normally require
preservative treatment (see BS 5268-5).
8.3.2 Masonry cavity walls
Masonry cavity walls are shown in Figure 3. The thermal response is slow to medium, except where
internal insulation is used [Figure 3b)], and therefore regular low output heating is recommended to
minimize surface condensation. Interstitial condensation is likely to occur on the inner surface of the outer
leaf but, in general, it will be inconsequential. Careful detailing is necessary at all openings to minimize
thermal bridging.
Internal insulation [Figure 3b)] provides a fast response surface, which reduces the risk of surface
condensation in intermittently heated dwellings. With this construction it is essential to incorporate a
vapour control layer between the insulation and the plasterboard lining to prevent severe interstitial
condensation on the inner masonry leaf. Vapour control layers are normally included in proprietary
insulating plasterboard products.
External insulation [Figure 3d)] can deal with external surface defects, alleviate rain penetration problems
and reduce the risk of thermal bridges causing surface condensation and mould growth. Interstitial
condensation is likely on the warm side of an impermeable cladding if a permeable insulation is used; in
these circumstances a vapour control layer should be provided on the inside of the wall construction and/or
a vented airspace provided on the immediate inside of the cladding.
Damp-proof courses should be carefully designed, and it is essential that cavity trays should have stop
ends. Detailed design should follow the recommendations of BS 8215.
© BSI 23 December 2005
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BS 5250:2002
3
4
2
2
2
2
5
5
a) Full cavity fill
3
2
4
4
b) Internal insulation
2
2
3
4
5
2
1
5
c) Partial cavity fill
d) External insulation
3
6
2
5
e) Insulating masonry inner leaf
Key
1 External render
4 Insulation
2 Masonry
5 Internal lining
3 Cavity
6 Light weight block
Figure 3 — Masonry cavity wall
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8.3.3 Solid walls: internal insulation
Internally insulated solid walls are shown in Figure 4. This type of construction provides a fast response
structure and so surface condensation is therefore unlikely in buildings with adequate heating and
ventilation.
Regular heating is preferable, but infrequent heating may also be used with this construction.
Thermal bridges at external wall/floor junctions, should be minimized and insulation and vapour control
layers, where provided, should be returned into the reveals of any opening.
There is a risk of interstitial condensation occurring on the inner surface of the masonry that can wet
timber studding or insulation in contact with the wall. In these cases, a vapour control layer should be
provided on the warm side of the insulation. However, the use of a vapour control layer can create a risk of
reverse condensation on its outer surface. If this is likely to be severe enough to cause damage, a vented
airspace should be provided on the cold side of the insulation and/or a weatherproof surface finish to the
wall (see 5.3.3).
Alternatively, an external weather protection of low vapour resistance should be applied to reduce
rainwater penetration. This, in turn, reduces the risk of damage to parts of the construction and the risk
of reverse condensation.
Any fittings or spacers should be durable: where timber battens are used, they should be durable or
preservative treated.
2
1
3
1
4
2
3
5
4
5
6
Key
Key
1 Weather protection (if required)
1 External finish
2 Masonry
2 Insulation
3 Airspace (if required)
3 Masonry
4 Insulation
4 Vapour control layer (if required)
5 Vapour control layer (if required)
5 Internal lining
6 Internal lining
Figure 4 — Solid wall: internal insulation
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Figure 5 — Solid wall: external insulation
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BS 5250:2002
8.3.4 Solid walls: external insulation
Externally insulated solid walls are shown in Figure 5. To minimize surface condensation, the heating
system should be matched to the construction. The thermal response is slow and, therefore, constant
low-output heating is recommended.
To avoid thermal bridging the insulation should be returned into the reveals of any openings. Particular
attention should be paid to wall/roof and wall/floor junctions.
Interstitial condensation is unlikely if the external insulation system (insulant and finish) is of low vapour
resistance.
If the external insulation system incorporates high vapour resistance cladding, interstitial condensation is
likely to occur. A vapour control layer should be provided on the inside of the wall construction and/or a
vented airspace provided on the immediate inside of the cladding.
8.3.5 Timber framed walls
Timber framed walls are shown in Figure 6 and Figure 7. If thermal bridges are avoided, this type of
construction provides a fast response structure. Surface condensation is therefore unlikely and, while
regular heating is preferable, intermittent heating may be used.
Thermal bridges should be minimized, particularly at external wall/floor and external wall/roof junctions.
While timber studs cause repeated thermal bridges that have to be taken into account when calculating
heat loss, their effect on internal surface temperature is not sufficient to increase the risk of condensation.
A vapour control layer is essential on the warm side of the insulation to reduce the risk of damaging
interstitial condensation on the inner surface of the sheathing. Service penetrations should be avoided;
where this is not possible, they should be kept to a minimum and the openings sealed.
Because of the risk of interstitial condensation occurring on the inner surface of the cladding, it is essential
that the construction be vented, preferably by a cavity. If the cladding is of high vapour resistance material,
e.g. metal or plastic, a drained cavity is essential. Where the cladding also functions as the sheathing, a
vented airspace is recommended. If this is not possible, the cladding should have as low a vapour resistance
as possible or have open joints at horizontal laps.
8.3.6 Metal framed walls
Metal framed walls are shown in Figure 8 and Figure 9. This type of construction provides a fast response
structure. Surface condensation is therefore unlikely and, while regular heating is preferable, intermittent
heating may be used. However, detailing should ensure that insulation is positioned in such a way to avoid
significant thermal bridging through the metal.
Two types of construction are common.
a) In the warm frame construction, the entire steel frame is on the inside of the insulation layer
(see Figure 8). This means that the steel is kept above the dewpoint so there is no need of a vapour control
layer. Some interstitial condensation will occur on the external cladding but, if the cavity is vented, this
will be inconsequential. A breather membrane should be included on the face of the insulation to repel
any water that penetrates the cladding.
b) The other type of steel-framed wall has the frame within the main insulation layer with an insulated
sheathing outside (see Figure 9). As the steel is bridging part of the insulation layer the thermal
sheathing must be designed to provide sufficient insulation to avoid the risk of interstitial condensation.
A vapour control layer is necessary on the warm side of the insulation to reduce the risk of damaging
interstitial condensation on the inner surface of the sheathing and unacceptably high humidities causing
corrosion to the frame.
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4
4
3
3
5
2
6
2
5
6
1
1
7
7
Key
Key
1 Masonry
1 Vertical tiling on battens
2 Vented cavity
2 Counterbattens
3 Breather membrane
3 Breather membrane
4 Sheathing
4 Sheathing
5 Frame/insulation
5 Frame/insulation
6 Vapour control layer
6 Vapour control layer
7 Internal lining
7 Internal lining
Figure 6 — Framed wall
© BSI 23 December 2005
Figure 7 — Framed wall with tile cladding
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BS 5250:2002
3
2
4
1
6
5
Key
1 External cladding
4 Insulation
2 Vented cavity
5 Steel frame
3 Breather membrane
6 Internal lining
Figure 8 — Warm steel frame wall
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1
2
3
4
5
Key
1 One or two layers of plasterboard
4 Wall tiles
2 Vapour control layer
5 Insulating sheathing board
3 Light steel studs with mineral wool between
Figure 9 — Steel frame wall with frame within the insulation
8.3.7 Profiled metal walls
8.3.7.1 Site assembled metal wall
Site assembled twin skin systems (see Figure 10), in which the principal thermal insulation layer is placed
at or immediately inside the internal lining, result in the external profiled sheeting being substantially
colder (in winter) than the interior of the building. A vapour control layer should be included at or
immediately inside the internal lining. This may be achieved by sealing the side and end lap joints of metal
liner sheets or by the use of a vapour control membrane with sealed joints.
To alleviate any condensation where high internal humidity is predicted, the void between the insulation
and external profiled sheeting should be through-ventilated to the outside air.
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8.3.7.2 Composite panel wall
In a composite panel (see Figure 11) the voids in profiled sheeting are completely filled by insulation. Local
condensation cannot occur in sandwich panels with a vapour impermeable undersheet (e.g. metal).
However, in practice, small voids will still occur at side and end laps where vapour leakage can occur and
local condensation can develop. Therefore, sandwich panel systems should be capable of being sealed at
side and end laps to prevent moist air entering the joints between panels.
1 2 3
2
1
3
5
4
4
6
5
Key
Key
1 Profiled metal external sheet
1 Profiled metal external sheet
2 Insulation
2 Insulation
3 Vapour check or sealed liner sheet joints
3 Metal liner sheet
4 Metal liner sheet
4 Structural support
5 Structural support
5 Vapour tight seal at panel joints
6 Spacer system with thermal break
Figure 10 — Site assembled metal wall
Figure 11 — Composite panel wall
!8.4 Roofs
8.4.1 General
8.4.1.1 Sources of moisture entering the roof
8.4.1.1.1 The designer should take account of the following sources of moisture in buildings:
a) water incorporated during the construction process (including precipitation);
b) precipitation after construction;
c) water vapour arising from the occupants and their activities;
d) temporary condensation which may occur when cold weather conditions are followed by warm humid
weather."
22
© BSI 23 December 2005
BS 5250:2002
!8.4.1.1.2 In order to avoid damage from condensation, the designer should minimize the amount of
moisture entering the roof by:
a) selecting materials and forms of construction with low moisture content;
b) minimizing moisture from the construction process by allowing the building to dry out adequately
where wet forms of construction have been used;
c) protecting the building from precipitation during the construction process;
d) ensuring wall cavities do not interconnect to cavities in the roof;
e) providing a weatherproof covering designed in accordance with the guidance given in codes of practice
for the chosen materials and form of construction;
f) providing the means to minimize the amount of water vapour which reaches the roof from the occupied
space within the building, by:
1) removing it at source, by means of passive or mechanical extract ventilation, and
2) providing a well sealed ceiling (see 8.4.1.2);
g) designing the roof in accordance with the recommendations in 8.4.2, 8.4.3, 8.4.4 and 8.4.5.
8.4.1.2 Air tightness of sloping and horizontal ceilings
Air leakage through gaps in a ceiling transfers more moisture into the roof by convection than passes
through the ceiling materials by diffusion (see 5.3.1); it also transfers a substantial amount of heat into the
roof. Sealing the ceiling will reduce both moisture transfer and heat loss, thus minimizing the risk of
condensation in the roof whilst at the same time improving the energy efficiency of the building, however
a totally airtight ceiling is extremely difficult to achieve in practice.
A well sealed ceiling requires the following.
a) The design avoids constructional gaps, especially at the wall/ceiling junction with dry lining
construction, and holes in the ceiling.
b) No access door or hatch should be located in rooms where large amounts of moisture are produced,
including kitchens or bathrooms.
c) The air leakage rate through an access hatch, including its frame, when tested to
BS EN 13141-1:2004 4.3 is less than 1 m3/h at a pressure difference of 2 Pa. It can be assumed that “pushup” wooden hatch covers in a frame, constructed in-situ, with continuous compressible seals, will meet
this criterion provided the weight of the door is at least 5.5 kg. Hatch covers should either be heavy
enough to compress a seal or be clamped, with a closed cell compressible seal, or “O-ring” between it and
the frame. Drop-down hatch covers are more difficult to seal; it is recommended that proprietary units
with a supplied hatch cover in a frame are used. Manufacturers can provide third party evidence that
the leakage criterion is met.
d) Penetrations, such as those for services and rooflights, are permanently sealed with suitable
proprietary products.
e) The ceiling is sealed to the external walls to limit any leakage through cracks.
f) Recessed light fittings should either comply with BS EN 60529 and be rated IP60 to IP65 (depending
on room use), or incorporate an appropriate sealed hood or box which meets the following test
criteria. The total leakage through all downlighters should not exceed 0.06 m3/h·m2 of ceiling
at 2 Pa. The leakage of individual downlighters can be tested using the method specified
in 4.3 of BS EN 13141-1:2004.
g) The head of any cavity in any wall or partition should be sealed to prevent transfer of warm moist air
into the loft.
At the design stage, it is important to consider how construction details that are robust over the lifetime of
the building can be achieved. A well-sealed ceiling requires high standards of workmanship by the trades
involved in installing plasterboard or other ceilings, plumbing and electrical services.
When existing buildings are being upgraded, refurbished or reroofed, the advantages of improving the
airtightness of the existing ceiling should be considered; however, it might not be possible to achieve a well
sealed ceiling in those circumstances and that should be borne in mind when determining the form of
construction and selecting materials."
© BSI 23 December 2005
23
BS 5250:2002
!8.4.1.3 Installation of vapour control layers
Vapour control layers should be formed on the warm side of the insulation; it is essential they are
adequately lapped and the laps sealed to maintain their integrity. Care should be taken to seal around the
perimeter, at junctions and at penetrations such as electrical cables, ducts and pipes, and access hatches.
It is good practice to seal off the tops of the external walls to prevent water vapour from the wall entering
the roof void, see 8.4.1.2.
General guidance on the design and installation of vapour control layers is given in 8.2.
8.4.1.4 Condensation risk
The designer should ensure that any moisture in the roof will be able to disperse to the external atmosphere
without forming harmful condensation.
Where buildings have been completed without the relevant advice in 8.4.1.1 being followed, provision
should be made for removing the large quantities of water the structure is likely to contain.
In ventilated constructions, it is important to ensure airpaths remain unobstructed during the life of the
building. Particular attention should be paid to avoiding potential restrictions in airpaths at changes in
roof slope, at changes in constructional details, at penetrations and at junctions with walls.
All sources of moisture should be considered, taking account of the expected building occupancy
(see Annex B). Where there is little or no airflow through the roof from the occupied space to the outside,
condensation risk analysis can be undertaken in accordance with D.3 to establish the risk of harmful
condensation occurring within the roof. In cases such as pitched roofs with accessible lofts, where moisture
transport is dominated by air transport, the method of D.3 no longer applies; guidance for appropriate
analysis of these situations is given in D.4.
The risk of condensation in a roof depends upon:
a) the building humidity class (see Annex B);
b) the vapour resistance and air permeability of the ceiling, taking account of the presence and integrity
of any VCL or air barrier;
c) the thermal conductivity and vapour resistance of the thermal insulation;
d) the presence of voids within the roof and whether they are connected to atmosphere;
e) the vapour resistance and air permeability of the underlay, including the effect of any sarking boards;
f) the air permeability of the weatherproof covering.
Design guidance on different types of roof construction is given in 8.4.2, 8.4.3, 8.4.4, 8.4.5 and 8.4.6.
8.4.2 Pitched roofs
8.4.2.1 General
8.4.2.1.1 Introduction
Pitched roofs have a slope of between 10º and 70º; they can be duo-pitch, mono-pitch or mansard and can
incorporate features such as valleys, hips, gables, dormers, roof lights and chimneys. A wide variety of
materials is used to form the outer covering.
The weatherproof covering of domestic pitched roofs is usually formed with small discontinuous units such
as tiles, slates or shingles, fixed to battens, above an underlay which reduces wind uplift forces on the outer
roof covering and forms a secondary barrier against wind-driven precipitation.
Roofs of commercial, retail, hospital and schools may be constructed with discontinuous units similar to the
domestic pitched roofs, or they may have either large discontinuous profiled sheeting or flat continuously
supported coverings. BS 6229 gives recommendations for avoiding the risk of condensation in flat
continuously supported covered roofs.
Profile sheeted roofs where the outer skin provides the weatherproof covering can be either:
a) site assembled twin skin construction composed of an outer sheet, rigid liner sheet, spacers and
insulation;
b) factory assembled composite panels with insulation filling the space between outer and liner sheets; or
c) site assembled single skin construction composed of an outer sheet and underlay."
24
© BSI 23 December 2005
BS 5250:2002
!8.4.2.1.2 Roof coverings and batten spaces
Research has shown that moisture conditions in the space between the outer roof covering and the underlay
are dominated by the external environment, the air openness of the outer covering and the vapour openness
of the underlay. If an HR underlay is used there will be relatively little moisture transfer from within the
building to the batten space. In the case of an LR underlay more moisture will be transferred: in order to
avoid a build up of moisture in the batten space it is good practice to ensure adequate air movement
through the void between the outer covering and the underlay. Most traditional unsealed slating and tiling
methods are sufficiently air open.
Should it be necessary to determine if the roof covering is sufficiently air open, it should be tested using a
rig as described in Annex L of BS 5534:2003 but with equipment designed to measure low pressure
differences down to 2 Pa.
If the airflow at 2 Pa is, or could be reasonably expected to be, greater than 7.8Ar (in m3/h, where Ar is
the effective test rig area in m2) the roof covering will allow sufficient air movement in the batten space
without additional ventilators. If the airflow is not, or could not reasonably be expected to be, greater
than 7.8Ar (in m3/h), it is necessary to provide either:
a) ventilation openings to the batten space which are equivalent to a continuous slot 25 mm wide in the
eaves and 5 mm wide at the ridge and 25 mm counterbattens; or
b) ventilation openings to the roof void or air void below the underlay as specified in 8.4.2.2.3.
In case of uncertainty, the air tightness of the roof covering should be tested, as above, or ventilation
openings to the batten space or loft installed.
If there is any temporary condensation when cold dry conditions are followed by humid weather
(see 8.4.1.1.1), adequate ventilation of the batten space, either by infiltration around the tiles or through
special vents will allow any condensation to disperse rapidly.
8.4.2.1.3 Types of underlay
BS 5534:2003 recognizes two types of underlay.
a) Type LR underlay has a water vapour resistance of less than or equal to 0.25 MN·s/g.
b) Type HR underlay has a water vapour resistance greater than 0.25 MN·s/g.
For the purposes of condensation control the important considerations are the vapour resistance of the
underlay and the amount of vapour transfer that might take place through unsealed laps of the underlay.
