Table of revisions
Rev 1
Rev 2
Software tie-ins BuildDesk U value calculator added
ISO Standards added
BS EN 1745: 2002, BS EN 410: 1998, BS EN 673: 1998 added
Current problems arising when designing the Thermal Envelope
Heat Loss
 poor airtightness /draughts 15%
 walls 30%
 windows 20%
 roof 25%
 floors 10%
from Lambeth Council (UK) document Building For The Future
Thermal Bridging
The heat loss statistics above do not account for thermal bridging. Unfortunately I do not have
a complete data set for this, the only data that do I have is from Portugal, this notes that
thermal bridging in insulated Portuguese construction can be attributable for up to 15% of the
fabric heat losses. If this is true of other countries then thermal bridging accounts for a serious
quantity of energy that is being lost through poor detailing; hence the EN ISO 10211 Thermal
Bridging Pt1 + Pt2.
Another key issue is the condensation that can occur at thermal bridges, this can result in
mold growth and as a consequence have a negative impact upon the health of the buildings
occupants.
U Values
Prior to the current U value calculation method things were fairly straight forward i.e
determine the resistance of each material, simply add them up and divide into 1. The reason
the old calculation was superceded was due to its gross inaccuracy i.e. it was found that a U
value calculated using the old method a roof may achieve 0.2w/m2k however this bore little
reflection of the constructed reality. The revised calculations (and their ensuing complexity)
were developed to overcome this inaccuracy under the revised method the roof would only
achieve a U value of 0.35w/m2k. This means that the old method was inaccurate by as much
as 57% when compared to the revised method!
So its no surprise that the old method has been superceded and that the calculations have
been improved. As a consequence the calcs are based upon the permutations of the various
heat path that that are possible through a given wall. I don’t want to go into detail but believe
me when I say that it can get very complex very fast i.e. a traditional masonry cavity wall
(block/ cavity/ insulation/ block) has up to 16 heat paths in the current U vlue calculation the
resistance of each pathway has to be calculated and factored into the final U value
calculation. All thanks to BS EN ISO 6946 Building components and building elements! Now
I’m no mathematicial and this certainly gives me a headache!
ISO Standards
The new software would be developed so to help designers to address the following: U values: Thermal Performance of the Building Envelope
BS EN ISO 6946 Building components and building elements
BS EN ISO 10077 Pts 1+2: Thermal performance of windows, doors and shutters – Simplified
method
EN ISO 13370 (1998) - Thermal performance of buildings - Heat transfer via the ground Calculation methods
EN 673 (1997) - Glass in buildings - Determination of thermal transmittance (U-value) -
10.11.06
EN 13187 (1998) - Thermal performance of buildings - Qualitative detection of thermal
irregularities in building envelopes - Infrared method
prEN 13947 (2000) – Thermal performances of curtain walling - Calculation of thermal
transmittance - Simplified method
BR 443 Conventions for U-value calculations
BRE IP 10/02: Metal cladding: assessing the thermal performance of built-up systems which
use ‘Z’ spacers
BRE Digest 465: U-values for light steel-frame construction
SCI P312
BS EN 1745: 2002: Masonry and masonry products. Methods for determining design thermal
values (London: British Standards Institution) (2002)
BS EN 410: 1998: Glass in building. Determination of luminous and solar characteristics of
glazing (London: British Standards Institution) (1998)
BS EN 673: 1998: Glass in Building. Determination of thermal transmittance (U-value).
Calculation method. (London; British Standards Institution) (1998)
Ψ Values - Thermal Bridging
EN ISO 9346 (1996) - Thermal insulation - Mass transfer - Physical quantities and definitions
EN ISO 10456 (1999) - Thermal insulation - Building materials and products Determination of declared and design thermal values
EN ISO 13789 (1999) - Thermal performance of buildings - Transmission heat loss coefficient
- Calculation method
BS EN ISO 13370 Thermal performance of buildings - Heat transfer via the ground
BS EN ISO 10211 Thermal Bridging Pt1 + Pt2
EN ISO 14683 Thermal bridges in building constructions. Linear thermal transmittance.
Simplified methods and default values.
BRE IP 106 Assessing the effects of thermal bridging at junctions around openings
Condensation
BS EN ISO 5250 Control of condensation in buildings
BS EN ISO 12524:2000 Building materials and products: Hygrothermal properties
BS EN ISO 13788:2002 Hygrothermal Performance of building components and elements
prEN ISO 15927-1 (1999) - Hygrothermal performance of buildings - Climatic data - Part 1 :
Monthly means of single meteorological elements
Energy Use
prEN ISO 13790 (1999) - Thermal performance of buildings - Calculation of energy use for
heating
prEN 12831 (2002) - Heating systems in buildings - Method for the calculation of the design
heat load
CIBSE TM33 Tests for software accreditation and verification
BS EN ISO 13791: 2004: Thermal performance of buildings. Calculation of internal
temperatures in a room in summer without mechanical cooling. General criteria and validation
procedures
BS EN ISO 13792: 2004: Thermal performance of buildings. Calculation of internal
temperatures in a room in summer without mechanical cooling. Simplified methods(
prEN 15255: Thermal performance of buildings. Sensible room cooling load calculation.
