U-values express the rate of heat transfer through any element of a building, such as a wall, roof or window. The units used to express U-values are watts per m² Kelvin (W/m²K). This means that if a roof had a U-value of 1.0 W/m²K, for every degree of temperature difference between the air on the surface inside the roof and the air on the surface outside, 1 watt of heat would pass through any m² of the roof.
The lower the U-value is, the better the insulation provided by the material. Read more