Psi Value Calculations

Using Psi (Ψ) values in SAP

The Government’s Standard Assessment Procedure for Energy Rating of Dwellings (SAP) is due to be revised in 2019, following its last update in 2012.

SAP Calculations are used to demonstrate the energy performance of dwellings in the UK and are a key part of building regulations compliance. The SAP document defines exactly how the assessment works. In July 2018, the Department for Business Energy & Industrial Strategy (BEIS), in conjunction with the technical authors the Building Research Establishment (BRE), published the latest version of SAP, known as SAP 10.

Although the new methodology won’t come into play until Part L of the building regulations has been revised (likely to be later in 2019 or 2020), the information gives the industry a head start in preparing for the changes that will be made.

Removal of the ACD in Thermal Bridging

One major change in SAP 10 will be the withdrawal of The Accredited Construction Details (ACDs) in the calculation of default Psi values as they are now considered to be inaccurate based on current building practices.

Psi values are a measure of heat loss along a metre of junction between two elements where a material with a high thermal conductivity value (Lambda value λ (w/mK)) bridges a material with low thermal conductivity. An example would be the line between a ground floor and an external wall. These thermal bridges can account for up to 30% of heat loss in a dwelling, which has a significant impact on SAP assessment.

The withdrawal of the ACDs means that assessors will need to either use alternative schemes (possibly from the insulation manufacturers) or calculate bespoke heat loss values for each junction. Moving forward, planning requirements and future versions of the Building Regulations will require calculated Psi values as default values are either being removed or worsened.

The benefits of calculated Psi values

Thermal modelling, provided by C80 Solutions ensures that the bespoke Psi values of all thermal bridging lengths can be measured. Our specialist software delivers a 2D and 3D analysis which allows us to identify, assess and rectify areas of thermal bridging in construction detailing. This means we can make recommendations on how to reduce a building’s carbon emissions, lower energy and construction costs and ultimately ensure that dwellings are SAP 10 compliant.

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