Psi-values: What are they and what’s changing?

U-value Calculator

Psi-values: What are they and what’s changing?

Psi-values are an important part of building design because they (along with U-values) are key to determining thermal efficiency. And with recent updates to Part L of the Building Regulations for England introducing greater scrutiny of as-built performance, now is a good time to get to grips with them. 

What is a Psi-value? 

Psi-values measure the heat loss properties of junctions where two elements meet, for example where a floor meets an external wall, or a window interrupts a wall. Psi-values are measured in watts per metre Kelvin (W/mK). 


How is a Psi-value different to a U-value? 

U-values measure the rate of heat lost through thermal elements of a building, such as a wall, roof or floor, as opposed to the junctions where they meet. U-values are measured in watts per square metre Kelvin (W/m2K). 


Both Psi-values and U-values measure the heat escape from a building, so both must be taken into account, when assessing its energy efficiency. 

What’s changed with the new Part L? 

To quantify the heat loss through junctions in a building’s design, Psi-values must be inputted into Standard Assessment Procedure (SAP) software.  

In the past, SAP assessors have had the option to produce these Psi-value calculations using the Government’s ‘approved’ Accredited Construction Details (ACDs) – a pre-set list of standardised details, based on different types of thermal element and junction. However, these became outdated and have been removed from the latest version of the SAP method. Assessors can still use non-government ACDs, as well as SAP’s default values, but the latter have been adjusted to be less favourable.   

The revisions encourage more accuracy, with less reliance on assumptions and default information. This can be best achieved by working with suppliers who provide Psi-value calculations based on up-to-date, technical information, specific to the products being specified.   

Your Next Steps

For more help and advice on how to calculate a Psi-value, contact our Technical Support Team. You can also use our Psi-value pattern book and free, online Psi-value calculator: The important thing to remember when it comes to Part L, is that you have options.  

Contact our Technical Support Team

Download our Psi-value Pattern Book Use Knauf Insulation's online Psi-value calculator