SAP 10: What you need to know about Psi Values

With the arrival of SAP 10 drawing ever closer, we are urging clients to prepare for the upcoming changes. Thermal bridging is one element of SAP Calculations that will be different going forwards as new legislation seeks to ensure that newly constructed dwellings are more energy efficient.

What are Psi Values?

Psi values measure linear heat loss across a junction between two thermal elements, for example, the line between a ground floor and an outer wall. They are calculated in W/mK.

They are an element of thermal bridging, which occurs where two adjoining materials exist within a construction (e.g. mortar and blocks) or where two constructions come together (e.g. wall/floor junction). Alongside Psi values, you may come across U-values which account for the heat loss across surface area.

Psi values are important because all SAP assessments must contain a thermal bridging component. If Psi values are not calculated manually, default values are assumed and these can be significantly worse than the true values, and will more than likely lead to non-compliance with the target emission ratings and target fabric energy efficiency.

What is changing in SAP 10?

In the current version of SAP, the heat loss assigned to all the junctions in a dwelling can come from a combination of sources:

  • A government approved Accredited Construction Detail (ACDs)
  • A calculated Psi value carried out by a thermal modeller trained in BR497
  • A default Psi value
  • A default Y-value of 0.15

To comply with the current Part L of the Building Regulations, a combination of ACDs and a calculated Psi value is often adopted as a means to reduce emissions and improve fabric performance.

In the new version of SAP, the option to use ACDs will be removed because they are now considered to be inaccurate and not fit for purpose. It will also become significantly more difficult to achieve compliance using assumed values. This will likely lead to a greater emphasis on calculated Psi values.

Want to know more about SAP 10?

Before June 15th arrives, we’ll be exploring more differences between the current version of SAP and SAP 10.

In the meantime, you might be interested in some other articles we’ve written on the subject:

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