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New Superglass Psi-value calculation tool

When considering insulating buildings, U-values are the commonly used unit of measurement, as they represent 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, for example where a floor meets an external wall, or a window interrupts a wall. 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.


To help housebuilders and SAP assessors calculate their Psi-values more accurately, Superglass has developed a new calculation service for building specifiers, which includes product-specific details for a range of common junction types.

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 will retain heat and therefore conserve valuable energy.

How the Superglass Psi-value calculator works

The Superglass Psi-value calculator includes a range of external wall and party wall details. Each junction includes an illustration to demonstrate the recommended Superglass Insulation product plus other components and detailing considerations.

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 technical-uk@tn-i.com 

Find out more about Superglass Insulation Ltd on their member profile page here

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