Thermal Bridging - Technical Detail

House Builder Knauf Insulation

Thermal Bridging

The difference between U-values and PSI-values 

U-values measure the rate of heat lost through thermal elements of a building, such as a wall, roof or floor. These are measured in watts per square metre Kelvin (W/m2K). 

PSI-values measure the rate of heat lost through junctions where two elements meet, for example where a floor meets an external wall, or a window interrupts a wall. PSI-values are measured in watts per metre Kelvin (W/mK). 

Both U-values and PSI-values must be taken into account, when assessing the energy efficiency of a building. The lower the U-values and PSI-values of a build, the better the building is detailed and will retain heat. 

How the ADL updates affect calculations 

In the past, SAP assessors have had the option to use ‘Accredited Construction Details’ to calculate PSI-values. These are standardised details, based on different types of thermal element and junction, which the assessor can select from a pre-set list (Annex K).  

Under the updated regulations, assessors will be required to produce PSI-value calculations using SAP 10.2 software, which no longer includes the option to use Accredited Construction Details. Instead, housebuilders and SAP assessors will need to work with suppliers who can provide up-to-date, technical information, specific to their products.  

Help is at hand 

To help housebuilders and SAP assessors calculate their PSI-values more accurately, we have developed a ‘PSI-value Pattern Book’, including product-specific details for a range of different junction types, and a PSI value calculator which can both be accessed below.