What is a U-value?

Thermal transmittance, also known as U-value, is a measure of heat loss through the structure. It is measured in W/m2K. The lower the value, the better the insulation.

The UK Building Regulations require a maximum of 2.2 W/m2K in existing dwellings and an average of 1.2 - 1.7 W/m2K for new dwellings on rooflights.