U-Value

A measure of the rate at which heat passes through a building element (wall, roof, floor, or window). Expressed in W/m²K (watts per square metre per degree Kelvin temperature difference): the lower the U-value, the better the thermal performance. Building regulations Part L sets maximum U-values for new extension elements - for example, 0.28 W/m²K for a new wall, 0.18 W/m²K for a roof, and 1.6 W/m²K for new windows.