The Importance of Thermal Bridging (Psi-Values) and their effect on SAP Calculations

What is Thermal Bridging?

Thermal bridging sometimes called cold bridging occurs when heat escapes through weak points in a building’s insulation. These typically appear where different elements of the structure meet, such as walls to floors, roofs to walls, or around window sills.

This heat loss is measured using a Psi Value (Ψ-Value), expressed as the rate of heat loss per metre of junction. The principle is simple: lower Psi values mean better performance.

Why Thermal Bridging Matters

With today’s stricter regulations and improved insulation standards, thermal bridging has a much bigger influence on energy performance than in the past. Poor thermal detailing can:

• Create cold spots inside homes.
• Increase energy demand and heating bills.
• Allow condensation to form, leading to damp and mould.
• Cause long-term damage to finishes and even building structure.

From a compliance perspective, SAP 10.2 gives far more weight to Psi values. In short: good thermal bridging design makes the difference between passing and failing Part L.

How Thermal Bridges Affect SAP

Under SAP 10.2, new dwellings must meet three key criteria:

1. Dwelling Emissions Rate (DER): Measures the home’s carbon emissions.
2. Dwelling Primary Energy Rate (DPER): A newer metric tracking energy efficiency per square metre per year.
3. Dwelling Fabric Energy Efficiency (DFEE): Captures the performance of the building fabric U-values, airtightness, and Psi values all included.

Practical Example

At Energy Report Limited, we tested a typical house design using two different sets of Psi values:

• Recognised construction details (good practice)
• Default SAP values (worst case)

The outcome was stark:

• Ground floor perimeter insulation (1A): 8.88% variation
• Ground floor to wall junction (2A): 16.38% variation
• Eaves edges (3A): 3.13% variation

Switching from defaults to recognised details improved overall fabric efficiency by up to 50%. This lifted the property from a 76C to an 81B EPC rating without extra insulation or renewables.

The Bigger Picture

Heat moves like water: always looking for the easiest route out. Thermal bridges are exactly those weak points. Once heat escapes, energy demand rises, bills go up, and emissions increase.

• For developers: Poor Psi values can force costly add-ons to hit compliance.
• For homeowners: Cold spots and mould risk undermine comfort and health.
• For architects and consultants: Thermal bridging, if ignored early, can compromise designs and delay projects.

Why Act Early?

• Self-builders: Improve Psi values and enjoy a warmer, healthier, lower-cost home for years to come.
• Developers: One investment in bespoke junction details pays off across multiple plots.
• Architects & Consultants: Build in performance at concept stage and avoid compliance headaches.

Conclusion

Thermal bridging may be hidden in the details, but it plays a critical role in SAP performance, compliance, and long-term building quality.

At Energy Report Limited, we guide projects through Psi value optimisation, compliance testing, and practical advice. The result: lower costs, stronger ratings, and better outcomes for both clients and occupants.

Ready to see how Psi values could impact your project? Contact Energy Report Limited today.