The calculation of ψ-value(s) is carried out by using specialist software, able to perform 2D and 3D thermal modelling. A good working knowledge of BR 497: 2007 Conventions for calculating linear thermal transmittance and temperature factors is essential for performing modelling at an appropriate level. It is also essential to have a good working knowledge of calculating U value(s) of plain elements as well as an understanding of basic Building Physics principles.

Constructive Details Limited recommends the use of THERM and BISCO and/or TRISCO for modelling thermal bridges. These software packages have been used extensively in the construction industry and are in line with the requirements of BS EN ISO 10211: 2007.

Currently, a number of organisations, including the BBA are offering a modelling service for calculating ψ-value(s). If you want to know more about this thermal modelling service with a view to achieving likely beneficial and accurate psi-values, please contact the BBA on

When a product covered by a BBA certificate is also part of a Constructive Detail, this will be referred to within the body of the BBA certificate, under the Additional Information section. This has been implemented in BBA certificates covering aircrete blocks for the existing CDL handbooks. You can find the certificates relating to the APA products on the BBA website, using the product search function.