The U-value used in PSI calculations must represent the “clear field” thermal transmittance of the building element – essentially the performance of the construction if it continued uninterrupted without any junctions, corners, or discontinuities.
What makes it different:
When calculating a typical U-value for a cavity wall or similar construction, you’d normally apply the “combined method” from BR 443 (or the proportional area method from BS EN ISO 6946). This accounts for repeating thermal bridges such as mortar joints in masonry, timber studs in frame construction, or mechanical fixings penetrating insulation layers. The resulting value is a bridged U-value that incorporates these systematic thermal bridges.
For PSI calculations under BR 497, however, you require an “unbridged” U-value. This is calculated assuming homogeneous layers without accounting for repeating thermal bridges – effectively the thermal transmittance of the construction with perfect continuity of all materials.
Why this matters:
The PSI (Ψ) value quantifies exclusively the additional linear thermal transmittance attributable to the junction geometry and construction – where two or more building elements meet, change direction, or where the construction is interrupted. The fundamental calculation is:
Ψ = L₂D - Σ(U × l)
Where:
L₂D is the two-dimensional thermal coupling coefficient from finite element analysis
Σ(U × l) is the sum of the one-dimensional heat flows (unbridged U-value × length)
If you used bridged U-values in this calculation, you’d be accounting for repeating thermal bridges twice: once in the U-value and again in the 2D model. This would artificially reduce the calculated PSI value and misrepresent the junction’s thermal performance.
Using unbridged U-values ensures proper separation of:
Base fabric heat loss (one-dimensional flow through elements)
Repeating thermal bridges within elements (accounted for in bridged U-values for SAP)
Non-repeating thermal bridges at junctions (captured in PSI values)
In practice:
Thermal modelling software compliant with BR 497 will typically calculate and output unbridged U-values (for use in the PSI formula)
This methodology ensures each heat loss mechanism is accounted for once and only once in the overall fabric heat loss calculation, maintaining accuracy and avoiding double-counting of thermal bridge effects