A U-value measures how much heat passes through a building element, expressed in watts per square metre per degree of temperature difference (W/m²K). The lower the U-value, the less heat is lost, so insulation standards are written as maximum permitted U-values for walls, roofs, floors and windows.
The U-value is the rate of heat transfer through one square metre of a construction for each degree of temperature difference between inside and outside. It combines the thermal resistance of every layer in the build-up, plus the surface resistances on each face, into a single number.
Because it accounts for the whole assembly, a U-value calculation also has to allow for repeating thermal bridges such as timber studs, wall ties and mortar joints. This is why two walls with the same insulation thickness can have different U-values.