The U-value is the rate of thermal insulation (also called thermal transmittance) and is expressed in Watts per square metre per Kelvin, or W/m²K. It refers to the quantity of heat that passes every second through one square metre of a structure 1 metre thick, where the air on 2 sides of the structure has a difference of 1 K in temperature.

The rule is: the lower the U value of the structural element is, the less heath it disperses. Therefore well-insulated products have a low thermal transmittance value.

Thermal insulation is expressed in W/m2K. If an entrance door has a thermal insulation value U< 1,68 W/m2K, this means that 1,68 Watt pass through the door every square metre of its surface for every K degree of difference between the internal and external temperature.