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Following the introduction of Building Regulations 2021, you have likely noticed an increase in inquiries from your SAP assessor about ‘PSI Values’. What exactly are they? Why is it important to understand them? How can a better understanding of these help to meet our goal of achieving Building Regulations compliance?

What Are PSI Values?  

PSI Values (Ψ), also known as ‘thermal bridging’, represent the heat losses across a linear length between different elements of construction, such as from the ground floor to an external wall. While U values account for heat loss across an area of a construction element, separate U Values (W/m²K) are applied to the ground floor and the external wall. The PSI value measures the heat lost between the junction of these two elements (W/mK). 

PSI Values will form an overall ‘Y-Value’ (W/m²K) which contributes to the overall fabric heat losses from the dwelling. Like U Values, where SAP assessments are concerned, the lower the better. 

Why am I hearing more about them now?  

Attention to detail within SAP has increased with each revision of the Building Regulations and the impact of thermal bridging has followed suit. The Building Regulations of 2006, for example, allowed for a “default” Y-value of 0.08 which was permissible by simply adhering to the use of Accredited Construction Details; a very broad and generalised set of details that applied to numerous construction types and variations. Fast forward to Building Regulations 2021 and the landscape has certainly changed:  

• “Default” Y-values near impossible to achieve Building Regulations compliance 

• Heavy focus on achieving a “Fabric First” approach with tougher targets for FEEs (Fabric Energy Efficiency) 

• Higher default PSI Values under SAP Table K1 encouraging use of more calculated values 

• Invalidation of older industry values, such as the accredited details, with later calculations focussed on more accurate results following BR497 v2.  

Where do PSI Values come from?  

Your SAP assessor will need to review the provided build specification as well as any available construction details to determine the applicable values. These can be obtained from the following resources:  

• SAP Table K1 – These are default values and represent any construction for a given junction. While these are flexible, the values are poor and generally require improvement to achieve Building Regulations compliance.  

• Industry Values – Non-government organisations have put together independently assessed thermal junction details, such as Recognised Construction Details, that cover a range of different constructions. These are freely available however the construction on site must reflect the details used to be valid.  

• Manufacturer Values – Manufacturers of various construction elements have seen the value of carrying out psi values and commissioned calculation of their own to accompany their specific products; adding increased value to opting with them to supply your development.  

• Independently assessed calculations – Where a psi value cannot be sourced for a junction from the previous options there is always the option of getting the junction calculated yourself. AES Sustainability Consultants Ltd can calculate these psi values for your project – See below for more details.  

Where do I see the value from these calculations?  

Getting psi values calculated for your bespoke junctions are certainly worthwhile and provide many benefits to the SAP calculations:  

• Improvements to thermal bridging will add benefit to all three main assessment criteria within SAP (Carbon, Primary Energy and Fabric Energy Efficiency). As a result, savings could be made in a variety of different areas following the calculations.  

• Calculation of specific junctions will provide an insight into the thermal performance of the detail and prompt improvements where applicable. When AES Sustainability Consultants Ltd calculate a psi value, demonstrating heat loss through the junction, we also provide confirmation of the temperature factor which determines whether a junction is likely to be at risk of accumulating surface condensation or mould growth.  

• Psi value calculations can be an investment rather than a cost. Once you’ve paid for a detail to be calculated it is yours. Provided the same detail is used on future schemes you can continue to claim the psi value that has been calculated – The more you use it, the less expensive it becomes. 

If you require PSI Value calculations for your projects, feel free to contact us at AES Sustainability Consultants Ltd. We will be happy to help provide you with a quotation for the necessary scope of works.