Thermal Bridging Psi (Ψ) Calculations

Psi Values vs U-Values: What’s the Difference?

Clients often ask how a Psi value differs from a U-value, since both relate to heat loss. The difference matters because they measure two different things, and both feed into the same SAP or SBEM calculation.

A U-value tells you how well a flat section of wall, roof, or floor performs, measured in W/m²K, based on BS EN ISO 6946. A Psi value tells you how well a junction performs, where two elements meet, measured in W/mK, based on BR 497 and BS EN ISO 14683. Junctions are very often the weak point. On a typical UK dwelling, thermal bridging at junctions can account for 15 to 30% of total fabric heat loss, so getting Psi values right has a direct, measurable effect on the SAP or SBEM result.

Our Process for Accurate Psi Calculations

Thermal performance depends on details. That is where linear thermal bridging (Psi calculations or Ψ-values) play a key role. They measure heat loss at junctions. This includes wall-to-floor, roof edges, and window reveals. Our team follows a simple and reliable process to ensure accuracy.

We begin by reviewing your design drawings. This helps us to understand each junction in detail. We then build 2D or 3D thermal models in line with BR 497 and BS EN ISO 10211, the recognised standards for calculating thermal bridging Psi calculations in the UK.. These models follow approved standards and guidance. Every step is checked for compliance and precision.

What We Need to Get Started

To produce accurate Psi values for your junctions, we typically need:

1. Section drawings through each junction, showing all construction layers.
2. Material specifications, including insulation type, thickness, and thermal conductivity.
3. Confirmation of whether you want a standard detail or a bespoke junction modelled.

Haven’t finished the detailed drawings yet? That’s fine. We can work from sketch details and reasonable assumptions early on, then refine the figures once construction details are confirmed.

What Our Service Covers

• Detailed review of the construction drawings.
• Identification of all key junctions.
• Thermal models creation for each junction.
• Accurate calculation of thermal bridging PSI values.
• Reports are easy to read and comprehend.
• Guidance on thermal improvement.

Each report is structured to assist with compliance. It also helps your design team make better decisions early.

Why PSI Calculations Matter

Thermal bridging leads to heat loss. It can also cause condensation and mould. With proper thermal bridging PSI calculations, you can avoid these issues. This improves energy efficiency and thermal comfort.

Our approach keeps things simple. We use proven techniques and reliable industry-approved software. We are also aware of the current regulations, including the changes brought in under SAP10 and the withdrawal of ACDs. This ensures that your project is done to the required standard, first time, without relying on default values that can push a SAP calculation toward failure. This ensures that your project is done to the required standards.

Junction Types We Cover

Not all thermal bridges behave the same way, and the calculation method depends on the junction type. Our assessors work across the full range found in residential and non-domestic schemes:

Linear (non-repeating) bridges, found around windows and doors, at eaves, and where walls meet floors or roofs

Geometric bridges, found at external corners and other points where building planes meet, increase heat loss due to shape alone.

Support from Start to Finish

We work hand-in-hand with architects and builders. Our goal is to reduce risk and save time. We provide clear advice at every stage. If changes are needed, we guide you step by step.

Choosing the right team for thermal bridging Psi calculations can make a big difference. It supports compliance, improves performance, and adds value to your project.

FAQs (Frequently Asked Questions)

Q: What is a Psi value in simple terms?

A Psi value (Ψ-value) measures how much heat escapes along a junction, the line where two parts of a building meet, such as a window reveal or where a wall meets the floor. It’s expressed in W/mK and is fed into the SAP or SBEM calculation alongside U-values to give a full picture of fabric heat loss.

Q: Do I actually need Psi calculations, or can I use default values?

You can use default values, but since Accredited Construction Details were withdrawn under SAP10, the defaults are deliberately set high to discourage their use. On most modern builds, relying on defaults makes it very difficult to pass Part L. Calculated Psi values, based on your actual construction details, give a more favourable and more realistic result.

Q: How much do thermal bridging Psi calculations cost?

Pricing depends on the number of junctions and whether 2D or 3D modelling is required, since a bespoke 3D junction takes more time to model than a standard 2D linear bridge. Get in touch with your drawings and junction list, and we’ll provide a fixed quote before any work starts, so there are no surprises.

Q: What’s the difference between a Psi value and a U-value?

A U-value measures heat loss through a flat element like a wall or roof, in W/m²K. A Psi value measures heat loss along a junction, in W/mK. Both are needed for a complete SAP or SBEM calculation, but they apply to different parts of the building fabric.

Q: Can I still use Accredited Construction Details (ACDs)?

ACDs have been withdrawn and are no longer an accepted route to compliance under the current SAP methodology. If your construction happens to match a legacy ACD junction exactly, it carries no formal weight in current SAP runs. Calculated Psi values are now the standard approach for any junction not covered by a current default.

Q: How long does a Psi calculation take?

Turnaround depends on the number and complexity of junctions, but most residential projects with a typical set of junctions are turned around within a few working days once we have section drawings and material specifications. Let us know your programme, and we’ll confirm a realistic timescale upfront.

Q: What information should I send to get an accurate quote?

Section drawings showing each junction, your insulation specification (type, thickness, conductivity), and a note of which junctions need calculating. If you haven’t finished the details yet, send what you have. We can work from early sketches and refine the figures as the design develops.

Q: Will Psi calculations actually improve my SAP score?

In most cases, yes. Moving from default Psi values to calculated, project-specific values improves the Y-value, the overall thermal bridging heat loss figure used in SAP, which in turn helps the Fabric Energy Efficiency Standard and Dwelling Emission Rate. The scale of improvement depends on your specific junctions and construction type, so it varies project to project.