A measurement of insulating capacity
A window’s U-value is a good measurement of how its energy characteristics compare to those of other windows. The U-value specifies the product’s total thermal resistance and applies to the entire window, i.e. the combination of glazing, frame and sash together. The lower the U-value, the better its insulating capacity.
Easy to compare different windows
Since the purpose of energy rating is to enable rapid comparisons of windows from different manufacturers, the U-value is based on a standardised size. The dimensions used in the European product standard are 1230 x 1480 mm.
The U-value is described by the unit: W/m2K (watt per square metre per kelvin).
An outer wall in a modern house should have a U-value of 0.10 to 0.20 W/m2K.
A good energy window should be well under 1.0 W/m2K.