All about glass > Thermal transmittance : U value

Heat loss is normally measured by the thermal transmittance or U value, calculated according to EN 673. The lower the U value, the greater the thermal insulation

If a glazing has a thermal transmittance U = 2 W/m².K
It will be necessary to produce 2 watts of heat per square meter to make up for 1 degree Kelvin difference between the inside and outside temperature.

Ug (g=glass) is the thermal transmittance of the glass expressed in  W/m2.K
Uw (w=window) is the thermal transmittance of the window expressed in W/m2.K
We will use U=Ug