The designer must consider the system as a whole and take account of the total resistance offered by the
underlay and any supporting material. Thus, an LR membrane, which is fully supported on material which
offers a high resistance to the passage of water vapour, such as plywood, oriented strand board (OSB) or
chipboard, should be treated for design purposes as an HR underlay. LR underlays laid on open jointed
sarking boards (see 3.23) may be regarded as LR underlays.
In principle, the lower the water vapour resistance of the LR underlay, the greater its ability to lower the
risk of condensation.
Certain LR underlays are air permeable to some degree, which may allow some air movement in the roof
void, and can reduce the risk of condensation on the underlay. The degree of air permeability of an underlay
material can be determined by testing to BS EN 13141-1:2004 at 2 Pa. Designers should ensure that
any potential increase in wind loading on the slates and tiles is taken into account in accordance
with BS 5534:2003.
Both type HR and type LR underlays, with a smooth underside, can create problems with condensate run
off. Underlays which can hold or absorb moisture, and re-evaporate it when conditions are more favourable,
are beneficial (see Table 2).
Table 2 — Maximum retention of condensate before running
or dripping on different underlay types, in g/m2
Type of underlay
Slope of roof
15°
Sanded bitumen felt
280
Underlays with underside fleeces
160
Underlays without undersides fleeces 80
30°
230
130
70
45°
180
100
60
"
© BSI 23 December 2005
25
BS 5250:2002
!8.4.2.1.4 Location and type of insulation
Pitched roofs normally include a ceiling lining and thermal insulation, situated either:
a) directly above a horizontal ceiling, with a large void (the loft) above the insulation; or
b) in the plane of the rafters with a small void (or no void at all) above the insulation.
Roofs containing habitable spaces may incorporate both forms of construction and dwarf stud partitions.
Design guidance appropriate to particular roof constructions is given in 8.4.2.2 and 8.4.2.3.
8.4.2.2 Pitched roofs with a large void above the insulation
8.4.2.2.1 Introduction
These roofs contain a large void or loft above thermal insulation. The void is often accessible through a
hatch in the ceiling, may contain water tanks and other plumbing and may be used for storage. This
category includes “room-in-the-roof ” constructions where some insulation follows the rafter slope whilst
the remainder follows the line of a horizontal ceiling and any dwarf stud partitions.
8.4.2.2.2 Condensation risks
8.4.2.2.2.1 Surface condensation and mould growth are unlikely to occur on the underside of the ceiling
provided that:
a) the insulation is continuous over the whole ceiling including the access hatch (see BRE Report 262 [16]
for further guidance); and
b) thermal bridging is minimized, particularly at the junction of the roof with the external walls.
8.4.2.2.2.2 High moisture levels and interstitial condensation can cause problems in this type of roof.
a) Condensate can form on the underlay overnight at any time of the year; problems arise if conditions
allow an accumulation of condensate sufficient to run and drip onto the insulation, the roof timbers
and/or household goods stored in the loft (see Table 2).
b) Condensate can accumulate on the underside of the outer covering of the roof in sufficient amount to
lead to increased moisture content of the tiling battens.
c) Persistently high levels of relative humidity will cause hygroscopic materials, including the rafters,
battens, and goods stored in the loft, to absorb sufficient water vapour to promote rot in the timbers and
the growth of moulds.
8.4.2.2.2.3 Those problems may be minimized by:
a) providing the means to remove water vapour generated within the building at source; and
b) reducing the amount of water vapour which can pass into the loft by:
1) ensuring that the ceiling is well sealed (see 8.4.1.2); and
2) ensuring that loft and access hatches are not located in rooms in which large amounts of moisture
are produced, including kitchens and bathrooms.
8.4.2.2.2.4 The designer then has the option to:
a) use a type HR or LR underlay and ventilate the loft space (see 8.4.2.2.3); or
b) use a type LR underlay either unsupported or supported on sarking boards (see 8.4.2.2.4).
8.4.2.2.3 Roofs with a type HR underlay (unsupported or fully supported) or fully supported type LR
underlay
8.4.2.2.3.1 Roofs of this type are shown in Figure 12.
Problems from interstitial condensation are unlikely to occur provided that there is adequate provision for
ventilation of the loft as detailed in 8.4.2.2.3.2 and 8.4.2.2.3.3.
Note that, when an existing roof is being recovered, whilst desirable, it might not be possible to achieve a
ceiling that is as well sealed as in new build."
26
© BSI 23 December 2005
BS 5250:2002
!
1
2
3
4
1
2
3
4
A
5
A
Section A - A
5
6
7
a) Large void above the insulation and a type HR underlay (unsupported)
Key
1 Roof covering
3 Type HR underlay, with 10 mm 4 Rafters
approx. drape (see BS 5534:2003) 5 Large ventilated void (loft)
2 Tiling battens
1
2
3
4
5
A
6
7
7 Well sealed ceiling
(see 8.4.1.2)
1
2
3
4
5
6
7
A
6 Insulation
Section A - A
8
9
Alternative Section A - A
b) Large void above the insulation and a type HR underlay on sheet or board sarking or type LR underlay fully supported on sheet
sarking (see BS 5534:2003 4.10.2)
Key
1 Roof covering
2 Tiling battens
3 Counterbattens
4 Type HR or LR underlay with
10 mm approx. drape where
appropriate (see BS 5534:2003)
5 Rigid sarking (sheet sarking 7 Large ventilated void (loft)
or sarking boards)
8 Insulation
6 Rafters
9 Well sealed ceiling
Figure 12 — Pitched roof with insulation on a horizontal ceiling — Ventilated below the
underlay
"
© BSI 23 December 2005
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BS 5250:2002
!8.4.2.2.3.2 Ventilation openings should be provided at low level on the longer sides of a typical
rectangular roof, or by some equivalent openings on the shorter sides that will allow good through
ventilation, avoiding stagnant air pockets. The openings should be equivalent in area to a continuous
opening of not less than:
a) 25 mm × length at eaves for pitches of 15º or less;
b) 10 mm × length at eaves for pitches of more than 15º.
8.4.2.2.3.3 Additional ventilation openings, equivalent in area to a continuous opening of 5 mm, should be
provided at high level in:
a) roofs where the pitch exceeds 35º;
b) roofs of any pitch with a span greater than 10 m;
c) lean-to and mono-pitch roofs.
8.4.2.2.3.4 Designers should ensure that ventilation openings provide a continuous weatherproof path
between the loft and outside atmosphere without compromising the weatherproof function of the underlay
or of the roof covering. This can be achieved by ensuring that:
a) all airpaths between the insulation and the underlay, including those provided by proprietary eaves
ventilation products, are at least 25 mm deep, irrespective of opening size (see 8.4.1.4);
b) ventilation openings are not blocked by insulation material, dust, airborne debris, paint or frost;
c) the ingress of rain, snow is prevented;
d) entry of birds and large insects is prevented; to achieve that, and to avoid excessive airflow resistance,
a nominal mesh/grill size of 4 mm is recommended;
e) ventilation openings do not form thermal bridges at the eaves (see the Robust Construction
Details [5]).
8.4.2.2.4 Roofs with a type LR underlay, (unsupported or fully supported on sarking boards)
8.4.2.2.4.1 Roofs of this type are shown in Figure 13, Figure 14 and Figure 15.
A
1
2
3
4
1
2
3
4
A
5
Section A - A
5
6
7
a) Side view
b) View along rafters
Key
1 Air open roof covering
4 Rafters
2 Tiling battens
5 Large ventilated void (loft)
3 Unsupported type LR underlay, with a minimum drape of 10 mm 6 Insulation
7 Well sealed ceiling (see 8.4.1.2)
Figure 13 — Pitched roof — Large ventilated void above the insulation and a type LR
underlay unsupported with an air-open roof covering (see 8.4.2.1.2)
"
28
© BSI 23 December 2005
BS 5250:2002
!
1
2
3
4
5
1
2
3
4
5
A
Section A - A
A
6
6
7
8
a) Side view
b) View along rafters
Key
1 Tight roof covering
5 Rafters
2 Battens
6 Large ventilated void (loft)
3 Ventilated counterbatten space
7 Insulation
4 Unsupported type LR underlay
8 Well sealed ceiling (see 8.4.1.2)
Figure 14 — Pitched roof — Large ventilated void above the insulation and a type LR
underlay unsupported with a tight roof covering (see 8.4.2.1.2)
1
2
3
4
5
A
6
7
1
2
3
4
5
6
7
Section A - A
A
8
9
Alternative Section A - A
a) Side view
b) View along rafters
Key
1 Air open roof covering
6 Rafters
2 Tiling battens
7 Large ventilated void (loft)
3 Counterbattens
8 Insulation
4 LR underlay, installed beneath or over counterbattens
9 Well sealed ceiling (see 8.4.1.2)
5 Sarking boards
Figure 15 — Pitched roof — Large ventilated void above the insulation and a type LR
underlay supported on sarking boards
"
© BSI 23 December 2005
29
BS 5250:2002
!Problems from interstitial condensation are unlikely to occur provided that the ceiling is well sealed as
described in 8.4.1.2, and either:
a) the eaves are designed with at least the minimum loft ventilation option depending on the ceiling type
as shown in Table 3; in new build situations with constructions which are expected to produce a
significant additional moisture load, consideration should be given to installing an additional 5 mm high
level ventilation; or
b) a minimum of 5 mm ventilation slot is provided at high level.
Table 3 — Minimum equivalent continuous low level loft space
ventilation openings with LR underlays
Ceiling type
Minimum equivalent continuous low
level loft space ventilation openings
mm
Normal ceiling
Well-sealed ceiling (see 8.4.1.2)
7
3
These figures have been derived following the principles in D.4 sssuming an underlay
vapour resistance of 0.25 MN·s/g. Use of an underlay with lower vapour resistance will
reduce the risk of condensation.
NOTE Proprietary low-level ventilation products which usually provide an area equivalent
to a 10 mm continuous eaves opening will satisfy these requirements.
The provisions in Table 3 apply to roofs over buildings typical of housing. Much larger roofs of this type,
such as those over supermarkets, hospitals or schools should include the minimum equivalent
of a 10 mm opening in the eaves for a normal ceiling or a 5 mm opening if the ceiling is well sealed as
well as a 5 mm opening at high level.
8.4.2.2.4.2 The following points should also be considered.
a) To achieve adequate cross ventilation and avoid stagnant air pockets, any ventilation should wherever
possible be placed on the longer sides of a typical rectangular roof.
b) The values in Table 3 assume that the overlaps in the membrane are not sealed. If they are sealed then
larger amounts of eaves ventilation will be required.
c) If a tight roof covering is used (see 8.4.2.1.2) it is necessary to either provide counterbattens and
ventilate the batten space, as shown in Figure 14, or to ventilate the loft space to 8.4.2.2.3. With other
coverings, no counterbattens and batten space ventilation are necessary (see Figure 13).
d) If there is expected to be a significant moisture load from construction water in a new build house,
additional 5 mm minimum ventilation should be provided at high level.
8.4.2.2.4.3 Note that, when an existing roof is being re-covered, it may not be possible to achieve a ceiling
that is as well sealed as in new build. Consideration should therefore be given to increasing the area of low
level ventilation openings as shown in Table 3, or providing a minimum of 5 mm ventilation slot at high
level.
8.4.2.2.4.4 Roof with an LR underlay with limited or no ventilation
This Code does not consider the situation where it is proposed to provide no ventilation to the roof void, or
ventilation more limited than recommended in 8.4.2.2.4.1 or 8.4.2.2.4.2.
Where no ventilation to the roof space is proposed, it is recommended that reference be made to the
conditions attached to Technical Approvals given by UKAS (or European equivalent) accredited technical
approval bodies."
30
© BSI 23 December 2005
BS 5250:2002
!8.4.2.3 Pitched roofs with a small void or no void above the insulation
8.4.2.3.1 Introduction
In these roofs the insulation follows the line of the rafters and there is either a small void (no deeper than
the rafters), or no void separating the insulation from the underlay. The presence of a void will depend upon
the position of the insulation and its depth relative to the roof framing.
NOTE Roofs commonly called “hybrid roofs”, which contain both sloping and horizontal insulation, are classified as “roofs with large
voids”, if any part of the void is greater than the depth of the rafters (see 8.4.2.2).
Detailed guidance for the design of this type of roof is given in Annex B of BS 5534:2003.
8.4.2.3.2 Condensation risks
8.4.2.3.2.1 Surface condensation and mould growth are unlikely to occur on the underside of the ceiling,
provided that:
a) no gaps have been left in the insulation covering the ceiling; and
b) thermal bridging is minimized, particularly at the junction of the roof with the external walls and
around roof windows.
8.4.2.3.2.2 High moisture levels and interstitial condensation can cause problems in this type of roof.
a) Condensate might form on the underside of the underlay overnight at any time of the year; problems
arise if conditions allow an accumulation of condensate sufficient to run and drip onto the insulation and
the roof timbers.
b) Condensate can accumulate on the underside of the outer covering of the roof in sufficient amount to
lead to increased moisture content of the tiling battens.
c) Persistently high levels of relative humidity will cause hygroscopic materials, including the rafters and
battens, to absorb sufficient water to promote rot in the timbers and the growth of moulds.
8.4.2.3.2.3 Those problems may be minimized by reducing the amount of water vapour which can pass into
the roof from the occupied space, by:
a) providing the means to remove it at source;
b) providing a well sealed ceiling (see 8.4.1.2);
c) installing an effectively sealed VCL between the insulation and the ceiling (see 8.4.1.3); and
d) sealing all gaps/joints between insulation boards.
8.4.2.3.2.4 The designer then has the option to:
a) use a type HR underlay with a ventilated void beneath it (see 8.4.2.3.3); or
b) use a type LR underlay fully supported on the insulation (no void) (see.8.4.2.3.4); or
c) use a type LR underlay unsupported (small void) (see 8.4.2.3.5).
When this type of roof is being designed, it is essential to do an interstitial condensation analysis in
accordance with BS EN 13788, especially when the insulation is made up of layers of materials with
different thermal and vapour transmission properties.
8.4.2.3.3 Roofs with a type HR underlay, unsupported over a small void
Roofs of this type are shown in Figure 16."
© BSI 23 December 2005
31
BS 5250:2002
!
1
2
3
4
5
6
7
8
9
A
1
2
3
4
5
6
7
8
9
Section A - A
A
a) Side view
b) View along rafters
Key
1 Roof covering
6 Insulation
2 Tiling battens
7 Rafters
3 Unsupported type HR underlay with 10 mm approx. drape
8 VCL
4 50 mm counterbattens
9 Well sealed ceiling (see 8.4.1.2)
5 50 mm deep ventilated void (minimum of 25 mm at centre of
underlay drape)
Figure 16 — Pitched roof — Small ventilated void above insulation and a type HR underlay
Problems from interstitial condensation are unlikely to occur provided that a well sealed ceiling is provided
and vented voids at least 25 mm deep at the centre of the drape are maintained between the insulation and
the underlay. It is important those airpaths remain unobstructed during the life of the building: particular
attention must be paid to potential restrictions at eaves, at changes in roof slope, at valleys and hips, and
at changes in construction details where such a void may be difficult to achieve. Obstructions such as
dormers, roof windows, compartment walls, fire-barriers, or changes in pitch create separate voids in the
roof slope.
Ventilation openings should be provided to each void, at both high and low level, to allow free air movement
through the gap between the insulation and the underlay.
a) Low level openings should be equivalent in area to a continuous opening of not less
than 25 mm × length at eaves; and
b) High level openings should be equivalent in area to a continuous opening of not less
than 5 mm × length at the ridge or hips. Where there is no cross communication between each
roof slope, 5 mm should be provided on both sides of the ridge."
32
© BSI 23 December 2005
BS 5250:2002
!8.4.2.3.4 Roofs with no void with a type LR underlay (fully supported on insulation)
Roofs of this type are shown in Figure 17.
1
2
3
4
5
6
7
8
B
1
2
3
4
5
6
7
8
B
Key
1 Roof covering
5 Insulation
2 Tiling battens
6 Rafters
3 Counterbattens
7 VCL
4 Type LR underlay supported on the insulation material
8 Well sealed ceiling (see 8.4.1.2)
Figure 17 — Pitched roof — No void above insulation and a type LR underlay
It is not necessary to provide a void beneath the underlay in roofs of this type, but a well sealed VCL should
be provided below the insulation.
Problems from interstitial condensation are unlikely to occur if a well sealed ceiling is provided and there
is sufficient air movement between the underlay and the roof covering to allow moist air to migrate to the
atmosphere. For air-open outer coverings no specific provision for batten-space ventilation is required, but
for tight outer coverings batten space ventilation should be provided (see 8.4.2.1.2)."
© BSI 23 December 2005
33
BS 5250:2002
!8.4.2.3.5 Roofs with a small void with draped type LR underlay
Roofs of this type are shown in Figure 18.
1
2
3
4
5
6
7
1
2
3
4
5
7
Section
6
1
2
3
4
5
6
5
7
Key
1
2
3
4
Air open roof covering
Tiling battens
Type LR underlay with 10 mm approx. drape
Rafters with small voids between
5 Insulation
6 Vapour control layer with taped joints (VCL to be carefully cut
and sealed around struts and ceiling joist penetrations)
7 Well sealed ceiling (see 8.4.1.2)
NOTE This does not apply to trussed rafter roofs where the VCL is interrupted.
Figure 18 — Pitched roof — Small void above insulation and a type LR underlay
There are many forms of roof where, for constructional reasons, there is a small void above the insulation.
Problems from interstitial condensation are unlikely to occur if a well-sealed ceiling is provided and there
is sufficient air movement between the underlay and the roof covering to allow moist air to migrate to the
atmosphere. For air-open outer coverings no specific provision for batten space ventilation is required, but
for tight outer coverings batten space ventilation should be provided (see 8.4.2.1.2). If the integrity of the
roof and wall VCL can be maintained, there is no need to provide ventilation in the void between the
insulation and the LR membrane. If there is any doubt about the ability to provide and maintain an
effectively sealed VCL, ventilation should be provided in accordance with 8.4.2.3.3."
