Indoor Air Quality –
Relative Humidity In Dwellings
Mike Davies
University College London
Health & well-being for occupants:
Ventilation, Condensation & Internal Air Quality
Today
• The move to ‘tighter’ dwellings - why is control of
moisture important?
• (Surface) Relative Humidity - a key parameter
• How can surface RH be controlled?
• Development of guidelines
• Building Regulations
• Summary
The health benefits of tackling climate change
• Household energy use is an important
target for actions to tackle climate change
• Properly designed and implemented, such
actions could have major co-benefits for
public health
• To investigate, we examined the effect of
strategies to improve energy efficiency in
UK housing stock
3
Why is control of moisture important?
• Moisture gives rise to two types of problems:
– condensation on internal surfaces
– accumulation of moisture within a structure in areas where it may
cause corrosion of metal components, decay of timber based
components or reduction of the performance of insulants
• I will focus today on the first of these two issues
Why is control of moisture important?
• “Persistent dampness and microbial growth on interior
surfaces and in building structures should be avoided or
minimized, as they may lead to adverse health effects.”
• “Indicators of dampness and microbial
growth include the presence of
condensation on surfaces or in
structures, visible mould [and] perceived
mouldy odour….”
Why is control of moisture important?
• There are many potential moisture related problems e.g:
– Mould
– Bacteria
– Dust mites
• However ADF refers to mould so I’ll focus on mould
today. The ADF performance criterion for moisture is as
follows:
“There should be no visible mould on external walls
in a properly heated dwelling with typical moisture
generation.”
Terminology
•
•
•
•
•
•
Water vapour
Saturation
Dewpoint
Vapour pressure
Saturated vapour pressure
Relative humidity (RH)
• The RH of the ambient air determines how much moisture
will be absorbed by hygroscopic materials
• This in turn determines the risk of mould growth on
surfaces and the decay of materials
• Note that RH close to the surface of a material is the same
as ‘water activity’ which is often used to describe mould
growth conditions
CIBSE
How can surface RH be controlled?
To control RH, it is necessary to:
• limit moisture input to the building
• achieve high surface temperatures by
– providing more insulation
– limiting thermal bridges
– if necessary, increasing the heat input
• provide effective ventilation
How can surface RH be controlled?
The following factors should be taken into account in
design:
•Occupant activity and heating/ventilation regime
•Built-in water
How can surface RH be controlled?
• Thermal insulation
• Thermal bridging
• Thermal response
How can surface RH be controlled?
• Ventilation
• Low energy design – we must ensure that adequate
ventilation remains
Under what conditions will mould grow?
• RH
• Temperature
• Time
•
•
•
•
•
Nutrients
pH value
Substrate salt content
Light
Oxygen content
Under what conditions will mould grow?
• You do not need to have condensation for mould growth
to occur
Under what conditions will mould grow?
Clarke
Under what conditions will mould grow?
• The challenge is to devise guidance that captures
enough of the science but yet is straightforward enough
to be implemented
• That was our task for ADF 2010
Development of guidance
• Analysis of field data
• Laboratory work
Laboratory work
Mould species in study
Ulocladium consortiale
Penicillium chrysogenum
Aspergillus repens
Aspergillus versicolor
Cladosporium sphaerospermum
Aspergillus niger
Building Regulations
• So, what did we recommend for ADF 2010?
• We gave recommendations for RH limits in order to meet
the primary criterion of no mould on external walls
• The guidance for the ventilation systems given in ADF is
intended to ensure that these (and other) limits were met
Building Regulations
• For all dwellings (existing and new) the moisture criterion
is likely to be met if the average surface water activity of
the internal surfaces of external walls is less than the
values noted in table 1 during the heating season
Table 1. Surface Water Activity
Building Regulations
• However, as a guide, for new dwellings, the moisture
criteria in table 1 are likely to be met if the average
relative humidity in a room is less than the values noted
in table 2 during the heating season
Table 2. Room Air Relative Humidity
Summary (1)
• Moisture related problems are important
• With reduced infiltration we need to be careful to control
moisture carefully
Summary (2)
• However….it is not just moisture related issues that we
have to worry about - e.g:
•
•
•
•
•
PM2.5
Combustion products (e.g. CO and NO2)
Radon
ETS
VOCs
• Temperature - increased exposure to heat?
End
michael.davies@ucl.ac.uk