Conventions for Calculating Linear Thermal Transmittance and Temperature Factors (BR 497)

Amongst other measures that address the need to reduce CO2 emissions, that of having better insulated buildings requires more sophisticated methods such as numerical modelling to calculate the heat loss caused by thermal bridging at wall, roof and floor junctions and around openings. This guide gives the information needed by designers and numerical modellers to carry out these calculations, so that users of the same software package or users of different software packages can obtain consistent, reproducible results. For building regulation purposes two key modelling outputs, temperature factor and linear thermal transmittance, are identified. These key outputs will enable designers to confirm the adequacy of particular junction details and help with the development of novel solutions to improve the thermal performance of junctions. This second edition has been comprehensively updated and includes: - additional junction types with guidance on how they should be modelled- 7 new worked examples giving the determined values of linear thermal transmittance and temperature factors.

1. Introduction. 2. Numerical modelling. 3. Thermal bridging at junctions. 4. Junction types - Roof junctions, Room-in-roof junctions, Junctions around openings, Corner junctions, Intermediate-floor or party wall junctions (with external wall), Exposed floors, Ground-floor junctions, Special-case junctions. 5. Point thermal bridges, chi-values. 6. Reporting of calculations. 7. References. Appendix A: Detailed Input and Output from a numerical model. Appendix B: Worked examples with calculated values of psi and f

Tim Ward, Senior Scientist, Thermal Performance, BRE Scotland, UK Graeme Hannah, Senior Consultant, Energy and Sustainable Development, BRE Scotland, UK Chris Sanders, Formerly Director, Centre for Research on Indoor Climate and Health, Glasgow Caledonian University, UK