- Project plans
- Project activities
- Legislation and standards
- Industry context
Last edited 17 Sep 2017
Conventions for calculating linear thermal transmittance and temperature factors
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:
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.
 Find out more
 Related articles on Designing Buildings Wiki
- Air tightness in buildings.
- Cavity wall insulation.
- Computational fluid dynamics.
- Computational fluid dynamics in building design: An introduction FB 69.
- 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
An architectural technologist in Germany.
3 World Trade Center designed by RSH+P
The struggle to achieve net-zero emissions by 2050.
What is 'agent of change' and who does it protect?
A consistent and measurable approach to home adaptation.
Acknowledging and challenging the realms and interpretations of heritage.
Embodied carbon in construction steel.
A prototype for assessing circularity in buildings.
New Wiki site is set to make BIM mainstream.
FMEA is a step-by-step approach for collecting knowledge about possible points of failure.
The various types and everything else.