BR497 conventions for thermal bridging modelling for compliance with UK Building Regulations and the Passivhaus Standard

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In the UK, psi values from thermal bridging calculations are carried out differently to the standard used for Passivhaus: in the UK, internal building dimensions are used, while for Passivhaus external building dimensions are used. For the purposes of consistency for UK Regulation, all calculations must be carried out in accordance with BR497, ‘Conventions for calculating linear thermal transmittance and temperature factors’, (Ward. T, Sanders. C, BRE 2007), although it is accepted that external dimensions will be used for Passivhaus psi calculations. (UK calculations can be converted to external dimensions in PHPP as required). At present (March 2013) there is no ‘competent person scheme’ for numerical thermal modelling of junctions to demonstrate the competence of those carrying out thermal transmittance and temperature factor calculations. However, Regulatory bodies in the UK (e.g. Local Authority Building Control, Local Authority Planners) will expect to see evidence that psi values submitted in building energy calculations (using SAP or SBEM) will comply with BR497, which requires some junctions to be modelled in 3D tools to be appropriately represented. Further details are available at: