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Improve your SAP rating with our expert support
Psi values, also known as Ψ values, quantify heat loss where building elements meet, such as walls, floors, and roofs. These junctions are non-repeating thermal bridges, and the wireframe of the house can significantly impact your SAP score and overall energy efficiency. Psi value calculations are required for new build homes to demonstrate compliance with energy efficiency standards under Building Regulations. A key advantage is that calculated Psi values can be reused on subsequent sites that use the same construction details.
Crucially, accurate Psi value calculations are essential for ensuring the fabric efficiency of a building is compliant with Building Regulations. At Expert Energy Consultancy Ltd, we provide highly accurate Psi value calculations using industry-leading thermal modelling software. Whether designing to meet basic Building Regulations or low-carbon standards, our service helps reduce energy loss and achieve compliance.
Ready to improve your SAP rating? Contact us today for a free quote and expert advice tailored to your build.
What is a Psi value (Ψ-value)?
Psi values measure the rate of heat loss (in W/m·K) at linear junctions - areas where building elements like walls, floors, roofs, or windows connect. Common junctions include wall-to-floor, wall-to-roof, window reveals, and external corners. These areas can contribute significantly to a building’s heat loss when not properly designed or calculated.
Using default Psi values will almost always result in a building failing fabric efficiency requirements, making accurate Psi value calculations essential for achieving compliance with Building Regulations.
In SAP calculations, default Psi values are conservative and often overestimate energy loss. Replacing them with assessed values can dramatically improve your project’s energy performance and compliance with Part L of the Building Regulations.
Why do Psi values matter?
Psi values are crucial to improving the accuracy and efficiency of SAP calculations. Using default values can lead to a poor SAP score, increased design revisions, or even project delays. Calculated Psi values help:
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Reduce the DER/TER gap (energy performance targets)
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Improve your EPC rating
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Strengthen your fabric-first design strategy
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Support airtightness and U-value enhancements
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Default values lead to a poor fabric efficiency (rather than SAP score), which can be detrimental to compliance
Accurate Ψ-values are especially important in complex or high-performance builds, where precision can make or break compliance.
Our Psi value calculation service includes:
We offer a complete Ψ-value modelling service tailored to your build:
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2D thermal bridging analysis like THERM or Physibel
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Custom Ψ-value calculations based on your actual junction details
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Fully accredited reports accepted by Building Control and SAP assessors
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Advice on how to redesign junctions for better thermal performance
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Collaboration with your existing SAP, or design consultants
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Calculated Psi values can be reused on subsequent sites that use the same construction details
We make the process straightforward and ensure your results are submission-ready.
Who needs Psi value calculations?
You should consider Ψ-value modelling if:
✓ You’re using non-standard construction methods
✓ You want to improve SAP performance on a challenging plot
✓ Your development targets low-carbon credentials
✓ You want to reduce dependence on renewables to meet compliance
✓ You’re applying a fabric-first design approach and want to maximise efficiency
Whether you're a self-builder or part of a development team, calculated Psi values are a smart step towards sustainable, cost-effective design.