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BS 5250:2002
!8.4.2.4 Tiled and slated roofs containing rooms: fully inclined ceilings
These roofs are shown in Figure 19. Roof constructions should comply with the recommendations for roofs
with HR underlays, given in 8.4.2.3.3, and for roofs with LR underlays given in 8.4.2.3.4.
If an HR underlay is used, ventilation should be provided at the positions shown in Figure 19. If an
obstruction in the ventilation paths occurs, such as at skylights or roof windows, compartment or fire-break
walls or at changes in pitch, the roof void should have additional ventilation openings:
a) immediately below the obstruction equivalent to 5 mm × length of obstruction; and
b) immediately above the obstruction equivalent to 25 mm × length of obstruction.
25
5
5
25
5
5
2
a) At skylight windows
1
5
5
25
25
25
5
2
b) At compartment or fire-break wall with insulation following
slope of roof
1
25
25
c) At compartment or fire-break wall with large void at
ridge level
Key
1 Fire-break wall
2 Clear airway
Figure 19 — Ventilation positions for room in the roof construction requiring
ventilation
8.4.2.5 Tiled and slated roofs containing rooms: partially inclined ceilings
Roof constructions should comply with the recommendations for roofs with HR underlays given in 8.4.2.3.3,
and for roofs with LR underlays given in 8.4.2.3.4.
Where the roof construction is of the type described in 8.4.2.3.3 with an HR underlay, ventilation should
be provided at the positions shown in Figure 20. If an obstruction in the ventilation paths occurs, such as
at skylights or roof windows, dormer sills or at changes in pitch, the roof void should have additional
ventilation openings:
a) immediately below the obstruction equivalent to 5 mm × length of obstruction; and
b) immediately above the obstruction equivalent to 25 mm × length of obstruction.
NOTE A large void at the apex may be treated in the same way as 8.4.2.2.
"
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BS 5250:2002
!
5
1
25
2
5
25
25
Key
1 At least 50 mm clear airway
2 Clear airway
Figure 20 — Ventilation positions for room-in-roof construction including a flat roofed
dormer window
8.4.2.6 Pitched roofs containing rooms: dormers
Roof constructions should comply with the recommendations for roofs with HR underlays given in 8.4.2.3.3
and for roofs with LR underlays given in 8.4.2.3.4.
Where the roof construction is of the type described in 8.4.2.3.3 with an HR underlay, ventilation should
be provided at the positions shown in Figure 21. The following additional provisions are also recommended.
a) The dormer roof should be ventilated from eaves to eaves if it is a pitched type with insulation
horizontal. Ventilation openings should be as 8.4.2.2.3.
b) The dormer roof should be ventilated from eaves to ridge if it is a pitched type with insulation inclined.
Ventilation openings should be as 8.4.2.3.3. Consideration should also be given to providing low level
ventilation on either side of the valley of the dormer.
c) The dormer roof should be ventilated from eaves to ridge of the main roof if it is a cold deck flat type.
Ventilation openings should be as recommended in 8.4.3.2.2.
d) Where dormers are in excess of one rafter width and/or abut gable or separating walls, the eaves
“triangle” below the dormer should have ventilation outlets immediately below dormer sill level,
equivalent in area to 5 mm × length of dormer. Ventilation openings should be located so as to avoid
static air.
In conversions where the main roof is as shown in 8.4.2.3.3 and the dormer roof is of a flat warm deck type,
the dormer eaves will not be suitable as a ventilation inlet. In this situation the ridge “triangle” should be
ventilated through openings in the roof slope located just above the dormer.
NOTE A large void at the apex may be treated in the same way as in 8.4.2.2.
"
36
© BSI 23 December 2005
BS 5250:2002
5
5
10
25
10
25
5
a)
5
b)
Figure 21 — Ventilation positions for roofs with dormers
8.4.3 Continuous membrane roofs
8.4.3.1 General
Continuous membrane roofs have a continuous weatherproof covering which is also impermeable to water
vapour from below. They are often, but not invariably, of low pitch, between 0° and 10°. They can be
subdivided into three types.
a) Cold deck roofs have the insulation located below the roof deck, usually at ceiling level. It is essential
that both a vapour control layer and adequate ventilation of roof voids is provided. Specific
recommendations on the overall size and location of ventilation openings are given in 8.4.3.2.2
and 8.4.3.3.1.
b) Warm deck roofs have insulation located above the roof decking. This type of roof relies on a high
resistance vapour control layer below the insulation.
c) The inverted roof, also known as the protected membrane or upside down roof, is a type of warm roof
deck construction in which the insulation is above the weatherproof finish. The weatherproof finish also
functions as a vapour control layer. Design guidance appropriate to this particular roof construction is
given in 8.4.3.2.3 and 8.4.3.3.3.
8.4.3.2 Framed continuous membrane roofs
8.4.3.2.1 General
These roofs have a structural frame, which may be metal or timber, supporting a continuous deck. There
is a cavity between the frame, which can be ventilated.
8.4.3.2.2 Cold deck
A cold deck roof is shown in Figure 22. This type of roof should be avoided because interstitial condensation
is likely and its effect on the structure and insulation can be severe. Where construction of this type of roof
cannot be avoided, it is essential that moisture entering the roof is minimized and that adequate
ventilation is provided. If condensation does occur, it will normally be at the underside of the roof decking
or waterproof finish.
The vapour control layer should have a resistance of at least 250 MN·s/g, and should have sealed laps to
preserve the integrity over the whole roof. Gaps in the ceiling should be minimized and service openings
should be avoided; if they cannot be avoided it is essential that they are sealed.
Ventilation openings should be provided to every roof void along two opposite sides of the roof and should
be equivalent in area to a continuous opening of not less than 25 mm at each side.
A 50 mm (minimum) unrestricted airspace should be maintained between the underside of the roof
deck and the top of the insulation. Adequate cross ventilation can be difficult to achieve with spans in
© BSI 23 December 2005
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BS 5250:2002
excess of 5 m; in these situations, both the openings and airspace over the insulation should be
substantially increased. Where fire stops occur which obstruct cross ventilation, it will be necessary
to provide ventilation through the roof covering; to avoid this the use of a warm roof should be
considered.
Timber not rated moderately durable or better should be preservative treated. During its life, it is
likely that the decking will be subjected to periods of moist conditions; material should be chosen
accordingly.
Surface condensation on the ceiling is unlikely to occur due to the fast thermal response, provided there is
adequate insulation over the whole ceiling and steps have been taken to minimize thermal bridging,
especially at external wall/ceiling junctions (see 8.6).
1
2
3
4
5
6
7
Key
1 Weatherproof roof finish
5 Vapour control layer
2 Roof decking
6 Ceiling
3 Ventilated air space
7 Structural member
4 Insulation
Figure 22 — Framed flat roof: cold type
8.4.3.2.3 Warm deck
A warm deck roof is shown in Figure 23. Surface condensation is unlikely to occur due to the fast
thermal response, provided there is sufficient insulation to maintain the vapour control layer above
dewpoint over the whole roof and thermal bridging is minimized, particularly at external wall/ceiling
junctions (see 8.6).
Interstitial condensation can occur on the underside of the weatherproof finish. To minimize this, a vapour
control layer with a vapour resistance of at least 250 MN·s/g is essential, laid in hot bitumen with sealed
laps. The VCL should be turned up around the insulation and bonded to the weatherproof finish at all edges
and penetrations such as roof lights.
38
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BS 5250:2002
1
2
3
4
5
6
7
Key
1 Weatherproof roof finish
5 Unventilated air space
2 Insulation
6 Ceiling
3 Vapour control layer
7 Structural member
4 Roof decking
Figure 23 — Framed continuous membrane roof: warm type
8.4.3.2.4 Inverted
An inverted roof is shown in Figure 24. Surface condensation is unlikely to occur due to the fast thermal
response, provided that there is sufficient insulation to maintain the weatherproof finish above the
dewpoint over the whole roof and thermal bridging is minimized, particularly at external wall/ceiling
junctions. With this type of roof, rainwater seeping below the insulation will cool the waterproof membrane
intermittently, increasing the risk of condensation on the membrane. This should be allowed for by
reducing the thickness of insulation by 20 %, when calculations of the risk of interstitial condensation are
carried out.
© BSI 23 December 2005
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BS 5250:2002
1
2
3
4
5
6
7
Key
1 Topping or ballast layer
2 Insulation
3 Weatherproof roof finish
4 Roof decking
5 Unventilated air space
6 Ceiling
7 Structural member
Figure 24 — Framed continuous membrane roof: warm type inverted
8.4.3.3 Concrete continuous membrane roofs
8.4.3.3.1 Cold deck
A cold deck roof is shown in Figure 25. This type of roof should be avoided because interstitial condensation
is likely to be serious. Where construction of this type of roof cannot be avoided, it is essential that moisture
entering the roof is minimized and that adequate ventilation is provided. If condensation does occur, it will
normally be at the underside of the roof decking or waterproof finish.
Surface condensation is unlikely to occur due to the fast thermal response provided there is sufficient
insulation to maintain the vapour control layer above the dewpoint over the whole ceiling and thermal
bridging is minimized, particularly at external wall/ceiling junctions (see 8.6).
The vapour control layer should have a vapour resistance of at least 250 MN·s/g with sealed laps. Gaps in
the ceiling should be minimized and service openings should be sealed.
Ventilation openings should be provided to every void along two opposite sides of the roof and should be
equivalent in area to a continuous opening of not less than 25 mm at each side. A 50 mm (minimum)
unrestricted airspace should be maintained between the underside of the roof deck and the top of the
insulation. Adequate cross-ventilation can be difficult to achieve with spans in excess of 5 m; in these
situations, both the openings and airspace over the insulation should be substantially increased.
It is essential that construction water should be allowed to dry out, e.g. by delaying the installation of the
vapour control layer and internal finishes.
40
© BSI 23 December 2005
BS 5250:2002
1
2
3
4
5
6
7
Key
1 Weatherproof roof finish
5 Insulation
2 Roof screed (if required)
6 Vapour control layer
3 Structural concrete deck
7 Ceiling
4 Ventilated air space
Figure 25 — Concrete continuous membrane roof: cold type
8.4.3.3.2 Warm deck
A warm deck roof is shown in Figure 26. To minimize surface condensation, the heating system should be
matched to the construction, because the thermal response is slow and therefore constant low output
heating is recommended. Sufficient insulation should be provided to maintain the vapour control layer
above dewpoint and as thermal bridging is difficult to avoid at the perimeters; care should be taken at
external wall/roof junctions, gutters and roof lights (see 8.6).
Interstitial condensation can occur on the underside of a weatherproof finish. To minimize the amount of
condensation, a vapour control layer is essential and it should have a vapour resistance of at least
250 MN·s/g with sealed laps and be turned up around the insulation and bonded to the weatherproof finish.
If possible construction water should be allowed to dry out, for example by delaying the installation of
vapour control layer and internal finishes. Suspended ceilings can delay drying out or cause condensation
problems (see 8.4.6).
1
2
3
4
5
6
Key
1 Solar reflective chippings
4 Vapour control layer
2 Weatherproof roof finish
5 Roof screed (if required)
3 Insulation
6 Structural concrete deck
Figure 26 — Concrete continuous membrane roof: warm type
© BSI 23 December 2005
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BS 5250:2002
8.4.3.3.3 Warm deck inverted
An inverted warm deck roof is shown in Figure 27. To minimize surface condensation, the heating system
should be matched to the construction, because the thermal response is slow and therefore constant
low output heating is recommended. Sufficient insulation should be provided to maintain the
waterproofing layer, which acts as the vapour control layer, above dewpoint and as thermal bridging is
difficult to avoid at the perimeters; care should be taken at external wall/roof junctions, gutters and roof
lights (see 8.6).
With this type of roof, rainwater seeping below the insulation will cool the waterproof membrane
intermittently, increasing the risk of condensation on the membrane. This should be allowed for by
reducing the thickness of insulation by 20 %, when calculations of the risk of interstitial condensation are
carried out.
Suspended ceilings can delay drying out or cause condensation problems (see 8.4.6).
1
2
3
4
5
Key
1 Paviors or ballast layer
4 Roof screed (if required)
2 Insulation
5 Structural concrete deck
3 Weatherproof roof finish
Figure 27 — Concrete flat roof: warm type inverted
8.4.4 Profiled metal roof cladding
8.4.4.1 General
Lightweight sheeted roofs can be subject to particular low temperatures on clear winter nights due
to radiation loss to the cold night sky. In these circumstances, it is possible that if the underside of
the sheeting is ventilated with outside air, condensation will occur without there being a source of
moisture within the roof void. This phenomenon is most frequently seen on lightweight roofs of garages or
factories.
If this condensation is regarded as harmful, or an unacceptable nuisance, then the construction should be
changed.
Profiled metal roofs are laid from 4° pitch using standard through fix systems and down to 1.5° pitch using
special concealed fix profiles. Where liner sheets other than profiled metal are used, it is recommended that
the risk of condensation be calculated by the method given in Annex D.
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8.4.4.2 Site assembled built-up roofing
The following methods of alleviating the harmful effects of local and interstitial condensation in built-up
roofing (see Figure 28) should be considered with reference to the risk category based on occupational use
classes that are given in B.2.
1
2
3
4
5
7
6
Key
1 Profiled metal external sheet
5 Metal liner sheet
2 Breather membrane (if required)
6 Structural support
3 Insulation
7 Spacer system with a thermal break
4 VCL or sealed liner sheet joints
Figure 28 — Site assembled metal roof
Method a) is suitable for dry conditions of occupancy classes 1, 2 and 3. Method b), in addition to method
a), should be adopted for humid conditions of occupancy class 4. Method c), in addition to methods a) and
b), should be adopted for high humidity conditions of occupancy class 5 including special risks such as
swimming pools.
a) A vapour control layer is essential, which in normal environments may consist of sealed side and end
laps of the metal liner sheet with continuity of the seal at penetrations and junctions. Alternatively, a
separate vapour control membrane may be laid over the metal liner sheets with minimum 100 mm width
sealed laps. The liner or vapour control layer should discharge any condensate externally.
b) The voids should be vented to the outside air at both ends of the sheeting through profile fillers
incorporating venting openings. Experience has shown that proprietary ventilated fillers with an
opening of not less than 5 % of the profiled sheet void above the sheet support are generally satisfactory.
Ventilation openings should also be resistant to ingress of rain, birds and large insects and not prone to
blockage by dust or debris, which can be achieved with a mesh size of 4 mm.
c) Where vented voids are used on roofs to buildings with high internal humidity and the insulation is
likely to be affected by the combination of local condensate, consideration should be given to the inclusion
of a breather membrane on top of the insulation.
8.4.4.3 Composite panel roof
A composite panel roof is shown in Figure 29. Composite panels form a warm roof type construction, in
which the principal thermal insulation layer is placed immediately inside the outer profiled sheeting,
resulting in the supporting structure and any voids being at a temperature close to that of the interior of
the building. The condensation risk plane is also at the outer face of the insulation. Unless special
precautions are taken, it is impracticable to expect airtight construction.
Where voids in profiled sheeting are completely filled by insulation, such as in sandwich panels with a
vapour impermeable undersheet (e.g. metal), local condensation cannot, in principle, occur. However, in
practice, small voids will still occur at side and end laps where vapour leakage can occur and local
condensation can develop, and therefore sandwich panel systems should be capable of being sealed at side
and end laps to prevent moist air entering the joints between panels.
© BSI 23 December 2005
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BS 5250:2002
1
2
3
5
4
Key
1 Profiled metal external sheet
4 Structural support
2 Insulation
5 Air tight seal at panel joints
3 Metal liner sheet
Figure 29 — Composite panel roof
8.4.5 Fully supported metal roofs
Fully supported metal roof finishes include aluminium, copper, lead, stainless steel and zinc. Copper, lead
and other metal roof finishes require provision for expansion and contraction of the sheet material; some
form of rolled or lapped joints usually provides this. In existing warm deck flat roof designs, this has given
rise to problems; the temperature within the roof void can be lowered during a rain shower, resulting in a
drop in pressure which can cause rainwater to enter the roof joints. This, in turn, can result in corrosion of
the roofing material.
In this type of roof, it is recommended that a ventilated air space be provided on the cold side of the
insulation and beneath the support structure, primarily to reduce this effect, in addition to a high
performance vapour control layer near the inner surface.
8.4.6 Suspended ceilings under roofs
The provision of a suspended ceiling will tend to lower the plane of condensation and, if the ceiling is
insulated or has insulating properties, it is essential to check the condensation risk by the calculation
procedure described in Annex D. It is also essential that the insulation above the vapour control layer is
balanced with the insulation below to ensure that the plane of condensation does not occur at or under the
vapour control layer.
Where a suspended ceiling is provided under a deck that needs to dry out (such as in situ concrete), the
ceiling should be of low vapour resistance and capable of withstanding large moisture fluctuations without
damage.
Alternatively, the void space above the suspended ceiling should be ventilated to the outside to remove
moisture vapour. A vapour control layer at ceiling level should not be used with a “wet” construction where
ceilings may have to withstand large moisture fluctuations for several years.
44
© BSI 23 December 2005
BS 5250:2002
Where the suspended ceiling is below a roof deck of lightweight construction, it is essential to minimize
the amount of water vapour penetrating to the underside of the decking. The decking might be susceptible
to localized surface temperature variations at weaknesses in the insulation layer, such as at eaves,
gutters and rainwater outlets. These surface temperature changes could be sudden, for example as a
result of a shower of rain, or more prolonged due to radiation to the clear night sky. If water vapour is
present, these conditions can lead to heavy deposits of condensate that can rain onto the suspended ceiling.
8.5 Floors
8.5.1 General
Floors can be grouped into three main types:
a) suspended, where the floor structure spans a void or crawlspace;
b) solid, where the floor rests directly on prepared ground;
c) externally exposed floors, where the building shape results in the underside of a floor being exposed to
the outside air.
