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What is a PSI value?

Completion & Post-Completion (RIBA 6-7)
Completion & Post-Completion (RIBA 6-7)

What is a PSI value?


A psi-value (also written as Ψ-Value) is a measurement of linear thermal heat transmittance, usually through the junctions of a building. Psi-values are measured in W/mK, showing the energy lost per metre between two thermal elements, for example, the join between a ground floor and an external wall. Psi-values are used to assess the overall thermal bridging performance of a building.

How to calculate a psi-value

A psi-value is calculated using specialist thermal modelling software, where the assessor replicates the construction junction from detailed drawings supplied by the Design Team. The model also includes the specific construction materials used. Whenever the materials or the design of the junction are changed, the psi-value will change also. A set of calculations are completed to measure the psi-values for all common junctions. These results are then entered into the SAP calculation as an alternative, better, approach than using thermal bridging worst-case defaults.

Why focus on thermal bridging

Thermal bridging is one of the most significant sources of heat loss within a building. A conductive material, such as metal, or a gap in the insulation can form a ‘bridge’ between the inside of a home and the outside world, where heat can escape and be wasted. The use of bespoke psi-value calculations is expected to generate better performance SAP results. Building junctions which are well designed and properly insulated can significantly reduce the amount of heat escaping through air gaps, which means less energy is wasted through the fabric, reducing fuel bills overall.

Air leakage and psi-values

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.

Psi-value in property development

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.

How can Energist help

Producing bespoke psi-values can really pay dividends if you regularly build to the same specifications. Once you have a set of Thermal Bridging Calculations, you can use them repeatedly on your next sites. As well as the standard calculation sheets, we also produce a plain English report which summarises the results and clearly indicates performance. For each junction assessed we will check both the thermal bridging heat losses and the surface condensation risks as standard. Should any junctions fall short of the target, we will include recommendations on how the thermal performance of the junction could be improved.

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