| Junction Type | Default ψ | Good Practice ψ | Best Practice ψ |
|---|---|---|---|
| Wall/Ground Floor | 0.32 | 0.16 | 0.08 |
| Wall/Intermediate Floor | 0.14 | 0.07 | 0.02 |
| Wall/Roof (Ceiling Level) | 0.12 | 0.06 | 0.04 |
| Window Lintel | 0.50 | 0.30 | 0.10 |
| Window Sill | 0.10 | 0.04 | 0.02 |
| Window Jamb | 0.10 | 0.05 | 0.02 |
| External Corner | 0.15 | 0.09 | 0.04 |
Values in W/mK. Use manufacturer or calculated values where available.
Of fabric heat loss in well-insulated buildings
U-value increase from timber frame bridging
Reduction possible with good detailing
Psi (ψ) value is the linear thermal transmittance of a thermal bridge, measured in W/mK. It represents additional heat loss per metre length of the junction beyond what the U-value calculation predicts.
In well-insulated buildings, thermal bridges can account for 20-30% of total fabric heat loss. As U-values improve, the relative impact of thermal bridges increases significantly.
The y-value is the sum of all linear thermal bridges divided by the total floor area of the dwelling. It represents the additional heat loss due to thermal bridging per m² of floor area.
Key strategies include: continuous external insulation, thermal breaks at structural connections, careful detailing at junctions, and avoiding metal fixings through insulation. Accredited Construction Details (ACDs) provide proven solutions.