Ground floors are commonly types a) and b).
Intermediate floors are not normally a concern when considering condensation risk, unless conditions vary
considerably from one side to another as e.g. an upper floor over a car park.
8.5.2 Considerations
The designer should take account of five sources of dampness: ground moisture, surface condensation,
interstitial condensation, construction water and spillage. Whilst the principal source of dampness is from
the ground, the selection and arrangement of materials to eliminate this problem will have implications in
dealing with the other four sources.
Design recommendations on differing types of floor construction are given in 8.5.3, 8.5.4 and 8.5.5.
However, the following factors should also be considered.
a) To minimize surface condensation, it is essential to provide a construction such that the inner surface
is maintained above dewpoint for the design conditions. For a solid ground floor, the risk of condensation
is normally higher in the area around the perimeter where thermal bridging has an effect; insulation
should be included to avoid this.
b) For the control of interstitial condensation risk in suspended floors or externally exposed floors, the
principles described in 8.1 also apply. However, for solid slabs on the ground incorporating a damp-proof
membrane of high vapour resistance, sub-floor ventilation cannot be considered. For an uninsulated
floor, calculation can show that, as well as surface condensation risk, there is a risk of condensation on
the damp-proof membrane under the slab. If the quantity deposited is high (although this is unlikely), it
should be considered harmful.
A vapour control layer should be provided on the warm side of the insulation in floors with insulation
placed either above or below the slab. Where insulation is placed beneath the slab, the damp-proof
membrane can also act as a vapour control layer, provided that the insulation is unaffected by ground
water. Where insulation of high vapour resistance is placed above the slab, a vapour control layer might
not be necessary.
c) The drying out of wet construction materials, such as screeds and precast units as well as the slab, can
lead to degradation of floor finishes, such as wood blocks or chipboard. It is therefore recommended that
sufficient time is allowed for drying out.
d) Care should be taken to avoid spillage or leaks from appliances or services that can result in
damage from the retention of water within the floor construction. This is particularly important where
vapour control layers or plastics insulants are used immediately below the timber floor finishes or
deckings.
© BSI 23 December 2005
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BS 5250:2002
8.5.3 Suspended ground floors
8.5.3.1 Timber
A timber suspended ground floor is shown in Figure 30. Surface condensation is unlikely to occur on the
upper surface of the floor deck especially if there is adequate insulation beneath with no gaps.
As the thermal response is fast, regular heating is preferable but infrequent heating may be used with this
construction.
Thermal bridging should be minimized, particularly at the floor/external wall junction.
Interstitial condensation is unlikely to occur. However, subfloor cross ventilation should be provided by
openings not less than either 1 500 mm2/m run of external wall or 500 mm2/m2 of floor area whichever gives
the greater opening area. This is primarily to avoid high timber moisture contents due to high relative
humidities in the subfloor space.
Insulation and its supports should be of low vapour resistance and, preferably, allow the free passage of
water. Where rigid insulants or sheet material supports are used, attention is drawn to the warning given
in 8.5.2 regarding water spillage or leaks. Vapour control layers should not be used in this form of
construction.
1
2
3
4
5
Key
1 Timber floor deck
4 Ventilated air space
2 Air space (optional)
5 Oversite concrete
3 Insulation
Figure 30 — Timber suspended ground floor
8.5.3.2 Precast concrete (beam and block)
A precast concrete suspended ground floor is shown in Figure 31. Surface condensation in unlikely to occur
provided that sufficient continuous insulation is included over the concrete structural floor to give a fast
thermal response. Regular heating is preferable but infrequent heating may be used with this construction.
Thermal bridges should be minimized, particularly at the wall/floor junction.
Interstitial condensation is likely to occur on the upper surface of the concrete beams and filler blocks. It
is unlikely to be harmful. However, if an insulant that can absorb water is used, a vapour control layer
should be provided on the warm side of the insulation.
NOTE Attention is drawn to the recommendation given in 8.5.2 regarding water spillage or leaks.
Subfloor cross ventilation should be provided by openings not less than either 1 500 mm2/m run of external
wall or 500 mm2/m2 of floor area whichever gives the greater opening area.
46
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BS 5250:2002
1
2
3
3
5
4
6
Key
1 Floor finish
3 Precast concrete beams and filler blocks
2 Rigid insulation
4 Ventilated airspace
Figure 31 — Precast concrete suspended ground floor
8.5.4 Ground bearing floors
A ground-bearing floor is shown in Figure 32. Surface condensation is unlikely to occur on the floor surface
if insulation is provided over the slab. Due to the fast thermal response of the construction, regular heating
is preferable but infrequent heating can be used with this construction. If insulation is provided below the
slab, surface condensation can occur due to the slab’s slow thermal response. This would normally be
considered nuisance condensation and regular heating is recommended.
Thermal bridging should be minimized, particularly at floor/wall junctions.
Interstitial condensation can occur on the upper slab surface where insulation is over the slab. If considered
harmful, a vapour control layer of at least 500 gauge polyethylene or equivalent should be provided on the
warm side of the insulation.
Interstitial condensation is unlikely to occur if insulation is installed beneath the slab with a vapour control
layer that also acts as a damp-proof membrane which should be at least 1 000 gauge polyethylene or
equivalent.
Insulation should be able to withstand superimposed loads and where used below the damp-proof
membrane, should not absorb water.
NOTE Attention is drawn to the recommendation given in 8.5.2 regarding water spillage or leaks.
The damp-proof membrane used must be compatible with the insulation. Where the damp-proof membrane
is below the slab, construction water should be allowed to dry out, e.g. by delaying the installation of vapour
control layer and floor finishes.
8.5.5 Externally exposed floors
8.5.5.1 Joisted floors
A joisted externally exposed floor is shown in Figure 33. Surface condensation is unlikely to occur on floors
with timber joists, or on joisted floors where insulation is provided over the joists, due to the fast thermal
response. Regular heating is preferable, but infrequent heating may be used with this construction.
Thermal bridging should be minimized, particularly where the floor passes over external walls, and at the
floor edges.
Interstitial condensation is unlikely to occur if insulation supports or soffit linings are of low vapour
resistance. Where rigid insulants or sheet material supports are used, attention is drawn to the warning
given in 8.5.2 regarding water spillage or leaks.
Vapour control layers should not be used with joisted floors because water spillage could cause decay and
subsequent collapse.
© BSI 23 December 2005
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BS 5250:2002
1
2
3
4
5
Key
1 Floor finish
4 Damp-proof membrane (may be below slab)
2 Vapour control layer (if required)
5 Concrete slab
3 Rigid insulation
a) Ground bearing floor with insulation above slab
1
2
3
4
Key
1 Floor finish
2 Concrete slab
3 Damp-proof membrane also acting as vapour control layer
4 Rigid insulation
b) Ground bearing floor with insulation below slab
Figure 32 — Solid ground floors
8.5.5.2 Solid floors
A solid externally exposed floor is shown in Figure 34. Surface condensation is unlikely to occur on floors
where the insulation is provided above the structure due to the fast thermal response. If insulation is
provided below the structure, the thermal response is slow, so surface condensation can occur and regular
heating is recommended.
Thermal bridging should be minimized, particularly where the floor passes over external walls, and at the
floor edge.
Where insulation is above the slab, interstitial condensation can occur on the upper surface of the slab. If
considered harmful, a vapour control layer of at least 500 gauge polyethylene or equivalent should be
provided on the warm side of the insulation.
Interstitial condensation is unlikely to occur if insulation is provided below the structure. Such insulation
should be of low vapour resistance and should preferably allow the free passage of water. Where rigid
insulants or sheet material supports are used, attention should be drawn to the recommendation given
in 8.5.2 regarding water spillage or leaks.
If external layers have high vapour resistance, adequate ventilation should be provided to an airspace
between the insulation or the structure and soffit lining. This should be provided by openings on opposite
soffit edges equivalent to 25 mm continuous opening with a minimum ventilated airspace of 50 mm.
48
© BSI 23 December 2005
BS 5250:2002
1
2
3
4
Key
1 Timber deck
2 Airspace (optional)
3 Insulation
4 External finish of low vapour resistance
Figure 33 — Timber deck with external finish of low vapour resistance
1
2
3
4
Key
1 Floor finish
2 Vapour control layer (if required)
a)
3 Insulation
4 Concrete
Suspended concrete slab with insulation above the slab
1
Key
1 Suspended concrete slab
2 Insulation
b)
2
3
3 External finish of low vapour resistance
Suspended concrete slab with insulation below the slab and external finish of low vapour resistance
1
2
3
4
≤ 50
Key
1 Suspended concrete slab
3 Ventilated air space of minimum depth 50 mm
2 Insulation
4 External finish of high vapour resistance
c) Suspended concrete slab with insulation below the slab and external finish of high vapour resistance
Figure 34 — Solid externally exposed floor
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BS 5250:2002
8.6 Thermal bridging
Thermal bridges are areas of the building fabric where, because of the presence of high conductivity
materials or the geometry of the detail, there is significantly higher heat loss than through surrounding
areas. Besides leading to increased energy use, they lower the internal surface temperature and are
therefore sites for condensation and mould growth. Thermal bridges fall into two categories.
a) Repeating thermal bridges, e.g. timber joists, mortar joints, mullions in curtain walling. These have a
significant effect on heat loss, and are required to be taken into account in the calculation of U-values,
using the methods specified in BS EN ISO 6946. They are, however, rarely severe enough to cause
surface temperatures to fall low enough to cause surface condensation or mould growth.
b) Non-repeating bridges, which commonly occur around openings such as lintels, jambs and sills and at
wall/roof junctions, wall/floor junctions and when internal walls or floors penetrate the outer building
fabric. If details to minimize thermal bridges are not used, they can add 10–15 % to the total heat loss
from the building.
The severity of a thermal bridge, in terms of its effect on internal surface temperatures may be expressed
by the surface temperature factor f defined under steady state conditions by:
T si – T o
f = -------------------Ti – To
Where:
Tsi is the internal surface temperature;
Ti is the internal air temperature;
To is the external air temperature.
This will be close to 1.0 for a well insulated structure, but will fall towards 0.5 or below at severe thermal
bridges. As it depends on the properties of the construction detail alone and is independent of the
environmental conditions, once the temperature factor has been found for a thermal bridge, it may be used
to calculate the internal surface temperature from any set of temperatures.
T si = T o + f ⋅ ( T i – T o )
This may then be used to calculate the internal surface relative humidity and risk of mould growth, if the
internal humidity is known.
A surface temperature factor of not less than 0.75 is considered to be sufficient to avoid mould growth, given
the range of conditions in UK buildings and the UK climate. This is discussed further in Annex D and
BS EN ISO 13788 and BRE Information Paper IP 17/01.
The surface temperature factor of a thermal bridge may be found from thermal bridge catalogues or by
calculation.
Thermal bridge catalogues have been produced that contain a representative sample of building details,
identify possible problems and give recommended solutions. These catalogues have the advantage of
simplicity and ease of use and cover most common constructions. The most comprehensive examples in the
UK are the Energy Efficiency Office Good Practice Guides 174 [3] and Good Practice Guide 183 [4], which
cover new and existing housing respectively, and the Guide to Robust Construction Details published in
association with the Building Regulations. A wide range of software packages are available which can be
used to carry out the appropriate calculations with two or three dimension thermal models. This software
should comply with BS EN ISO 10211-1. These models are complex and are currently used only by
specialist consultants.
To avoid problems, the continuity of the bridging element should be broken, with insulation inserted to
reduce heat flow and raise the internal surface temperature. If this is not possible, insulation should be
located so as to lengthen heat flow paths.
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8.7 Windows
8.7.1 General
Surface condensation regularly occurs on the inner face of glazing particularly on single glazed windows.
Sometimes there is sufficient water to run down and collect which should be mopped-up or provisions made
for drainage. If these recommendations are followed the condensation is merely a nuisance. However, if it
is allowed to remain in contact with a timber sill, or if it runs onto curtains, carpets or wall decorations, it
can be harmful.
Windows facing between north-east and north-west are more susceptible to prolonged condensation,
whereas southerly facing windows that take advantage of solar heat gain may not be so badly affected.
Condensation can also occur on window reveals, aggravating mould growth because of the thermal bridging
issues discussed in 8.6. This may be worse with single glazed windows because of radiation exchange
between the cold glazed surface and the reveal causing a consequent lowering of the surface temperature
of the reveal.
To reduce the incidence of condensation on glazing, double or multiple glazed units should be used either
as sealed units or separately glazed sashes. Condensate on single glazing may represent up to 5 % of the
moisture produced in a building; multiple glazing can reduce this, but accordingly can increase the amount
of water vapour in the air that can then condense elsewhere unless removed by ventilation.
Draught-proofing, replacement of window frames or the installation of secondary double-glazing will
reduce ventilation rates. It is essential, when carrying out such works, that adequate ventilation is
provided to remove water vapour.
To minimize surface condensation on frames, they should be made of low thermal transmittance material
or, if metal, should contain a thermal break.
Where severe condensation is unavoidable, drainage channels should be provided. Where the channel is
drained to outside, it is essential that the drain is not easily blocked.
External shutters or closures over windows will tend to raise the window surface temperature and thus
reduce condensation.
8.7.2 Sealed glazed units
The greater the insulation value of the unit, the more the likelihood of condensation is reduced. Thus
double units with low emissivity coatings or with gas filling and triple units will be better than ordinary
sealed units. Multiple glazed windows of the sealed unit type have the advantage of no condensation
forming between the panes. The optimum thermal performance is obtained with a spacing between panes
of about 16 mm. Roof windows, which fall into this category, are more at risk because of their
inclination.
Sealed units should be fitted so that the aluminium spacer is buried deeply enough in the rebate to
minimize a thermal bridge which could cause condensation (Figure 35). This effect will also be minimized
by the use of an insulated spacer. When stepped units are used, the thermal bridge effects can be reduced
by the use of beading. Drained systems are designed so that any water penetrating the glazing rebate from
the outside or condensate from the inside is drained to the outside of the building (Figure 36). Care should
be taken to ensure that the internal sealing of the glass to the frame is complete.
8.7.3 Separately glazed sashes of double windows
To minimize the incidence of condensation on the inner surface of the external glazing, it is essential that
the inner sash is sealed. A degree of ventilation should be provided between the outside air and glazed units
by the vents of 10 mm diameter placed at the bottom of the outer window at 300 mm centres. These vents
should be filled with open cell flexible foamed plastics to keep out dust and insects, and should slope
downward and outward through the window frame.
NOTE Condensation on the outer pane can occur with falling external temperature, but this usually disperses rapidly because of
the venting.
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Figure 35 — Standard glazing unit
Figure 36 — Drained glazing unit
8.8 Roof lights
8.8.1 Inline roof lights for profiled metal sheeting
Roof lights will generally have a higher U-value than the rest of the roof in which they are fixed since they
do not contain a layer of insulation material. In consequence there is a risk that condensation will form on
the cooler glazing skin, particularly if warm air penetrates into the cavity.
Roof lights in site-assembled (“built-up”) roof cladding usually consist of two or more layers; the inner sheet
must be sealed to the vapour control layer. Provided this is done, condensation on the upper sheet is seldom
a problem. Any intermediate layers included, for example to increase the thermal resistance of the roof
light, will not generally form a condensation risk.
Factory assembled roof lights comprise a closed box unit providing a cavity of static air, which minimizes
the risk of condensation forming on the inner face of the external glazing skin. This type of roof light is more
difficult to seal to the surrounding lining system and special care is needed to prevent warm air penetrating
into the roof cavity. The factory assembled roof light is more appropriate for use in sandwich panel
construction where the metal liner skin of the panel forms the vapour control layer and the perimeter gap
between the panels and the roof light can be filled.
8.8.2 Domed and vaulted roof lights
Domed and vaulted roof lights, projecting through the roof, should be fitted to insulated upstands.
Continuity of the vapour control layer should be maintained by sealing the inner skin of the roof light to
the upstand and to the liner system.
8.9 Condensation on internal fittings
Precautions should be taken to prevent damage by condensed water dripping from cold water storage
tanks, cisterns and pipes.
It is essential that water storage tanks in roofs are covered and that they and water pipes in roofs are
insulated. It is preferable to take advantage of the warmth in the dwelling by taking the ceiling insulation
up and over the water storage tanks and pipes, leaving the area beneath the tank uninsulated.
Condensation is very liable to occur on cisterns and cold water pipes within a dwelling. If the fitments are
located in poorly ventilated cupboards opening off bathrooms, kitchens or bed-sitting rooms, they should
be fully lagged and consideration should be given to the need to protect materials which might be damaged
by water dripping off cisterns or running down pipework.
It is recommended that water tanks are not located in structures which project above the normal roofing,
as there is considerable risk of condensation occurring either on the inner surface of the construction or as
interstitial condensation. Precautions suitable for general walling or roofs might not be sufficient because
the enclosure does not have the benefit of heating.
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8.10 Cupboards on external walls
Because they will shield areas from warmth but not moisture, cupboards should not, in general, be fitted
against external walls, to prevent surface or interstitial condensation occurring behind them.
If it is necessary to fit them in this way, the external walls should be insulated to a high standard or the
cupboards (and rear of cupboards if fitted with a back) ventilated.
9 Application of design principles: heating
9.1 General
Many condensation problems arise because the majority of buildings are not used 24 hours a day for every
day of the year and are, therefore, not heated continuously.
In new buildings, the building fabric should be insulated to the optimal level so that any necessary heating
can be provided as economically as possible. In existing buildings, the building fabric insulation standards
should preferably be upgraded so that any necessary heating can be provided in a cost-effective manner.
To minimize surface condensation the duration and amount of heating should be regulated to maintain the
internal surface temperatures above dewpoint. Ideally, this involves matching the heating system to the
thermal mass of the building fabric and to the way that the heating system is likely to be used by the
building occupants.
