The lower the U-value, the better the window

The U-value is a measurement of a building material's insulating capacity. It specifies how much heat passes through a square metre of the material at a given temperature. Unlike in many other contexts, the U-value improves the lower the figure. In other words, choosing energy windows with a low U-value is a smart idea if you want to save energy and have a pleasant indoor climate.

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.