Conventions for calculating linear thermal transmittance and temperature factors
BRE is an independent, research-based consultancy, testing and training organisation, operating in the built environment and associated industries.
On 3 June 2016, BRE published Conventions for calculating linear thermal transmittance and temperature factors (BR 497 2nd edition), written by Tim Ward, Graeme Hannah and Chris Sanders.
Well-insulated buildings require sophisticated techniques to model heat loss caused by thermal bridging at wall, roof and floor junctions and around openings. Thermal bridging at these junctions can add significantly to fabric heat loss and cause localised reduction in internal surface temperatures, which can lead to surface condensation and mould problems. A more detailed calculation method for U-values, as defined in BS EN ISO 6946:2007, has been introduced to take account of these repeating thermal bridges.
The BRE guide provides the information needed to carry out these calculations so that different users of the same software package or users of different software packages can obtain consistent and reproducible results. It is intended for use by designers and numerical modellers.
For building regulation purposes two key modelling outputs are identified:
These outputs enable designers to assess junction details and develop novel solutions to improve thermal performance.
The second edition of the guide has been updated to include additional junction types and information about how they should be modelled, as well as worked examples giving the determined values of linear thermal transmittance and temperature factors.
The contents of the guide are:
- Numerical modelling.
- Thermal bridging at junctions.
- Junction types.
- Point thermal bridges, chi-values.
- Reporting of calculations.
- Appendix A: Detailed Input and Output from a numerical model.
- Appendix B: Worked examples with calculated values of psi and f.
 Related articles on Designing Buildings Wiki
- Air tightness in buildings.
- Cavity wall insulation.
- Computational fluid dynamics.
- Conventions for U-value calculations (2006 edition) BR 443.
- Floor insulation.
- Heat transfer.
- Insulation specification.
- Limiting fabric parameters.
- Roof insulation.
- Solid wall insulation.
- Standard Assessment Procedure SAP.
- Thermal admittance.
- Thermal bridge.
- Thermal mass.
- Thermographic survey.
- U value.
- U-value conventions in practice: Worked examples using BR 443.
Featured articles and news
Eleven Magazine announce the winner and runners-up in their Moontopia competition.
As January is the time for hitting the gym, Designing Buildings Wiki lists the best gym architecture in the world.
London is at the top of the list of global construction megacities, beating Dubai and Abu Dhabi.
What are the innovative business models of the future, and how to incentivise supply chains to work on a whole life basis?
One of the largest churches in the world, the monumental St. Peter's Basilica.
How thermal comfort is quantified and how it can affect wellbeing.
Snøhetta complete a treehouse cabin that allows guests to lie beneath the Northern Lights.
Christiania is an anarchist 'freetown' in Copenhagen where strange and experimental architecture has flourished.
“UK waste data needs improving” say BRE specialists, in this summary of their report into construction waste.
UandI announce new joint venture with US developer to work on office refurbishment projects.
Why buildings crack, how cracks are categorised and what can be done.