Ignoring the comfort of the occupants, the aim should be to maintain an air temperature at or above 10 °C
to 12 °C in all parts of the building that are heated. In well insulated buildings, it is possible to maintain
these temperatures without any heating other than that given off by lighting and equipment, such as
computers.
Surface condensation is unlikely to be a problem in regularly heated buildings with a low vapour load, such
as office buildings, and is most commonly found in dwellings that are insufficiently heated and in other
buildings that are used, and consequently heated, intermittently, such as churches.
Surface condensation is likely in buildings such as swimming pools and laundries where large amounts of
moist air are generated on a regular basis. These types of buildings are beyond the scope of this standard
but the design principles of Clause 9 would apply to their design.
To achieve a satisfactory balance of internal temperatures, detailed calculations are necessary, taking
account of solar gains, internal gains, ventilation rates and local climatic conditions as well as the thermal
response of the building fabric.
Further guidance on heating provision is given in CIBSE Guide B1 Heating [6] and in CIBSE Domestic
heating — design guide [7].
9.2 Warm air heating
Wholly convective heat from forced warm air systems, heats room air very rapidly and is liable to be
operated intermittently to provide heat only when required for the comfort of the occupants.
Modern forced warm air heating systems are used in a similar fashion to water based central heating
systems, where by use of a time clock the warm air system is used to preheat buildings before occupation,
normally with a reduced preheat time when compared to water based systems. Current warm air heaters
have electronic controls, which proportionally vary heat output to avoid large temperature swings and
maintain preset temperatures. This allows warm air systems to be used in both low thermal mass buildings
and high thermal mass buildings.
Warm air systems should be installed in accordance with BS 5864 and also with due reference to British
System Design Manual (Gas fired warm air heating) [8].
A percentage outdoor air make-up as described in BS 5864 Section 3, can be incorporated into a warm air
system. This will provide a definite air change for the building, a positive air pressure to moisture
producing areas and thus a flow of moisture to outside.
An advantage of forced warm air heating can be that it creates sufficient air movement to reduce the risk
of stagnant air pockets, e.g. in the corners of rooms.
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9.3 Hot water radiators
These produce natural convective heating and as they do not respond as quickly as warm air systems, are
more suited to high mass buildings. They are normally heated before building occupation and give off
residual heat after the system is switched off thus prolonging the heating period. Hot water radiators are
also suitable for low mass buildings but surface temperatures in these types of buildings will fall more
rapidly after the heating is switched off as there is little thermal storage in the structure.
9.4 High temperature radiant heaters
These include radiant gas and electric fires, and produce easily controlled and almost immediately felt local
warmth. However, both are unlikely to be used for sufficiently long to warm the structure adequately and
they can also leave some parts of a room relatively unheated.
Gas fires need a satisfactory air supply for combustion and this, plus the warm flue, will create some
ventilation to the room in which they are sited.
9.5 Electric storage heaters
Electric storage heaters generally have a low rate of heat output. The charging period should be related
both to the needs of the occupants and to the need for an adequate reserve for heating the building fabric.
Storage heaters without fans emit heat almost continually to provide background heating, however they
can be difficult to control. Storage heaters with fans have some characteristics similar to forced warm air
heating. Some storage heaters incorporate a radiant heating element to provide rapid local warmth when
required. Because of their characteristics, storage heaters are particularly suited to buildings that are
occupied for long periods, and/or for buildings of high thermal mass.
9.6 Low temperature radiant heaters
These heating elements are normally embedded in the floor. Underfloor heating in a high mass floor gives
out heat for long periods, which is advantageous in controlling condensation.
9.7 Unflued oil and gas heaters
These are normally used as supplementary or temporary heating. Their use should be avoided as they
release large quantities of water vapour into the room. To avoid the need for such supplementary heating,
the building should be designed so that it can be adequately heated in an economical manner.
9.8 Open fires and solid fuel burning stoves
These require considerable air supplies for combustion and result in much heat loss via the flue. The effect
of the heated flue will be to draw air from other parts of the building including the moisture generating
areas. However, the high air change rate in the room will usually negate any increased risk of condensation
in that room that would have resulted from moist air being pulled in.
9.9 Heating controls
Two factors control the output of a heating system: the length of time that the system is operative and the
output temperature of the system. Heating systems that are capable of being automated should have time
controls to regulate the duration of heat output and temperature controls to ensure that the heating is
switched off in rooms or zones (where the heating system is zoned) when the demand temperature is met.
Some set-back control to maintain lower than comfort temperatures when the building is not in use are
particularly beneficial in avoiding large fluctuations in temperatures which would exacerbate the risk of
condensation.
Further advice is given in Good Practice Guide 302, Controls for domestic central heating and hot water —
guidance for specifiers and installers [9].
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10 Application of design principles: ventilation
10.1 General
The occupation of buildings, with associated activities and processes, produces moisture. This needs to be
removed to outside air by ventilation, to avoid high relative humidities that can result in problems of
condensation and mould growth.
For the purposes of this standard, the function of ventilation is to control the humidity of internal air to
between 40 % r.h. and 70 % r.h. However, short periods when the relative humidity is outside this range
need not necessarily lead to problems of discomfort or mould growth.
Some building uses, such as swimming pools and laundries can have normal relative humidities in excess
of 70 %. These types of buildings require specialist knowledge in design and use, but the design principles
of Clause 10 nevertheless apply.
Building occupants are sensitive to discomfort from draughts and therefore have a tendency to reduce
ventilation during cold weather. The effect of modern building, coupled with the provisions in the Building
Regulations is to reduce ventilation rates due to air leakage.
From the point of view of controlling condensation, the ideal ventilation system will provide either:
a) finely controllable background ventilation; and mechanical extraction of water vapour from moisture
producing areas such as kitchens and bathrooms; or
b) continuous ventilation either by use of passive stack ventilation or a mechanical ventilation system.
Further information on ventilation provision is given in CIBSE Guide B, Ventilation and air
conditioning [10]. The meteorological data needed for ventilation design is available in CIBSE Guide J,
Weather, solar and illuminance data [11].
10.2 Natural ventilation
In many existing buildings, the only control the occupier has of ventilation is the opening of windows. In
winter, this method is unlikely to be used unless the window incorporates provisions for controllable
background ventilation. Where such trickle ventilation is not provided, controllable slot ventilators should
be installed in the windows or in the walls. The ideal place to install these ventilators is in the top section
of an opening light in the window.
These background ventilation openings should be installed in all occupied rooms. Every room should have
background ventilation openings of at least 4 000 mm2. Habitable rooms in dwellings should preferably
have background ventilation openings of at least 8 000 mm2. In occupiable rooms in buildings other than
dwellings, the background ventilation provision of 4 000 mm2 should be increased by 400 mm2 per square
metre of floor area where the area of the room exceeds 10 m2.
While such small ventilation openings will normally provide adequate background ventilation, they are
unlikely to be able to cope with high amounts of water vapour production. Natural ventilation should
therefore be supplemented by mechanical extract ventilation or PSV in moisture producing areas to remove
water vapour to the outside of the building.
10.3 Passive stack ventilators
Passive stack ventilation (PSV) is a ventilation system using ducts from the ceiling or walls of rooms to
terminals on the roof which operate by a combination of the natural stack effect, i.e. the movement of air
due to the difference in temperature between inside and outside and the effect of wind passing over the roof
of the dwelling. In order to maximize the stack effect, ducts should be as near vertical as possible, and
should never be at an angle of more than 45° to the vertical. Ducts should be insulated, to prevent
condensation, where they pass through unheated spaces such as lofts. Humidity sensitive grilles are
available that provide increased flows when water vapour is being generated.
Trickle ventilators should be provided in the rooms where moisture is not produced so that air is drawn
through these dry rooms to exit via the moisture producing areas through the stack ventilator. Ideally,
when using natural ventilation, outside air should be drawn in through dry rooms located on the windward
side of the building to exit via the moisture producing areas, which should be sited on the leeward side of
the building. Further information on the performance of PSV is available in BRE IP 13/94 [12].
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10.4 Forced ventilation
Forced ventilation is more reliable than natural ventilation because air change rates and directions can be
better controlled. Where mechanical ventilation is provided in kitchens and bathrooms, it may be used for
short periods to assist natural ventilation.
With warm air systems the incorporation of a percentage of outdoor make-up air can induce satisfactory
ventilation throughout the building (see 10.2).
With heating systems that do not provide air movement, mechanical extract ventilation is recommended
in all moisture producing areas such as kitchens or bathrooms. However, extractor fans should not be
installed in the same room as an open flued heating appliance burning solid fuel or oil as they can draw
flue gases into the room.
The design of the forced ventilation system should be such that the action of fans will reduce moisture at
source, reduce the spread of moisture to the rest of the building, and cause outdoor air ventilation of other
rooms.
Fans should be of sufficient power to ensure that they can extract, if necessary, against the prevailing wind.
The sizing of fans depends upon the rate and amount of moisture generated in the room. It should be borne
in mind that when fans are not operating, the air movement patterns induced by the wind will be
re-established.
When air movement into moisture producing areas is from other parts of the building, care should be taken
to ensure that these rooms are not significantly cooler than the rest of the building as this could exacerbate
the risk of condensation in these areas. It should also be noted that air extracted will be replaced by
incoming air which in turn should be heated.
There is evidence to show that extractor fans are frequently not used and consideration should be given to
the use of humidistat controls. However, it should be borne in mind that certain types of humidistat
controls require periodic maintenance.
Speed controls on fans are an advantage so that extraction can be reduced to a trickle when large amounts
of moisture are not being generated. By operating fans in this way, the fans can assist in providing
background ventilation to the whole building.
A variety of mechanical ventilation systems are available (see BRE Digest 398 [13]), the simplest being
central mechanical extract ventilation which removes moisture laden air from several rooms and using
ductwork, discharges to the outside of the building from a single remote fan. Mechanical ventilation
systems with heat recovery provide outdoor air to the whole building and extract ventilation from all
moisture producing areas such as kitchens and bathrooms.
Incorporating a heat exchanger means that heat contained within the exhaust airstream can be partially
recovered and used to warm the incoming outdoor air. Individual heat recovery room ventilators are also
available which provide outside air and extract from individual rooms in buildings.
10.5 Mechanical supply ventilation systems
Supply ventilation systems are fans that are often mounted in the loft, which supply air continuously to
the centre of a house, usually through the ceiling of the landing. Besides providing a continuous supply of
outside air, these systems benefit from some degree of solar gain on the roof, supplying air that is warmer
than outside. Care should be taken to ensure that the fans are as quiet as possible and are not mounted in
a way to cause resonance. The inlet air supply vent should be arranged so that the effect of cold draughts
is minimized. Trickle ventilators should be provided in all rooms to allow for air to disperse to outside.
This system is unsuitable for unventilated lofts.
10.6 Mechanical ventilation with heat recovery
Whole house mechanical ventilation with heat recovery (MVHR) is a ventilation system that combines
supply and extract ventilation in one system. Whole house units comprising of two fans and a heat
exchanger are often mounted in the loft (wall mounted units are also available), the supply fan
continuously provides outdoor air to all habitable rooms using a system of ductwork. The extract fan
removes warm moist air from kitchens and bathrooms via a system of ductwork and passes it across the
heat exchanger before it is discharged to outside. The heat recovered preheats the incoming outdoor air. All
ducts in the loft should be insulated to minimize condensation, and care should be taken to ensure that the
fans are as quiet as possible and are not mounted in a way that could cause resonance.
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Individual room heat recovery ventilators are mounted in external walls and each ventilator comprises of
two fans and a heat exchanger. Warm air is extracted from the room and passes across the heat exchanger
before being discharged to outside. The heat recovered preheats incoming outdoor air to the room. These
units normally have two speed settings, low speed to provide continuous background ventilation and high
speed boost.
10.7 Heated drying cupboards
Where a heated drying cupboard is provided it should be ventilated to disperse moisture directly to the
outside air. Alternatively, it could be located in e.g. a bathroom, vented to that room and a humidistat
controlled extractor fan fitted in the room.
10.8 Dehumidifiers
Electric dehumidifiers that work on a closed refrigeration cycle both dry and heat the air. As some latent
heat is released when the water condenses, the heat output is 10 % to 30 % greater than electricity
consumed (typically 200 W to 300 W). An essential feature of all dehumidifiers is that the amount of water
they extract is dependent on the temperature and vapour pressure of the air. They are more effective in
warmer dwellings where condensation problems are caused by high vapour pressures, than in more typical
condensation prone houses where problems are caused by low temperature. They tend to be fairly obtrusive
and too noisy to run in bedrooms overnight and are not acceptable to all householders. However, their
portability means that they can be used on a trial basis and moved elsewhere if need be. Some
dehumidification systems can be mounted in the loft or a cupboard, but that makes the installation more
expensive.
11 Diagnosis and remedial work
11.1 General
The precautions to prevent condensation, which have been included in the building, should be determined
and retained, adapted and improved. Where a building is to be upgraded, the new design should follow the
guidance set out in Clauses 8, 9 and 10. If the building is merely to be repaired, then a diagnosis of its
existing condensation problems should be made and remedial action should be taken according
to 11.2, 11.3, 11.4, and 11.5.
In either case, it will be necessary to identify all other sources of dampness in the existing structure and to
eliminate these before dealing with condensation problems.
Building owner/occupiers should be given information about the control of condensation so as to enable
them to use the building within its design limits. The manual should contain advice for occupiers so that,
by following some simple rules, condensation can be minimized.
NOTE Further details of appropriate contents of the manual are given in 14.1.
11.2 Diagnosis of dampness
11.2.1 General
It is not possible to devise an infallible system for differentiating between condensation and other sources
of dampness. A building element can be damp for a number of reasons, e.g. because of rising damp,
condensation, water penetration, or presence of hygroscopic salts. The following guidance should be of
assistance in an investigation.
11.2.2 History
Enquiries should be made into the recent history of the building to determine whether it has been left
unoccupied for any length of time or whether it has been open to the weather or flooded.
If the building has remained unoccupied for some time, it will not be possible to determine every area in
which condensation problems relating to occupancy have previously occurred. Potential areas of risk
should therefore be discovered by examination of the structure taking into account the proposed use of the
building. The risk of interstitial condensation within the structure might need to be assessed using the
methods set out in BS EN ISO 13788 (see Annex D).
Areas showing the effects of condensation, such as mould growth and staining, are more easily identified
where a building has been occupied recently. If the inspection takes place during cold weather, while the
premises are occupied or soon after they are vacated, damp patches can be evident.
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The opportunity should be taken to obtain as much relevant information as possible from the occupants.
For example, the following factors can be considered.
a) The number and ages of the occupants.
b) The occupancy pattern.
c) The heating pattern.
d) Family economics, i.e. how much is spent on heating.
e) Types of domestic appliances, which are likely to generate water vapour such as cookers, washing
machines, driers and free-standing room heating equipment such as portable gas or paraffin heaters.
f) Whether the system for washing and drying clothes is one that will generate a lot of moisture within
the dwelling.
g) The weather and seasonal dependency of the phenomena.
h) What means of ventilation is installed and whether it is used.
i) Whether or not the family open the windows and when.
j) The extent and position of insulation.
k) Internal temperatures and humidities.
l) Comparison with adjacent or similar properties.
This information should be considered in the light of the principles outlined in 7.1 to assist in an
assessment of the building.
11.2.3 Causes of dampness
The causes of any dampness other than that resulting from condensation should first be determined.
The following is a suggested checklist.
a) Roof leaks, e.g. valleys, flashings around parapets and chimneys.
b) Defects in the rainwater drainage system.
c) Defects in wastepipes.
d) Leaks in the plumbing system.
e) Rain penetration around the door and window openings.
f) Defects in the damp-proof courses and membranes.
g) Area of wall surface affected by hygroscopic elements, e.g. parapets, balconies and porches.
11.2.4 Recognition of surface condensation
Surface condensation occurs where the temperature falls below the dewpoint temperature of the adjacent
air and can result in mould growth. This is likely to be found in the following locations.
a) Corners of rooms, especially corners of external walls.
b) Lintels, reveals and sills.
c) Behind furniture placed against external walls.
d) Within built-in furniture on external walls.
e) Floor/external wall junctions especially those containing ring beams.
f) On the internal surface of north facing walls.
11.2.5 Recognition of hygroscopic effects
Certain salts will absorb moisture from the atmosphere to such an extent that, if they are present in
brickwork, damp patches will appear in plaster covering the bricks every time the weather becomes
sufficiently humid and will fade away when dry weather returns.
The affected areas can relate to a single brick and such areas often show a well-defined edge. Typical
locations to be affected are chimney breasts, brickwork previously affected by rising damp or damp
penetration through the walls from adjoining structures.
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11.2.6 Recognition of rising damp
!Water travelling through a wall will contain dissolved salts and organic matter from the ground,
particularly sulfate, chloride and nitrate salts. Sulfates are efflorescent and as the water evaporates at or
just below the surface of the wall, these salts crystallize out. When this occurs at the surface, the crystal
will form a fragile feathery crystalline growth usually white in colour and often forming “tide” marks. When
it occurs below the surface, e.g. behind a paint film or plaster, the crystals will grow and can disrupt the
paint film or force the plaster off the wall. Chlorides and nitrates are hygroscopic and do not appear as
crystals on the surface, rather they tend to show as tide marks at the maximum height of rise. Decorations
are usually discoloured, wallpaper can be bleached or stained a brownish colour in these areas. The
presence of chlorides tends to inhibit the growth of moulds, in effect producing saline conditions locally.
Rising damp gives a decreasing amount of free moisture from ground level upwards, when measured by
gravimetric methods. Care should be taken with a chemical absorption type meter as it will give high
readings in regions contaminated by hygroscopic salts. Seawater flooding, for example, will give permanent
high readings by chemical absorption due to the salt content of seawater, and the use of contaminated or
unwashed sea-dredged materials in construction should also be borne in mind.
