0.18
New Build
W/m²K maximum
0.28
Extension
W/m²K maximum
0.30
Renovation
W/m²K maximum
Our AI calculator understands all common UK wall constructions and calculates U-values correctly
Traditional brick/block cavity construction with partial or full-fill insulation. Our calculator handles cavity closers and wall ties.
Typical U-value: 0.20 - 0.35 W/m²K
Timber stud walls with insulation between studs. Thermal bridging from studs calculated automatically using proportional area method.
Typical U-value: 0.15 - 0.25 W/m²K
Brick or stone walls with internal or external insulation. Common in period properties and retrofit projects.
Typical U-value: 0.25 - 0.35 W/m²K
Structural Insulated Panels with continuous insulation core. Minimal thermal bridging for excellent U-values.
Typical U-value: 0.12 - 0.18 W/m²K
The U-value of a wall is calculated using the methodology in BS EN ISO 6946 with conventions from BR 443. The basic formula is:
Where Rsi and Rse are the internal and external surface resistances (0.13 and 0.04 m²K/W for walls), and R₁ to Rₙ are the thermal resistances of each layer.
For walls with repeating thermal bridges (like timber studs), the calculation uses the combined method. Studs typically occupy 15% of the wall area at 400mm centres, or 12% at 600mm centres. The effective U-value accounts for heat flowing through both the insulation and the studs.
For new builds in the UK under Part L 2021, external walls should achieve a U-value of 0.18 W/m²K or better. For extensions, the target is 0.28 W/m²K, and for renovations, 0.30 W/m²K is acceptable. Lower U-values indicate better insulation performance.
To calculate cavity wall U-value, you need to account for all layers including the outer leaf, cavity (with or without insulation), inner leaf, and internal finishes. Each layer contributes its thermal resistance (R-value), and the total U-value is 1 divided by the sum of all R-values plus surface resistances.
An uninsulated cavity wall typically has a U-value around 1.5 W/m²K. With full-fill cavity insulation, this can be reduced to approximately 0.35-0.45 W/m²K. Modern cavity walls with PIR insulation can achieve U-values below 0.20 W/m²K.
Thermal bridging occurs where materials like timber studs or steel lintels create paths for heat to bypass insulation. In timber frame walls, studs typically occupy 15% of the wall area and have higher conductivity than insulation, increasing the overall U-value by 10-20%.