What Is a U-Value?
A U-value (thermal transmittance) measures how much heat passes through a building element such as a wall, floor, roof, or window. It is expressed in W/m²K (watts per square metre per degree Kelvin). The lower the U-value, the better the element is at preventing heat loss.
U-values are central to UK Building Regulations. Part L (Conservation of Fuel and Power) sets maximum U-values that walls, floors, roofs, and windows must achieve in new builds and extensions. For example, a new-build external wall must currently achieve a U-value of 0.18 W/m²K or better.
How U-Values Are Calculated
A U-value is calculated by adding up the thermal resistance (R-value) of every layer in the construction — bricks, blocks, insulation, plasterboard, air cavities, and surface resistances — then taking the reciprocal. In formula terms: U = 1 / (Rsi + R1 + R2 + ... + Rso), where Rsi and Rso are the internal and external surface resistances.
Why U-Values Matter
Better (lower) U-values mean less heat escapes through the building fabric, which reduces heating bills and carbon emissions. They also affect condensation risk — poorly insulated surfaces are more likely to suffer from condensation and mould growth on their interior face.
Typical UK U-Values
- Uninsulated solid brick wall: ~2.0 W/m²K
- Uninsulated cavity wall: ~1.5 W/m²K
- Insulated cavity wall (modern): ~0.18–0.25 W/m²K
- Double glazed window: ~1.4 W/m²K
- Triple glazed window: ~0.8 W/m²K
Related Calculators
- U-Value Calculator — calculate U-values for any wall, floor or roof build-up
- Insulation Thickness Calculator — determine the insulation depth to achieve your target U-value
- Heat Loss Calculator — calculate heat loss based on U-values