Psi-value calculations
for thermally efficient buildings

Are you losing heat through junctions?
We can now help, with our free Psi-value calculation service

When considering insulating buildings, U-values are the commonly used unit of measurement, as they represent the the thermal resistance of all the various layers that make up a construction.

Although U-value calculations take into account a number of factors, one aspect of heat loss they don’t consider is linear and geometric thermal bridges. These occur around openings such as windows and doors, or at junctions between the different building elements – providing a path of least resistance for heat to escape.

These areas are called thermal bridges and can account for a significant amount of heat loss from a property, particularly where the construction elements are insulated.

To determine how well a junction can prevent heat loss, we have to carry out additional calculations to generate what are known as Psi-values.



What is a Psi-value?

Unlike U-values, which represent surface area heat loss, Psi-values (also written as a ψ–value) are the measure of how much heat is expected to be lost through a particular junction. 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.

The Superglass Technical Services Team can carry out these calculations, consistent with the guidance set out in BRE document BR 497 – Conventions for calculating linear thermal transmittance and temperature factors (2nd edition).

For more information please email or Call our Technical Services team on 0808 1645 134