Psi Values & Thermal Bridging Explained
A guide to understanding heat loss at building junctions - what Psi values measure, why they matter, and how they affect your SAP rating.
What is a Thermal Bridge?
A thermal bridge (or cold bridge) is an area of a building envelope where heat flows more easily than through the surrounding construction. They typically occur at junctions where building elements meet - such as where a wall meets a floor, around windows, or at corners.
Why Thermal Bridges Matter
In well-insulated modern buildings, thermal bridges can account for 20-30% of total heat loss. They can also cause condensation and mould where cold surfaces meet warm, moist air.
What is a Psi (ψ) Value?
A Psi value (ψ-value, pronounced "sigh") measures the additional heat loss at a linear junction, beyond what the U-values of the connecting elements predict. It's measured in W/mK (watts per metre per Kelvin).
Key Principle: Lower Psi = Less Heat Loss
A junction with ψ = 0.05 loses half the heat of one with ψ = 0.10 per metre of junction length.
Common Junction Types
Building Regulations identify specific junction types, each with a reference code:
| Code | Junction Type | Typical ψ Range |
|---|---|---|
| E1-E5 | Wall-floor (ground floor) | 0.04-0.16 |
| E6-E7 | Wall-floor (intermediate) | 0.02-0.07 |
| E8-E9 | Corners | 0.02-0.09 |
| E10-E15 | Window/door reveals | 0.01-0.08 |
| E16-E17 | Eaves | 0.04-0.12 |
| E19-E24 | Party wall junctions | 0.02-0.12 |
Default vs Calculated Psi Values
Using Default Values
- Conservative assumptions
- Penalises SAP rating
- May need compensating measures
- Doesn't reflect actual design
Using Calculated Values
- Reflects actual details
- Typically improves SAP by 2-5 pts
- Can unlock compliance
- Demonstrates design quality
How Psi Values Affect Your SAP Rating
SAP uses a parameter called the "y-value" to account for thermal bridging. This is calculated from all the Psi values and junction lengths in your building. Better (lower) Psi values directly improve your SAP rating.
For a typical new build house, moving from default Psi values to calculated values can improve the SAP rating by 2-5 points - often the difference between meeting Part L or not.