U-value window - what does it depend on?

The U-value of the windows is of great importance for the overall energy balance of the building. Sometimes, however, the details of U-values ​​are polished. How they are calculated correctly, which standard applies, and what you have to pay particular attention to, you can read here.

Meaning of the U-value

The heat transfer coefficient - also referred to as the U value - indicates the amount of heat loss through a window. The requirements of the EnEV also apply to the permissible U-values ​​of the windows. These conditions will certainly get worse in the coming years.

Also for obtaining subsidies (e.g., KfW promotion), it is necessary for the windows to have a U-value of at least 1.4 W / (m²K) or less. This value even applies to the renovation of listed buildings when funding is requested.

Correct calculation of the U-value

There are clear normative specifications for calculating the U-value of a window. They can be found in EN 10077. It requires that the following values ​​be used for the window calculation:

  • U-value of the glazing and
  • Area size of the glazing
  • U-value of the frame and
  • Area measure of the frame
  • the so-called thermal bridge loss coefficient at the edge of the pane
  • the thermal bridge loss coefficient at the installation edge

Only if all these values ​​are considered, the calculation is correct.


In some cases, sales figures often indicate only the U (g) value (the U-value of the glazing). This value alone is not meaningful.

Above all, the thermal bridge loss coefficient at the edge of the window can have a very high effect on the overall U value of a window and massively worsen it. This value is usually denoted by? G, in contrast to the heat transfer coefficient s of the installation rim, which is indicated by?
is specified. These two values ​​must therefore be taken into account.

The specification of the previously used k-values ​​of windows is no longer meaningful today.

Energy saving windows

For energy saving, the U-values ​​of the windows are definitely decisive. Special energy saving windows today bring U-values ​​up to 0.4 W / (m²K). Values ​​of less than 0.8 W / (m²K) are important for a passive house, but other requirements also apply to the windows.

Those who renew their windows and rely on special energy-saving windows can often obtain even higher subsidies for special energy saving windows with lower U-values. For the isolation of the windows, however, the subsidies are lower.

Effects on heating energy consumption

The effect of better insulated windows on heating energy consumption depends entirely on the particular building and local conditions.

As a guideline, however, it can be assumed that in the case of a house with 40 m² of window surface, lowering the U-value from 1.3 to 0.9 can already result in savings of up to 200 liters of heating oil per year.

In addition, with windows on the south, west and east side of a building above a U-value of 0.6 already clearly noticeable heat gains can be achieved by the sunlight, which are higher than the heat loss through the window. It is here under optimal conditions about 20 times more energy absorbed than lost.

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