Thermal Bridge Modelling & Psi Values Explained

What is a Psi Value?

A Psi Value is a linear thermal bridging that occurs usually in the main building fabric and is a measure of heat loss expressed as W/m.k and is used within the SAP or SBEM calculation. A Psi Value is often referred to as a thermal model and are calculated in accordance with BRE BR 497 (conventions for calculating thermal linear transmittance and temperature factors). Below are a couple of example temperature graphics provied as part of a Psi Value calculation.

Timberframe Floor.PNG

What's the difference between a Psi Value & U Value?

A Psi Value is linear heat loss measurement which is measured in linear meters (m) and U-Value is a surface heat loss measured in meters squared (m²). Both measure heat loss through the building fabric but Psi Values usually occur at junctions around the building fabric and where diffrent materials abut or change direction.


Why calculate Psi Values?

1) The Building Research Establishment (BRE) estimate up to 30% of heat loss in buildings occur through thermal bridges, so for a building to be energy efficient these need to be reduced as much as possible. Typically a thermal bridge will result in a lower surface temperature which increase the risk of condensation, moisture and mould growth, all of which are recognised to cause harm to the building fabric and occupants within.

2) Currently Part L (2016) require all new "dwellings" to provide a SAP calculations and all "non-domestic" buildings to a SBEM calculation to demonstrate the building thermal performance and energy efficiency against the notional values set out with Part L of the building regulations. Within both calculations you have the choice of using default figures, accredited construction details (ACD's) or independently assess. For demonstration purposes the illustration below provides a comparison of the three options if all the linear heat losses were to be replaced with a hole in the building fabric. However, in 2022 the use of ACD's is being removed by the Building Regulations which will require designers and builders to provide bespoke Psi Value Calculations to avoid using default penalty figures within the SAP assessment. 


When should I get them calculated?

Psi Values should be calculated once a specification has been selected which will enable the designers and builder to check the proposed details are fit for purpose.


Obviously, should a proposed detail be preforming poorly our modelers can provide help and advice on how to improve the thermal performance of the detail.