PSI Values & Thermal Bridging

Psi values are a measure of the thermal efficiency of a ‘junction’ within a building such as edges around windows and doors, or corners in walls, or between walls and floor, or between walls and roof, or changes in materials such as insulation laid between joists or rafters (insulation-timber-insulation-timber …) they are the ‘thermal weak points’ of a building where up to 20-30% of the heat will escape from. 

Every junction of a building needs to be accounted for when producing SAP and SBEM calculations and will have a Psi value given to it depending upon the type of junction, the length of the junction and the materials involved. The default Psi values used by SAP and SBEM in calculating the thermal efficiency of the building are poor therefore it is increasingly important to have the actual Psi Values calculated as they are almost certainly going to be better than the standard defaults build into the software (particularly if you have gone to the trouble and expense of adding additional insulation to these thermally critical areas).

We provide bespoke Psi Value calculations to measure the actual heat loss at junctions rather than relying upon standard default values, which can be critical in ensuring compliance with Part L of the Building Regulations. Using actual, calculated Psi Values can also reduce the cost of the build by focusing your effort and cost on the thermally sensitive areas rather than ‘over insulating’ all areas of the building in order to achieve compliance.