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What is a PSI value?

21 Oct 2014


If we imagine a house, the U-Values take into account all of the areas where heat can escape. This includes the roof, external walls, windows and the ground floor. But this does not take into account where the various constructions meet.

For a house to be structurally safe there needs to be sections within the junctions of a building where the insulation is replaced with something more supportive. These points give heat a perfect place to escape. If you were to build the frame of a house using matchsticks, this symbolises the main, non-repeating thermal bridge areas. For example, where the wall meets the roof and the floors, window frames, corners… any other point where the insulation layer stops or changes.

A Ψ-Value measures how much heat can escape through these junctions. They are measured in watts per metre Kelvin (w/mK).

So where the U-Values measure heat losses through a given area, the Ψ-Values measure heat losses through a given length. There is also a Ψ or chi-Value which measures the heat loss of a specific point, but this is not something we need to worry about today! Most Ψ-Values for modern buildings are calculated to around 0.04 – 0.48. The biggest heat loss junctions tend to be around window frames. Ψ-Values are useful for improving SAP Calculations in the UK. Worst-case Ψ-Values are assumed in the assessment unless known otherwise – this would signify that a developer has not considered ways of reducing heat loss through the building junctions.

Developers can improve on this by building to freely available junction details (the most popular of which is Accredited Construction Details), or can choose to have bespoke calculations carried out. The SAP assessment is severely affected if the developer cannot prove that they have adopted some form of approved thermal bridging scheme. The calculation of Ψ-Values is far more complex than a standard U-Value, and requires specialists. Look out for our final blog in the series looking at Y-Values.

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