If, for example, because of the presence of a thermal bridge, or because of variations in air temperature or
humidity at different points in a room, water condenses on a non-absorbent surface, it will form droplets.
Rising damp does not appear in this form.
One of the most reliable ways of differentiating between dampness due to condensate and that due to rising
damp is to compare moisture contents of samples of masonry, or preferably mortar, from within the depth
of the wall and near the inner surface of the wall. Samples from within the wall will not be damp if surface
condensation is the sole cause.
More details of damp proofing treatments are given in BS 6576:1985.
11.2.7 Measurement of dampness
Accurate measurements of the moisture content of brick or mortar cannot be obtained by the use of
electrical moisture meters because the presence of salts increases the electrical conductance of the water,
giving falsely high readings. Gravimetric methods carried out on samples taken from the fabric give the
most reliable results. The use of chemical absorption type moisture meters will give a result in a short space
of time and be almost as reliable.
Measurement methods for dampness in walls are discussed in more detail in BRE Digest 245 [17] and BRE
Report 466 [18]."
11.2.8 Condensation on thermal bridges within the roof spaces
In many cases, wet patches on ceilings close to the wall are due to condensate running down from within
the roof space. This phenomenon occurs more often in warm deck flat roofs where the perimeter wall is, for
example, thermally bridged by the lintel of a window that extends into the roof space.
11.3 Damage caused by dampness
11.3.1 Damage to structure
The most likely forms of damage to a structure are decay in the timber or cellulose products, corrosion of
metals, or excessive moisture movement of materials. Dampness can cause distortion and in some cases
serious weakening of sheet or thin slab materials, e.g. in roof decks and ceilings. Moisture trapped beneath
impermeable roof finishes can, in hot weather, cause vapour pressure high enough to cause damage to the
roof finish.
Where inspection reveals damage, appropriate replacement, repair or preservative treatment should be
carried out.
Any precautions against future condensation should ensure that further structural damage will be
prevented. It is essential, therefore, that remedial treatment should be directed to overcoming harmful
interstitial condensation as well as surface condensation. Freedom from the latter does not necessarily
ensure that the former does not occur.
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BS 5250:2002
11.3.2 Damage to finishes
Damage to decoration occurs mainly from surface condensation, but occasionally can be caused by
interstitial condensation moisture, e.g. roof condensate dripping on to ceilings, or wall condensate drying
inward and damaging surface finishes.
Mould growth on room surfaces, particularly in room corners, first appears as spots or small patches, which
can spread to form a furry layer usually grey-green, black or brown in colour. On paint, it can show as pink
or purple.
Soft distempers are liable to flake if repeatedly wetted and dried, whilst emulsion paints are more likely to
remain undamaged. Moisture on the exposed surfaces tends to reduce the gloss of some impervious paints
but otherwise does not cause damage. If moisture penetrates behind impervious paint films, blistering of
the paint can occur. In extreme cases, the plaster can break down and/or lose its adhesion.
Repeated or prolonged absorption of condensate can cause distortion of sheet materials, e.g. plasterboard
or fibreboard. Water absorbed from a surface can reach and break down an adhesive by which a surface
finish is fixed. This can occur on floors or walls, but is more likely to be harmful in the case of ceilings when
the effect of gravity adds to the risk of displacement of adhesive fixed tiles.
11.4 Remedial works
11.4.1 Action to control condensation
It is essential that any action to control condensation takes account of the intended use of the building and
involves comprehensive consideration of heating, ventilation and thermal insulation.
11.4.2 Heating
All surface condensation problems can, in principle, be solved by the application of heat to raise
temperatures above the dewpoint and to evaporate existing dampness, coupled with adequate levels of
ventilation.
A common cause of harmful condensation in existing buildings is lack of adequate heating. If the existing
system is inadequate, a heating system should be installed with an output ample for the task required of
it, designed in accordance with the guidance given in Clause 9.
Heating and insulation should always be considered together since there is often scope for a saving in
capital cost as well as in running cost.
11.4.3 Ventilation
It should be recognized that some energy will have to be expended on the removal of water vapour if
condensation is to be controlled.
It is essential that adequate ventilation is provided to maintain the dewpoint temperature of the air below
the inside surface temperature of the building envelope at all times. Provision of ventilation or PSV to
kitchens and bathrooms is recommended and any replacement windows fitted in other rooms should
include trickle ventilators.
11.4.4 Insulation
Sufficient insulation should be added to the fabric of the building to allow the building to be heated
adequately at a reasonable cost. Care should be taken locating insulation so that problems of interstitial
condensation are not introduced and junctions detailed so that thermal bridging does not occur.
It is important to note that little change in condensation risk, if any, will result from improvements in
thermal insulation, however extensive, unless a satisfactory balance of heating and ventilation is also
achieved.
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11.5 Mould growth
11.5.1 General
Mould growth is often associated with surface condensation. Damp houses provide good conditions for its
development. Mould spores exist in large numbers in the atmosphere and, to germinate, need a nutrient,
oxygen, a suitable temperature and moisture. Sources of nutrition are widespread in buildings and the
internal environment provides a suitable temperature for growth. As oxygen is also always present, mould
growth is principally dependent upon the moisture conditions at surfaces and the length of time these
conditions persist. Studies have shown that moulds do not necessarily require the presence of water. As a
guide, if the average relative humidity within a room stays above 70 % for several days, the relative
humidity at external wall surfaces will be high enough to support the germination and growth of moulds.
11.5.2 Treatment
Although the symptoms of mould growth are fairly easily dealt with by either washing with a household
bleach diluted 1:4, followed by clean water, or the use of a proprietary toxic wash, it is better to remove the
cause of the mould growth, i.e. the high relative humidities. Proprietary anti-fungal paints and wallpaper
pastes, which can be used in areas where condensation occurs regularly, are also available (see BRE Digest
139 [14]).
12 Particular aspects
12.1 Initial period at commencement of re-use
It is important to remember that, if the building has been unoccupied for a number of years in a state of
disrepair, a great deal of water may be present in the fabric and one or two years can elapse before it has
all dried out. During this period, moisture may continue to appear on the walls, some of it as a result of
evaporation from the fabric and some of it as condensation forming on the surfaces of parts of the envelope
that have their insulating properties impaired due to absorbed water. It is essential to recognize the risk
of entrapping stored water behind new work such as dry lining to walls.
12.2 Thermal insulating materials
The insulation values of open cell or fibrous materials will be adversely affected if wetted. Remedial work
should include drying materials that are to be re-used. Precautions should be taken to prevent wetting of
all new materials introduced into the structure.
The provision of protection against mechanical damage to insulating materials should also be taken into
account.
12.3 Furniture
Consideration should be given to removing built-in furniture from an outside wall to allow heat to reach
the wall from the room, otherwise water vapour could penetrate to the wall and condense, resulting in
mould growth, not only on the wall but also on the contents of the furniture.
12.4 Larders and unheated stores
Particular attention should be paid to the ventilation of larders, unheated storage spaces and enclosed
porches. Water vapour migrates to these areas from adjacent heated spaces where the vapour pressure is
higher. Consideration should be given to the heating and insulation of such storage spaces.
12.5 Redecoration
The materials used for decoration during a period when the building is drying out should be capable of
allowing moisture to evaporate through them without incurring damage. Neither wallpaper nor impervious
paints should be used till the structure has achieved a near equilibrium condition with the prevailing
internal conditions. Occupants should be warned that the drying out period will be protracted and be
advised not to commence the decoration of walls until an equilibrium condition is attained.
12.6 Heating
Problems in existing buildings often arise because building owners do not provide the heating for their
buildings, but devolve heating and its costs to tenants. In rented property, it might benefit the owner to
make a provision for background heating, some of the cost of which can be included in rent, so as to ensure
that the building is not damaged by the accumulation of condensation.
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12.7 Ventilation
In existing buildings, problems often arise because higher levels of comfort expectations, combined with
rising costs of heating, have led to a reduction of unwanted air movement, which have become apparent as
draughts, for example by:
a) removal of fireplaces;
b) draught-proofing as a cost effective method of energy saving.
Fortuitous means of natural ventilation have often, therefore, been eliminated.
While the removal of moisture at source by mechanical extraction or PSV for example, in kitchens,
bathrooms and general moisture generating areas is an obvious remedy, it should be remembered that the
air extracted has to be replaced by incoming air, which in turn needs to be heated.
In dealing with ventilation rates and methods in existing buildings, all the technical and economic
considerations and methods of assessments given previously should be employed.
12.8 Drying clothes
Where possible, the drying of clothes inside should be avoided. If a heated cupboard is not provided or not
used, occupants should be advised to restrict clothes drying within a dwelling to a room fitted with an
extractor fan. When extraction is not continuous, controls should be provided to enable the extractor fan to
be switched on for clothes drying.
It is particularly important not to dry clothes in unventilated rooms, especially those kept at low
temperatures.
It is essential that tumble driers are vented outside or be of the condensing type.
13 Precautionary measures during construction
13.1 Construction information
The precautions to control condensation taken by the designer can be negated by lack of site supervision.
Having followed the advice in the preceding sections, the designer should ensure that his design
precautions are clearly shown in the specification, drawings and other construction information. All
supervisory staff should ensure that the design is strictly complied with and the work monitored. It is
essential therefore that easily understood information is given to the tradesmen executing the work.
The construction information should be given to the contractor and all precautions designed to prevent
harmful condensation should be brought to the attention of the supervisory staff on site.
13.2 Site checks
13.2.1 Materials and storage
All materials should be checked to ensure that they meet the specification and should be stored so that they
do not suffer damage.
13.2.2 Vapour control layers
It should be ensured that any vapour control layers are in the correct position, cover the whole area to be
protected and are fully lapped and/or sealed in accordance with the design. Special attention should be paid
to ensure that following trades do not damage the vapour control layers without repair. Where gaps are cut
through, e.g. for services, the hole should be sealed.
13.2.3 Dry construction
Where dry forms of thermal insulation are used, a check should be made to ensure that they cover all
specified positions, e.g. that lightweight sheet or slab insulation has close fitting joints and is free from gaps
caused by broken corners, etc.
In application, dry insulation should be protected from rain or other wetting. This is especially necessary
in positions such as roof insulation laid above a vapour barrier and beneath an impermeable roof finish,
and also in uncompleted insulated walls.
Insulation materials liable to mechanical damage should be fixed as late as other requirements allow. A
check for possible damage should be made immediately before the insulation is covered by other materials.
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13.2.4 Wet construction
Due to the presence of residual construction water, in situ concrete or masonry structures take a
considerable time to reach their ultimate “in-use” insulation values. Because it is normally impracticable
to allow sufficient time for complete drying out before buildings are occupied, it is important to prevent
unnecessary wetting of the materials during building.
The tops of walls should be protected from rain penetration at all times during construction. This is
especially important where walling materials would readily absorb water.
For floors and flat roofs full protection from rain might be impracticable but precautions should be taken
to avoid unnecessary wetting, e.g. by avoiding standing water. This is particularly necessary where, for any
reason, the construction is likely to be exposed for a long time.
When using wet roof screeds, where they have to dry inwards after the roof finish is complete or where they
would be dried outwards only by limited ventilation to the exterior, care should be taken to prevent the
ingress of excess water.
Adequate ventilation should be provided during both the construction and the drying out period.
13.2.5 Services
Gaps around services through walls and ceilings should be sealed to prevent moisture laden air getting into
cold voids.
13.2.6 Draught proofing
Where draught proofing is required it should be continuous with corners and junctions joined as specified.
13.2.7 Ventilation
Most forms of ventilation would be checked during normal building supervision. Aspects which may need
particular attention include ensuring that small ventilation openings, such as those to roof spaces, are
provided as specified and have not been made ineffective by unintentional obstructions or by late changes
in the design details, e.g. by roof insulation blocking the ventilation openings or by any interruption to
intended cross ventilation.
13.2.8 Final checks and maintenance
These should include the following.
a) Replacing any insulation which has moved out of position.
b) Checking that pipework and water tanks are fully lagged.
c) Checking that nothing has blocked ventilation airways.
d) Checking that the performance of dry insulation is not impaired by moisture.
e) Checking that seals on hatches, mechanical and electrical services, penetrating a ceiling under a roof
are intact.
14 Building user information
14.1 Owner’s manual
Building owner/occupiers should be given information on the control of condensation so as to enable them
to use the building within its design limits.
The manual should contain advice for the occupiers so that, by following some simple rules, condensation
can be minimized.
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For example, the following advice would be very helpful.
a) Keep all rooms warm and ventilated.
b) Keep the internal kitchen door closed and window open when cooking.
c) Keep the bathroom door closed and the window ajar when bathing or showering, or open the window
immediately afterwards.
d) Use an electric extractor fan, if installed, when cooking, washing clothes or bathing, particularly when
windows show a sign of misting. Keep doors and windows closed when the fan is in operation. Leave the
fan on until the mistiness has cleared; this is especially important where extractor fans have been
installed so as to counteract prevailing airflows within the dwelling.
e) Do not use paraffin heaters or flueless gas heaters in unventilated rooms, as every litre of paraffin
burnt can produce approximately one litre of water. Provide adequate ventilation where the use of these
heaters is unavoidable.
f) Keep heating on at all times in cold weather. (Intermittent heating causes condensation to be deposited
as the air and surfaces cool.)
g) Keep the heating on low if your home is unoccupied during the day.
h) If condensation has occurred:
1) as much as possible should be mopped up;
2) the room should be heated;
3) the window should be opened a little;
4) the door should be shut.
Unless such actions are taken, nuisance condensation can become harmful.
i) If condensation occurs in a room which has a heating appliance with a flue, check the heating
installation immediately as the condensation might have appeared because the flue has been blocked.
14.2 Particular aspects
14.2.1 Heating
In the case of owner-occupiers there is no conflict concerning responsibility for meeting the objective of
condensation control stated in the design. In landlord/tenant situations, agreements should define the
respective responsibilities of the parties for the provision of heat to minimize condensation.
These include the responsibilities for running and maintenance costs of the plant provided with the
building. The occupier should be given sufficient information to enable him to run the system in the manner
intended by the designer.
14.2.2 Heating appliances
The heating equipment provided should be satisfactory for the building and economic to use. The problems
of using flueless gas or paraffin heaters and the amount of moisture vapour they produce should be pointed
out to the building user and discouraged.
14.2.3 New buildings
New buildings often take a long time before they are fully dried out. While this is happening, they need
extra heat and ventilation; certainly during the first winter of use, many houses and flats require more heat
than they will need in subsequent winters.
Occupiers should be made aware of the extra care required during these first few months in allowing
ventilation for the release of moisture vapour, and the mopping up of condensation before damage occurs
to the decoration and fabric of the building.
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14.2.4 Dehumidifiers
There are various sizes of dehumidifiers commercially available that can be used to treat cases of severe
condensation. They are very useful for drying out new buildings or buildings where parts of the structure
are wet due to burst pipes or flooding. They are not normally suitable as a cure for regularly occurring
condensation.
14.2.5 Drying clothes
Where possible, the drying of clothes inside should be avoided. If a heated cupboard is not provided or not
used, occupants should be advised to restrict clothes drying within a dwelling to a room fitted with an
extractor fan. When extraction is not continuous, controls should be provided to enable the extractor fan to
be switched on for clothes drying.
It is particularly important not to dry clothes in unventilated rooms, especially those kept at low
temperatures.
It is essential that tumble driers are vented outside or be of the condensing type.
14.2.6 Curtains and internal blinds
The effect of a curtain or internal blind on a window is to further reduce the window surface temperature
and increase condensation on the glass. The use of trickle ventilators can help to alleviate the problem.
14.2.7 Furniture
Wardrobes, fitted cupboards and other large items of furniture should not be placed directly against
external walls. The resulting pockets of trapped air can lead to serious surface condensation and mould
growth, on the wall and furniture. The contents of wardrobes and cupboards can also be affected.
14.2.8 Ceiling airtightness
It is important, from the point of view of energy conservation and to limit the risk of condensation in the
loft, that airflow from the house into the loft is minimized. To achieve this, the ceiling should remain well
sealed.
If the householder installs products which penetrate the ceiling, the same precautions should be taken as
above.
The loft access trap can be a potential route for water vapour escape into the loft space. It should be kept
closed when access is not required.
14.2.9 Roof ventilation
If roof ventilation is installed, ensure that it is not obstructed by insulation or goods stored in the loft.
© BSI 23 December 2005
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Annex A (normative)
The interrelationship of moisture contents and temperatures
Table A.1 gives the saturated vapour pressure (SVP) over water above 0 °C and over ice below 0 °C. The
psychrometric chart (see Figure A.1 and Figure A.2) relates air moisture content, temperatures and
relative humidity and may be used in calculations made to check that condensation will not occur.
Figure A.1 is drawn for saturation vapour pressures over water; where a design is critical, it is preferable
to use the below zero values from Table A.1, which are calculated over ice.
As an example of the use of the chart consider point A in Figure A.1. This represents an air condition of
0 °C and 90 % r.h. with a vapour pressure of 0.55 kPa. This might well be the condition of outdoor air in
winter.
Point B indicates air with the same moisture content and, therefore, the same vapour pressure, but as it is
now at 20 °C its relative humidity has changed to approximately 24 %. This shows what happens to the
outdoor air after it enters a building and is warmed, if no other changes occur.
Point C indicates air also at 20 °C, but with vapour pressure increased to about 1.64 kPa. The increase in
moisture without change in temperature means the relative humidity has risen, and the curved lines show
this to be about 70 %. This is what might occur when the incoming air has picked up moisture from
activities within the building.
Reading horizontally to the left from C, point D indicates when saturation would occur, i.e. when the air is
cooled to a dewpoint temperature of about 14.3 °C.