General criteria and validation procedures
prEN 15265: Thermal performance of buildings. Calculation of energy use for space heating
and cooling. General criteria and validation procedures
Industry Standard Details:

Accredited Details for domestic construction only (supersedes the old Robust
Details)http://www.planningportal.gov.uk/england/professionals/en/1115314255826.html

Metal Cladding and Roofing Manufacturers Associate (MCRMA) have developed
guidance on the construction industrial/commercial buildings. www.mcrma.co.uk
Towards a Thermal Envelope Modelling System
Reflecting upon the increasingly rigorous design standards that are referenced in Building
Regulations Part L the text below seeks highlight a number of the issues that currently
concern building designers. Furthermore the documentation attempts to suggest a software
package that will assimilate these various design criteria bringing thus creating a powerful
design tool that will assist the designer in developing an understanding of their design whilst
also ensuring that good practice design is achieved with relative ease.
It is hoped that this document will encourage SQ1 to develop a software package that will
address these concerns and in doing so ensure that it continues to provide valuable design
software for architects in practice.
Software tie-ins
Ideally any new SQ1 software should seek to tie in with other software be it import, export or
utilising them as a plug-in somehow. Such software could include the following products:
 THERM is a state-of-the-art program that allows you to model two-dimensional heattransfer effects in building components such as windows, walls, foundations, roofs,
and doors; appliances; and other products where thermal bridges are of concern.
However it does not currently calculate Psi or Rsi values : -(
 WINDOW for analyzing the thermal and optical performance of windows (Required
plug-in for THERM).
 Radiance
 Energy Plus
 BuildDesk U value calculator http://www.builddesk.co.uk
 and all the usual suspects.
THERM is available from the following web site: http://windows.lbl.gov/software/
Ecotect and the modelling the Thermal Envelope Part 1
When it comes to modelling the thermal envelope a dream come true would be some form of
3D modelling system that could be linked into Ecotect. Before I describe a vision for this tool,
when it come to modelling the thermal envelope, I would like to examine the apparent
shortcomings of the current version of Ecotect (bearing in mind this is an impression from a
brief examination of the demo software only i.e. I am not yet a registered user, I apologise if I
have jumped to unfair conclusions about your software.)
Compared to the requirements of the BRE document IP 01/06 it would seem that Ecotect will
currently run into problems when modelling the thermal envelope. This is due to the fact that
there are gaps/voids between Zones (i.e. it is noted that there are the places that thermal
bridges are likely to occur i.e. wall/floor, wall/eaves interfaces).
I believe that by using an elementally based parametric tool for walls, floors and roofs
assessment this could be largely addressed.
NOTE: The current version of Ecotect could probably be adapted to address certain junctions
and openings by assigning thermal bridge characteristics to these various boundary
conditions i.e. at the head, jamb and cill of doors and windows.
SQ1 software and the modelling of U values
By integrating a U value calculator, or permitting the software to import of wall build ups/U
value calculations from the database of other U value calculator, parametric elements in the
form of wall, roof, floor, doors, windows and thermal bridge tools could all be assigned
thermal properties. Cold bridging calculations can then be conducted and it can be
determined whether the required Psi value is achieved if it is exceeded then the thermal
bridge detail will need have to be redesigned.
All modelling elements would be required to have a thickness so as to allow certain
dimensional properties to be calculated in accordance with IP 01/06 (similar to walls, floors
and roofs in ArchiCAD). The thermal bridge tool would operate much like a window/door (I
believe the Ecotect term is “child”) i.e. it is inserted into the master (parent?) element i.e. a
wall or roof.
In accordance with the intent of the building regulations many architectural details seek to
create thermal interruptions in any thermal bridges. As a result a sub-tool would be required
within the thermal bridge tool. This sub-tool would account for certain physical properties of
the thermal break and allow a design solution to be optimised i.e. where the insulation is best
located within the detail. (Reflecting on matters perhaps this sub-tool replicates aspects of the
U-value calculator; though it would also need to account for services penetrations, structural
fixings etc. that run transverse to the insulation place.)
Ecotect and the modelling the Thermal Envelope Part 2
As a result of the observations above an elemental 3D tool is required the (elemental tools
include walls, roofs, floors, doors, windows and thermal bridges/thermal breaks).
The need to model elementally the building has five functions:
1) To collate a project specific database of thermal bridges
2) To measure the linear length of the thermal bridges specific to the project
3) To allow the design of a thermal bridge free envelope i.e. to determine level of
insulation required to offset the negative losses caused by thermal bridges
4) To allow the collation of a non-project specific database of thermal bridging details
(as discussed in section 4.0, Note 4.)
5) To allow Zones to be exported to Ecotect (an thus avoid developing multiple models)
Though this could be imported from 3rd party software the SQ1 software could also allow:
1) U value calculations (walls floors and roofs)
2) Condensation risk calculations
Conclusion
A thermal envelope modelling tool that tackles the above as this would place designers in a
stronger position than they are at present, furthermore they would be able start to grapple
with issues that in the future will be of increasing import (i.e. as the energy consumption of
buildings falls cold bridges will become an increasingly significant proportion of a buildings
heat loss; hence voluntary standard of ψ 0.01 W/mK from PassivHausUK).
Over the course of my correspondence I hope that I have managed to give you a “practice
side view” of the issues that are being faced regarding modeling of the thermal properties of
buildings. Within this communication I hope to have successfully described the failings of
current thermal bridging software, demonstrated the need within the market for a suitable
product (and as a consequence wet your appetite on the matter developing a means for
modeling the thermal envelope) and demonstrated the need that such a piece of software
should be capable of integration, be it seamless or otherwise, with other modeling
environment software (such as Ecotect).