Thermal Bridging (Psi-Value) Calculations
What Are Psi Values
Psi values represent the rate of heat loss through thermal junctions—the points where two or more building elements meet, such as where an external wall connects to a ground floor or roof. Unlike U-values, which measure heat loss through individual surfaces, Psi-values quantify the heat lost at these junctions.
These junctions are areas of non-repeating thermal bridging, which means they do not occur in a regular pattern (unlike repeating bridges such as wall studs). Despite their relatively small size, they can contribute significantly to overall heat loss—potentially accounting for up to 25% of the total fabric heat loss of a building if not properly addressed. Accurate Psi-value calculations are therefore vital for ensuring energy efficiency and thermal comfort.
Why Choose ARN Architectural
Why Do I Need a Psi-Value Calculation
Changes to Building Regulations and the Standard Assessment Procedure (SAP) used to assess energy performance are making Psi-value calculations increasingly important. The UK government is implementing stricter targets for carbon emissions and thermal performance as part of its efforts to improve building efficiency and sustainability.
Under the updated framework
Accredited Psi-values (previously supplied by government-approved sources) have been phased out, meaning designers can no longer rely on these pre-approved figures. Default Psi-values, while still available, are overly conservative and make it very difficult to meet regulatory targets. As a result, using bespoke, calculated Psi-values is becoming essential. These provide a more accurate and realistic representation of junction performance, allowing for greater design flexibility and helping to ensure compliance with increasingly stringent standards.
Who Can Carry Out a Psi-Value Calculation Competently
- Accredited thermal modellers
- Building services engineers
- Energy consultants
- Architects or building physicists with appropriate training
The calculations must be performed using approved thermal modelling software and in accordance with established standards—most notably BR 497: Conventions for Calculating Linear Thermal Transmittance and Temperature Factors and BS EN ISO 10211.
Due to the complexity and precision required, it’s important that Psi-values are calculated by someone with both technical expertise and experience in modelling construction details accurately.
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