Air moisture contents and/or percentage relative humidity may be determined by using a wet and dry bulb
thermometer.
When undertaking calculation procedures for assessing condensation risk it is necessary to determine
vapour pressures. These can either be obtained from the psychrometric chart or from Table A.1.
In some circumstances, it can be beneficial to computerize the calculation procedure and in this case it
might be useful to have an equation for calculating saturation vapour pressure. A number of equations
exist; equations (A.1) and (A.2) are simplified equations that provide a close approximation to the
saturation vapour pressure over the range of temperatures typical in buildings. Saturated vapour pressure
is measured in kilopascals.
17.269 × T
SVP = 0.6105 exp ⎛ ----------------------------⎞
⎝ 237.3 + T ⎠
for T > 0
(A.1)
21.875 × T
SVP = 0.6105 exp ⎛ ----------------------------⎞
⎝ 265.5 + T ⎠
for T < 0
(A.2)
where T is the temperature given in °C.
The equation for T < 0 gives the saturation vapour pressure values over ice. These formulae are consistent
with BS EN ISO 13788.
These equations may be usefully inverted and used to calculate dewpoint temperature from saturated
vapour pressure.
66
p sat
237.3 log e ⎛ ---------------⎞
⎝ 6.105⎠
Tdp = -------------------------------------------------------- for SVP U 6.105 kPa
p sat
17.269 – log e ⎛ ---------------⎞
⎝ 6.105⎠
(A.3)
p sat
265.5 log e ⎛ ---------------⎞
⎝ 6.105⎠
Tdp = -------------------------------------------------------- for SVP < 6.105 kPa
p sat
21.875 – log e ⎛ ---------------⎞
⎝ 6.105⎠
(A.4)
© BSI 23 December 2005
BS 5250:2002
Relative humidity (%)
1.7
100
D
C
90
80
1.5
70
Vapour Pressure ( kPa )
60
1.3
50
1.1
40
0.9
0.7
0.5
B
A
0
2
4
6
8
10
12
14
16
18
20
22
24
Temperature ( ˚ C )
Figure A.1 — Example of use of the psychrometric chart
© BSI 23 December 2005
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BS 5250:2002
Table A.1 — Saturation vapour pressures for air temperatures 30.9 °C to –20 °C
Temperature
°C
30
29
28
27
26
25
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
0.0
0.1
0.2
4.243
4.005
3.780
3.565
3.361
3.167
2.983
2.809
2.643
2.486
2.337
2.196
2.063
1.937
1.817
1.704
1.598
1.497
1.402
1.312
1.237
1.147
1.072
1.001
0.935
0.872
0.813
0.757
0.705
0.657
0.611
4.267
4.029
3.802
3.586
3.381
3.186
3.001
2.826
2.659
2.501
2.352
2.210
2.076
1.949
1.829
1.715
1.608
1.507
1.411
1.321
1.235
1.155
1.079
1.008
0.941
0.878
0.819
0.763
0.711
0.661
0.615
4.292
4.052
3.824
3.607
3.401
3.205
3.019
2.843
2.675
2.519
2.366
2.224
2.089
1.961
1.841
1.726
1.618
1.517
1.420
1.329
1.244
1.163
1.087
1.015
0.948
0.884
0.824
0.768
0.716
0.666
0.620
Saturation vapour pressure
kPa
0.3
0.4
0.5
0.6
4.317
4.075
3.846
3.628
3.421
3.224
3.037
2.860
2.692
2.532
2.381
2.238
2.102
1.974
1.852
1.738
1.639
1.527
1.430
1.338
1.252
1.171
1.094
1.022
0.954
0.890
0.830
0.774
0.721
0.671
0.624
0.7
0.8
0.9
4.341
4.099
3.869
3.649
3.441
3.243
3.055
2.877
2.708
2.548
2.396
2.252
2.155
1.986
1.864
1.749
1.640
1.537
1.439
1.347
1.261
1.178
1.102
1.029
0.961
0.897
0.836
0.779
0.726
0.676
0.629
4.366
4.123
3.891
3.671
3.462
3.263
3.074
2.895
2.725
2.564
2.411
2.266
2.129
1.999
1.876
1.760
1.650
1.547
1.449
1.356
1.269
1.187
1.109
1.036
0.967
0.903
0.842
0.785
0.731
0.681
0.633
4.391
4.147
3.914
3.692
3.482
3.282
3.092
2.912
2.741
2.579
2.427
2.280
2.142
2.012
1.888
1.771
1.661
1.557
1.458
1.365
1.277
1.195
1.117
1.043
0.974
0.909
0.848
0.790
0.736
0.686
0.637
4.417
4.171
3.936
3.714
3.503
3.302
3.111
2.930
2.758
2.595
2.441
2.294
2.155
2.024
1.900
1.783
1.672
1.567
1.468
1.374
1.286
1.203
1.124
1.051
0.981
0.915
0.854
0.796
0.742
0.690
0.643
4.442
4.195
3.959
3.736
3.523
3.321
3.129
2.948
2.775
2.611
2.456
2.308
2.169
2.037
1.912
1.794
1.683
1.577
1.477
1.383
1.295
1.211
1.132
1.058
0.988
0.922
0.860
0.802
0.747
0.695
0.647
4.467
4.219
3.982
3.758
3.544
3.341
3.148
2.965
2.792
2.627
2.471
2.323
2.183
2.050
1.925
1.806
1.694
1.587
1.487
1.393
1.303
1.219
1.140
1.065
0.994
0.928
0.866
0.807
0.752
0.700
0.652
0.591
0.544
0.500
0.460
0.423
0.388
0.356
0.326
0.299
0.274
0.251
0.220
0.210
0.191
0.175
0.159
0.145
0.132
0.120
0.109
0.099
0.586
0.539
0.496
0.456
0.419
0.385
0.353
0.324
0.297
0.272
0.248
0.227
0.208
0.190
0.173
0.158
0.144
0.131
0.119
0.108
0.098
0.581
0.535
0.492
0.452
0.415
0.381
0.350
0.321
0.294
0.269
0.246
0.225
0.206
0.188
0.171
0.156
0.142
0.130
0.118
0.107
0.097
0.576
0.531
0.488
0.449
0.412
0.375
0.347
0.318
0.291
0.267
0.244
0.223
0.204
0.186
0.170
0.155
0.141
0.128
0.117
0.106
0.096
0.572
0.526
0.484
0.445
0.408
0.375
0.344
0.315
0.289
0.264
0.242
0.221
0.202
0.184
0.168
0.153
0.140
0.127
0.116
0.105
0.095
0.567
0.522
0.480
0.441
0.405
0.372
0.341
0.312
0.286
0.262
0.240
0.219
0.200
0.183
0.167
0.152
0.138
0.126
0.115
0.104
0.095
actual vapour pressure
saturation vapour pressure
r.h. = --------------------------------------------------------------------------- × 100 %
0
0.611
0.606
–1
0.562
0.558
–2
0.517
0.513
–3
0.476
0.472
–4
0.437
0.434
–5
0.402
0.398
–6
0.368
0.365
–7
0.338
0.335
–8
0.310
0.307
–9
0.284
0.281
– 10
0.260
0.257
– 11
0.238
0.236
– 12
0.217
0.215
– 13
0.198
0.197
– 14
0.181
0.180
– 15
0.165
0.164
– 16
0.151
0.149
– 17
0.137
0.136
– 18
0.125
0.124
– 19
0.114
0.112
– 20
0.103
0.102
Saturation point pressure over ice
0.601
0.553
0.509
0.468
0.430
0.395
0.362
0.332
0.304
0.279
0.255
0.233
0.213
0.195
0.179
0.162
0.148
0.135
0.122
0.111
0.101
0.596
0.548
0.505
0.464
0.426
0.391
0.359
0.329
0.302
0.276
0.253
0.231
0.211
0.193
0.176
0.161
0.146
0.133
0.121
0.110
0.100
actual vapour pressure
saturation vapour pressure
r.h. = --------------------------------------------------------------------------- × 100 %
68
© BSI 23 December 2005
BS 5250:2002
Note: BSI encourages the use of this chart and therefore permits reproduction freely
4.0
3.5
100
3.0
24
90
80
20
re
70
60
16
tem
pe
ra
tu
2.0
ulb
Vapour pressure (kPa)
2.5
tb
1.5
50
12
We
40
8
1.0
30
4
0
20
-2
0.5
10
0.0
-5
0
5
10
15
20
25
30
Temperature ( ˚ C )
Figure A.2 — Psychrometric chart
© BSI 23 December 2005
69
BS 5250:2002
Annex B (normative)
Moisture generation and ventilation in occupied buildings
The tables in this annex give guidance on moisture generation in occupied buildings.
B.1 Housing
Table B.1 — Typical moisture generation rates for
household activities
Household activity
People:
asleep
active
Cooking:
electricity
gas
Dishwashing
Bathing/washing
Washing clothes
Drying clothes indoor
(e.g. using unvented
tumble drier)
Moisture generation rate
40 g/h per person
55 g/h per person
2 000 g/day
3 000 g/day
400 g/day
200 g/person per day
500 g/day
1 500 g/person per day
Table B.2 — Typical moisture generation rates
from heating fuels
Heating fuel
Moisture generation rate
g/kW·h
gasa
Natural
Manufactured gasa
Paraffin
Portable LPG
Cokea
Anthracitea
Electricity
a
70
150
100
100
100
30
10
0
The majority of heating appliances using these fuels are
ventilated to the outside air. Consequently the water vapour
produced by combustion is not released directly into the
dwelling.
© BSI 23 December 2005
BS 5250:2002
Table B.3 — Daily moisture generation rates for households
Number of
persons in
household
Daily moisture generation rates
Dry occupancya
Moist
occupancyb
kg
1
2
3
4
5
6
a
b
c
3.5
4
4
5
6
7
Wet
occupancyc
kg
6
8
9
10
11
12
kg
9
11
12
14
15
16
Dry occupancy: where there is proper use of ventilation, it includes
those buildings unoccupied during the day; results in an internal
pressure of up to 0.3 kPa in excess of the internal vapour pressure.
Moist occupancy: where internal humidities are above normal; likely
to have poor ventilation; possibly a family with children, water vapour
excess is between 0.3 kPa and 0.6 kPa.
Wet occupancy: ventilation hardly ever used; high moisture
generation; probably a family with young children, water vapour
pressure excess is greater than 0.6 kPa
Table B.4 — Typical ventilation rates
Description of dwelling
Ventilation rate
ac/h
Well-sealed dwelling in
sheltered position
Average dwelling in
sheltered position
“Leaky” dwelling in
sheltered position
Well-sealed dwelling in
exposed position
Average dwelling in
exposed position
“Leaky” dwelling in exposed
position
0.5
1.0
1.5
1.0
1.5
2.0
B.2 Other buildings
Much less information is available covering buildings other than housing. However, the concept of classes
of internal humidity load can be helpful. This concept is based on the assumption that the difference
between the internal and external vapour pressure, the internal humidity load, depends upon the amount
of moisture produced within the building and upon the ventilation rate.
C
%p = p i – p e = -----------------------------------0.191 × N × V
(B.1)
where
V
is the total building volume in m3
N is the ventilation rate in ac/h
C is the daily moisture input in kg/day
As the external temperature falls, tp will rise because ventilation rates fall.
© BSI 23 December 2005
71
BS 5250:2002
Internal humidity load can be described by five humidity classes. Figure B.1 shows limit values of tv and
tp for each class as a function of external temperature, derived from measured data. Table B.5 shows the
types of buildings expected to fall into each class and the range of relative humidities covered by the class
in buildings with different internal temperatures, at an external temperature of 0 °C and a relative
humidity of 95 %.
For calculations, it is recommended that the upper limit value for each class be used unless the designer
can demonstrate that conditions are less severe.
∆ν
∆ρ
(kg/m3 )
(Pa)
0.008
1080
5
0.006
810
4
0.004
540
3
2
0.002
270
1
0
-5
0
5
10
15
20
25
Monthly mean outdoor air temperature, Θ e ( ˚ C )
Figure B.1 — Variation of internal humidity classes with external temperature
Table B.5 — Internal humidity classes: building types and limiting relative
humidities at Te = 0 °C
Humidity
class
Building type
Relative humidity at internal
temperature
15 °C
1
2
3
4
5
72
Storage areas
Offices, shops
Dwellings with low occupancy
Dwellings with high occupancy, sports halls, kitchens,
canteens; buildings heated with unflued gas heaters
Special buildings, e.g. laundry, brewery, swimming pool
20 °C
25 °C
<50
50 – 65
65 – 80
80 – 95
<35
35 – 50
50 – 60
60 – 70
<25
25 – 35
35 – 45
45 – 55
>95
>70
>55
© BSI 23 December 2005
BS 5250:2002
Annex C (normative)
Material properties
Table C.1 gives the thermal conductivity and vapour resistivity of a range of important building materials.
These values of material properties should be combined with component dimensions to give the thermal
and vapour resistances that are used in calculations. Further data are given in CIBSE Guide A3 [15] and
BS EN 12524.
NOTE The values given in this table are the best currently available. However, data from measurements, independent certification
or manufacturers’ literature should be used wherever possible.
Table C.1 — Thermal conductivities and vapour resistivities
Material
Density
kg/m
Airspace
Asbestos cement sheeting and substitutes
Asphalt (poured)
Bitumen
Blockwork
lightweight
medium weight
dense
Brickwork
common/inner leaf
common/outer leaf
engineering
Carpeting
with cellular rubber underlay
with synthetic underlay
Concrete (cast)
aerated, cellular
aerated
medium weight
dense
no fines
Fibre (glass or rock)
Glass
sheet
expanded or foamed
Metals
aluminium
copper
iron
lead
steel
stainless steel
tin
zinc
© BSI 23 December 2005
Thermal
conductivity
Typical
Range
W/m·K
MN·s/g·m
MN·s/g·m
3
Vapour resistivity
700
2 100
1 000
See Table C.3 See Note 1
0.36
300
1.20
—
0.20
10 000
600
1 400
2 050
0.22
0.60
0.90
30
50
100
20 to 50
30 to 80
60 to 150
1 700
1 700
2 000
0.62
0.84
1.25
50
50
120
25 to 100
25 to 100
100 to 250
400
160
0.10
0.06
200
200
100 to 300
100 to 300
400
850
1 350
2 200
1 800
12
0.15
0.29
0.59
1.70
0.96
0.04
50
100
150
200
20
5
2 500
140
1.00
0.05
—
10 000
—
2 700
8 600
7 900
11 340
7 800
8 000
7 300
7 000
230
384
72
35
60
16
65
113
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
—
200 to 1 000
—
2 000 to 60 000
73
BS 5250:2002
Table C.1 — Thermal conductivities and vapour resistivities (continued)
Material
Density
kg/m3
Thermal
conductivity
Vapour resistivity
W/m·K
Typical
Range
MN·s/g·m
MN·s/g·m
50
30
60
30 to 60
20 to 50
40 to 90
Plaster
gypsum
lightweight
Plasterboard
Plastic foams
phenol
polyisocyanurate
polyurethane
polyvinylchloride
urea formaldehyde
Polystyrene
expanded bead
expanded extruded
PVC (polyvinyl chloride) sheet or tile
Rendering
Roofing
clay tiles
concrete tiles
slates
Roofing felt
Rubber
natural
neoprene
butyl
foam rubber
EPDM
polyisobutylene
Screed
aerated
cast
Soil
clay or silt
sand or gravel
Stonework
basalt, gneiss, marble
granite
slate
limestone, hard
limestone, soft
sandstone
pumice
74
1 120
720
800
0.51
0.22
0.17
30
45
30
37
10
0.04
0.03
0.03
0.035
0.04
150 to 750
150 to 750
115 to 1 000
40 to 1 300
5 to 20
20
30
1 390
1 600
0.035
0.027
0.16
0.8
300
1 000
2 000
2 100
2 500
960
0.84
1.50
2.20
0.30
Table C.2
Table C.2
Table C.2
910
1 240
1 200
70
1 150
930
0.13
0.23
0.24
0.06
0.25
0.20
4 500
4 500
4 500
4 500
4 500
4 500
700
2 100
0.40
1.40
1 500
1 800
1.50
2.00
2 700
2 500
2 400
2 200
1 800
2 600
400
3.50
3.50
1.40
2.30
1.80
2.30
0.12
100 to 600
600 to 1 300
800 to 1 300
100
100
50
Z
150 to Z
150 to 450
350 to 450
130 to 160
75 to 450
30 to 50
© BSI 23 December 2005
BS 5250:2002
Table C.1 — Thermal conductivities and vapour resistivities (continued)
Material
Density
kg/m3
Tiling (ceramic)
Timber
Thermal
conductivity
W/m·K
Vapour resistivity
Typical
Range
MN·s/g·m
MN·s/g·m
2 300
1.30
750 to 1 500
500
700
260
0.13
0.18
0.07
90 to 700
200 to 1 500
Vermiculite
Wood based panels
cement bonded
particleboard
1 200
particleboard
300
particleboard
600
oriented strand board (OSB)
650
woodwool slabs
600
hardboard
880
sheathing plywood
500
decking plywood
700
fibreboard
250
medium density fibreboard (MDF)
600
cork board
110
0.23
0.10
0.14
0.13
0.10
0.12
0.13
0.17
0.07
0.14
0.04
15
19 to 50
300 to 500
500 to 700
200 to 500
15 to 40
250 to 1 000
150 to 1 000
1 000 to 6 000
150 to 400
300 to 600
25 to 50
NOTE 1 Although a value of 5 MN·s/g·m can be assigned to the vapour resistivity of still air (see Annex E), in practice the air in a
cavity is never still because of ventilation or convection. Consequently, the vapour resistivity of air in cavities should be assumed to
be zero, when carrying out interstitial condensation calculations.
The values in Table C.2 are vapour resistances of thin membranes and foils that are used directly in
calculations.
