Windows are often seen as the weak point of a house. But even here there are big differences. By enclosing air chambers, the insulating capacity of a building material is increased. These serve as a heat buffer, so to speak. This inclusion happens, for example, with multi-glazed windows. A double or triple-glazed window has air buffers between the individual panes, which considerably reduce the heat transfer coefficient. A single-glazed window, for example, has a U-value of approximately 5.3 W/m2K, whereas a double-glazed window only has a U-value of 1.3 W/m2K. With a triple-glazed window, values of up to 0.8 W/m2K and less can even be achieved. In general, the U-value of windows depends on the frame material, the glazing and the tightness. The U-value of windows can be divided into three different heat transfer coefficients.

Uf
Heat transfer coefficient of the window frame only.
Uf = U frame – the U value of the window frame.
The Uf value is determined for the frame-sash combination.

Ug
Heat transfer coefficients of the glazing only.
Ug = U glazing – the U value of the glazing.
It depends on the number of panes, the distance between the panes and the type of glass filling of the space between the panes.

Up
Heat transfer coefficient for one panel (filling) only.

Uw
Heat transfer coefficients of the window component including glazing are usually calculated according to DIN 10077-1.
Uw = U window – the U value of the entire window It includes the values for the window frame and the glazing.
A low U-value for windows and doors is particularly important because it allows energy costs to be saved. If you pay attention to the different insulation values of materials and their optimum composition when you buy them, you can save yourself the annoying extra costs for heating later on.

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Heat transfer coefficients of the front door component including glazing etc.

Ucw
Heat transfer coefficients of the curtain wall component.