**What is a Psi-value?**

A Psi-value (also written as a ψ–value) is the measure of how much heat is expected to be lost through a particular junction. Unlike a U-value, which measures heat loss across the surface area of an element, Psi-values are calculated for the line of a junction – whether it be the joint between a wall and floor or around a window cavity. This heat loss is measured in Watts per metre Kelvin (W/mK).

The Psi-value calculation is complex and considers several factors, including heat flow simulation of the junction and the impact of U-values in the surrounding or adjoining building elements. The output value is multiplied across the total length of a junction to create a total junction Psi-value.

Under the latest Building regulations post June 2022 for all new Residential Building applications, if the thermal bridging calculations cannot be produced by the Insulation manufacturer for the exact build-up of materials used then Bespoke PSI / Thermal Bridging calculation must be used within the SAP calculation.

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