NOTE 2 The values given in this table are the best currently available. However, data from measurements, independent
certification or manufacturers’ literature should be used wherever possible.
© BSI 23 December 2005
75
BS 5250:2002
Table C.2 — Vapour resistances
Material
Aluminium foil
Asphalt (laid)
Breather membrane
Building paper (bitumen impregnated)
Felt
(a) roofing felt laid in bitumen
(b) Type 1F felt
Glass (sheet)
Metals and metal cladding
Paint
(a) emulsion
(b) gloss
(c) vapour resistant
Polyester film (0.2 mm)
Polyethylene
(a) 500 gauge (0.12 mm)
(b) 1 000 gauge (0.25 mm)
Roof tiling or slating
Vinyl wallpaper
Vapour resistance
Typical
Range
MN·s/g
MN·s/g
1 000
10 000
0.5
10
200 to 4 000
0.1 to 0.6
1 000
450
10 000
10 000
0.5
15
25
250
250
500
2.5
10
8 to 40
200 to 350
400 to 600
0.5 to 3.0
NOTE 1
The values above are for the material alone and, when installed, may be considered lower.
NOTE 2
Thermal resistances of the above may be in general considered negligible for the purposes of these calculations.
NOTE 3
The values of asphalt, glass and metals are notional values for the purpose of calculation.
NOTE 4 Further information on material properties, particularly vapour resistances, should be made available to the Technical
Committee responsible for this standard for consideration.
76
© BSI 23 December 2005
BS 5250:2002
Table C.3 — Thermal resistances for surfaces and air spaces
Internal surface resistances
Walls
Ceilings, roofs (flat and pitched) and floors
Floors and ceilings
Heat flow
direction
Horizontal
Upwards
Downwards
Thermal resistance
m2·K/W
0.13
0.10
0.17
External surface resistances (normal exposure)
Walls
Roofs
Exposed floors
0.04
0.04
0.04
Unventilated airspace resistances
5 mm (high emissivity)
5 mm (low emissivity)
25 mm or more (high emissivity)
25 mm or more (low emissivity)
Heat flow
direction
All directions
All directions
Horizontal
Upwards
Downwards
Horizontal
Upwards
Downwards
0.10
0.17
0.18
0.16
0.19
0.44
0.34
0.50
Ventilated airspaces resistances (minimum 25 mm thickness)
Airspace in cavity wall construction
Airspace behind tiles on tile hung wall (includes resistance of the tile)
Loft space between flat ceiling and pitched roof lined with roofing felt or building
paper
Airspace between tiles and roofing felt or building paper (includes resistance of the
tiles)
NOTE 1
0.18
0.12
0.20
0.12
More detailed data are contained in CIBSE Guide A3 [15].
NOTE 2 In general the surfaces of most building materials are of high emissivity. Low emissivity values are applicable to cavities
adjacent to a reflective foil or foils.
© BSI 23 December 2005
77
BS 5250:2002
Annex D (normative)
Calculation methods
D.1 General
BS EN ISO 13788:2002 contains recommended procedures for the assessment of the risk of:
a) surface condensation and mould growth; and
b) interstitial condensation.
D.2 Surface condensation and mould growth
BS EN ISO 13788:2002 contains a method for calculating the internal surface temperature of a building
component or building element below which mould growth is likely, given the internal temperature and
relative humidity. The method can also be used to assess the risk of other surface problems including
condensation.
Besides the external climate (air temperature and humidity) the method uses three parameters to
determine the risk of surface condensation and mould growth:
a) the “thermal quality” of each building envelope element, represented by thermal resistance, thermal
bridges, geometry and internal surface resistance. The thermal quality can be characterized by the
temperature factor at the internal surface;
Ú si – Ú e
f Rsi = ------------------Úi – Úe
(D.1)
where:
Úi
is the internal air temperature in degrees centigrade;
Úe is the external air temperature in degrees centigrade;
Úsi is the temperature of the internal surface in degrees centigrade.
The internal surface temperature at any point will depend on the nature of the structure, especially the
presence of any thermal bridges causing multidimensional heat flow, and most importantly, the value of
the internal surface resistance Rsi (see Table C.3).
b) the internal moisture supply (see Annex B);
c) the internal air temperature.
To avoid mould growth the relative humidity at the surface should not exceed 80 % for several days. The
principal steps in the design procedure are to determine for each month of the year:
i) the internal air humidity;
ii) the acceptable saturation vapour pressure psat at the surface, based on the required relative
humidity at the surface;
iii) a minimum surface temperature and hence the required fRsi of the building envelope.
!D.3 Interstitial condensation
D.3.1 Principle"
BS EN ISO 13788:2002 contains a method for establishing the annual moisture balance and calculating
the maximum amount of accumulated moisture due to interstitial condensation within a structural
element. The method should be regarded as an assessment tool, suitable for comparing different
constructions and assessing the effects of modifications rather than as an accurate prediction tool. It does
not provide an accurate prediction of moisture conditions within the structure under service conditions,
and is not suitable for calculation of drying out of built-in moisture.
In building elements such as cold roofs, where there is air flow through or within the element, the
calculated results can be very unreliable and great caution should be used when interpreting the results.
Starting with the first month in which any condensation is predicted, the monthly mean external
conditions are used to calculate the amount of condensation or evaporation in each of the twelve months of
a year. The accumulated mass of condensed water at the end of those months when condensation has
occurred is compared with the total evaporation during the rest of the year. One-dimensional, steady-state
conditions are assumed. Air movements through or within the building elements are not considered.
78
© BSI 23 December 2005
BS 5250:2002
Moisture transfer is assumed to be pure water vapour diffusion, and the thermal conductivity and the
thermal resistance are assumed constant and the specific heat capacity of the materials not relevant. Heat
sinks/sources due to phase changes are neglected.
Calculation methods according to this principle are often called “Glaser methods”. More advanced methods,
which are not currently standardized, are available and are described more fully in the references quoted
in the bibliography.
There are several sources of error caused by these simplifications.
a) The thermal conductivity depends on the moisture content, and heat is released/absorbed by
condensation/evaporation. This will change the temperature distribution and saturation values and
affect the amount of condensation/drying.
b) The use of constant material properties is an approximation.
c) Capillary suction and liquid moisture transfer occur in many materials and this can change the
moisture distribution.
d) Air movements through cracks or within air spaces can change the moisture distribution by moisture
convection. Rain or melting snow can also affect the moisture conditions.
e) The real boundary conditions are not constant over a month.
f) Most materials are at least to some extent hygroscopic and can absorb water vapour.
g) One-dimensional moisture transfer is assumed.
h) The effects of solar and long-wave radiation are neglected.
NOTE Due to the many sources of error, this calculation method is less suitable for building components in which there is significant
storage of water and which can experience large diurnal changes in temperature. Further guidance is given in a BRE Information
Paper. In any case, neglecting moisture transfer in the liquid phase normally results in an overestimate of the risk of interstitial
condensation.
!D.3.2 External climate data
The calculation method specified in BS EN ISO 13788:2002 requires monthly mean external temperature
and vapour pressure data. Monthly mean temperature data can be obtained for many locations relatively
easily, however vapour pressure data are much more difficult to obtain. One useful source is the CD ROM
“International Station Meteorological Climate Summary, Version 3.0” available from the US National
Climate Data Centre, which contains information for 43 UK stations and many more around the world.
Table D.1 summarizes the mean temperatures and relative humidities, calculated from the mean
temperature and vapour pressure, for London, Manchester and Edinburgh.
Table D.1 — Monthly mean temperature and relative humidity for interstitial condensation
calculations (1983–2002)
Heathrow (London)
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
4.9
4.7
6.9
8.8
12.6
15.7
17.9
17.6
14.9
11.2
7.6
5.9
Ringway (Manchester)
Turnhouse (Edinburgh)
T
RH
T
RH
T
RH
°C
%
°C
%
°C
%
84
82
77
71
69
68
68
70
75
81
84
86
4.2
4.1
5.8
7.8
11.3
16.1
16.1
15.8
13.3
10.3
6.7
5.2
83
80
76
71
68
72
72
74
77
81
82
84
3.5
3.7
5.3
7.0
9.9
14.7
14.7
14.4
12.1
9.2
5.8
4.3
83
81
78
75
75
76
76
78
80
82
83
84
"
© BSI 23 December 2005
79
BS 5250:2002
!Most climatic data, including that in Table D.1, is derived from long-term means, consequently the
actual climatic conditions will be worse than shown for half the years: constructions which just pass the
BS EN ISO 13788 method may fail in those years. To evaluate the risk of such failures analysis may be
re-run under more severe climatic conditions, which may be expected to recur within a stated interval. The
number of years N should be selected according to the sensitivity of the building to condensation: a ten year
period is appropriate for most buildings, whilst a twenty year period is appropriate for particularly
sensitive buildings such as computer centres. The environmental conditions are then determined by
applying the corrections in Table D.2 to the mean values for the selected location.
Table D.2 — Corrections to monthly mean temperatures and relative humidities
from a mean year to achieve condensation risk years with various return periods
Risk
1 in 5
1 in 10
1 in 20
–1
–1
–2
Temperature
Relative humidity
°C
%
+2
+4
+4
NOTE At present there are no standard methods for transforming data measured at airfield locations to buildings in city centres or
in distant locations or at different altitudes. The best possible method at present is to define a number of regions similar, to the degree
day regions, and specify a condensation risk year for each, with perhaps a correction of altitudes above 100 m.
Where there is air flow through or within a building element, the results of calculations using the methods
described in BS EN ISO 13788:2002 can be very unreliable and should be interpreted with great caution.
Moisture transport in pitched roofs with a large void above the insulation are dominated by airflows
(see 8.4.2.2). When designing roofs of that type the designer is advised to follow the recommendations of
this code. Further guidance is given in D.4.
D.4 Calculation of condensation risk in pitched roofs with a large void above the insulation
The method for calculating interstitial condensation risk recommended in D.3 and described in
BS EN ISO 13788:2002 takes account only of water vapour transport by diffusion and ignores the effects
of airflow. Also, the method uses monthly mean temperatures, which means it is unable to include the
effects of radiation to the night sky or of solar gain. Consequently, the calculation method in D.3 is
unsuitable for use for determining condensation risk for the roofs with large voids above the insulation
(see 8.4.2.2). Methods for calculating that risk will be described in a future BRE information paper.
Most UK buildings are heated in wintertime and the use and occupation of those buildings generates water
vapour at a pressure greater than external atmospheric pressure: that pressure difference tends to drive
moisture from inside to outside. The transport of water vapour through the building fabric is dominated by
air flows between the interior and any voids within the construction (such as the loft and batten spaces of
a pitched roof) and between those voids and outside air.
Some water vapour will be removed from the occupied spaces by ventilation and some will pass into the loft
by diffusion through the ceiling, but the bulk will be transported by air currents (convection) through gaps
and holes in the ceiling. Tests of typical dwellings indicate that transfer of moisture by diffusion accounts
for 25 % and by convection 75 % of the total. If a well sealed ceiling can be achieved, transfer by diffusion
and by convection will be roughly equal.
Water vapour may in turn be removed from the loft by diffusion through the underlay and/or by the
movement of air through unsealed gaps in the construction, laps in the underlay and low level ventilation
provided specifically to encourage air flow.
In a pitched roof with an HR underlay and 10 mm low level ventilation (as 8.4.2.2.2) vapour diffusion
through the underlay material is negligible compared to vapour transport by air flow through unsealed
gaps in the construction, laps in the underlay and ventilation slots.
Pitched roofs with a large void above the insulation are subject to diurnal temperature cycles, particularly
in clear, calm weather. On clear nights, radiation to the sky can cause the temperatures of the roof covering
and of the underlay to fall to several degrees below those of outside air and of air in the loft. Condensation
may then form on the underside of the covering and of the underlay. On clear days solar radiation can raise
the temperatures of the roof covering and of the underlay above the temperature of the loft by as much as
20 ºC to 30 ºC."
80
© BSI 23 December 2005
BS 5250:2002
Annex E (informative)
Vapour resistances: Conversion factors for unusual units
There are two ways of quoting vapour transfer properties: either as the property of a component of the
material (expressed either as resistance or as its inverse, permeance) or the property of a metre thickness
of that material (expressed either as resistivity or as its inverse, permeability). Throughout this standard,
the vapour resistance or resistivity of materials has been used since this is analogous with heat transfer;
the vapour transfer characteristics are expressed in the following units:
Vapour resistance
Vapour permeance
Vapour resistivity
Vapour permeability
MN·s/g
Èg/N·s
MN·s/g·m
Èg·m/N·s
(g/MN·s)
(g·m/MN·s)
When undertaking calculations for interstitial condensation, it is suggested that the values of vapour
resistance or resistivity quoted in the annexes are used. However, on occasion, it might be necessary to
convert values that have been quoted in other documents and which quote units different from those above.
In most documents, vapour transfer characteristics are likely to be expressed as permeances. The Table E.1
lists conversion factors for converting values of permeance to 4g/N·s. Subsequently, take the inverse to
obtain a value of resistance.
In addition, many references quote the vapour diffusion resistance factor, which is dimensionless. In order
to convert these to vapour resistivities, they should be multiplied by five.
Table E.1 — Factors for converting unusual permeance units to Èg/N·s
Unit as originally expressed
2
g/(cm ·s·mbar)
2
Multiplication factor
1 × 10
8
Lb/(ft2·h·atm)
9.681 × 10–2
1.339 × 10
gr/(ft2·h·mb)a
1.937
gr/(ft2·h·inHg)a (perm)
5.719 × 10–2
Temperate (75 % r.h.; 25 °C) g/(m2·24h) (BS 2972)
4.874 × 10–3
Tropical (90 % r.h.; 38 °C) g/(m2·24h) (BS 2972)
mg/(N·h)
1.942 × 10–3
g/(m ·24h·mmHg) (Metric perm)
a
2.78 × 10–4
gr is short for “grains”.
European units:
European standards (including BS EN ISO 13788) and manufacturers’ data sheets commonly quote
vapour resistivities or resistances in the form of the water vapour resistance factor È, or the equivalent air
layer thickness, sd. These are defined by:
¸a
È = ------ and s d = È ⋅ d
¸
where:
¸a
is the vapour permeability of still air;
¸
is the vapour permeability of the material;
d
is the thickness of a sample of material in m.
The permeability of air, ¸a, varies with temperature and atmospheric pressure (further details are
given in BS EN ISO 12572:2001). However, a value of 0.2 g·m/MN·s should be taken as typical of UK
conditions. Therefore to convert a È-value to a vapour resistivity in the units given in Table C.1, MN·s/g·m,
divide by 0.2.
Similarly, to convert a sd value into a vapour resistance in the units given in Table C.3, MN·s/g, divide
by 0.2.
© BSI 23 December 2005
81
BS 5250:2002
Bibliography
Standards publications
BS 2972:1989, Methods of test for inorganic thermal insulating materials.
BS 4016:1997, Specification for flexible building membranes (breather type).
BS 5268-5:1989, Structural use of timber — Code of practice for the preservative treatment of structural
timber.
BS 8207:1985, Code of practice for energy efficiency in buildings.
BS 8211-1:1988, Energy efficiency in buildings — Part 1: Code of practice for energy efficient refurbishment
of housing.
BS EN 12524, Building materials and products — Hygrothermal properties —Tabulated design values.
BS EN ISO 12572:2001, Hygrothermal performance of building materials and products —Determination of
water vapour transmission properties
Other publications
[1] BRE Digest 296. Timbers: their natural durability and resistance to preservative treatment. Garston:
BRE.
[2] BRE IP 17/01. Assessing the effect of thermal bridging at junctions and around openings. Garston: BRE.
August 2001.
[3] Good Practice Guide 174, Limiting thermal bridging in new dwellings. London: TSO.
[4] Good Practice Guide 183, Limiting thermal bridging in existing dwellings. London: TSO.
[5] DEFRA/DTLR Report: Limiting thermal bridging and air leakage: Robust construction details for
dwellings and similar buildings. London: TSO. October 2001.
[6] CIBSE Guide B1. Heating. Available from the Chartered Institution of Building Services Engineers,
Delta House, 222 Balham High Road, London SW12 9BS.
[7] CIBSE, Domestic heating — Design guide. Available from the Chartered Institution of Building Services
Engineers, Delta House, 222 Balham High Road, London SW12 9BS.
[8] British System Design Manual (Gas fired warm air heating). 1988. Available from Benn Publications
Limited, Sovereign Way, Tonbridge, Kent TN5 1RW.
[9] Good Practice Guide 302, Controls for domestic central heating and hot water — guidance for specifiers
and installers. London: TSO.
[10] CIBSE Guide B. Ventilation and air conditioning. Available from the Chartered Institution of Building
Services Engineers, Delta House, 222 Balham High Road, London SW12 9BS.
[11] CIBSE Guide J. Weather, solar and illuminance data. Available from the Chartered Institution of
Building Services Engineers, Delta House, 222 Balham High Road, London SW12 9BS.
[12] BRE IP 13/94. Passive stack ventilation systems: design and installation. Garston: BRE. July 1994.
[13] BRE Digest 398. Continuous mechanical ventilation in dwellings: design, installation and operation.
Garston: BRE.
[14] BRE Digest 139. Controlling moulds and lichens. Garston: BRE.
[15] CIBSE Guide A3. Thermal properties of building structures. Available from the Chartered Institution
of Building Services Engineers, Delta House, 222 Balham High Road, London SW12 9BS.
![16] BRE Report 262. Thermal insulation: avoiding risks. Garston: BRE.
[17] BRE Digest 245. Rising damp in walls: diagnosis and treatment. Garston: BRE.
[18] BRE Report 466. Understanding dampness — effect, diagnosis and remedies. Garston: BRE."
Further reading
BS 5720:1979, Code of practice for mechanical ventilation and air conditioning in buildings.
82
© BSI 23 December 2005
blank
BS 5250:2002
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