Window glass: which heat transfer values ​​are there, and which ones make sense?

Of course, if you want to swap your old windows, the U-value of the new window glasses will play an important role. What to pay attention to, and what to look for in addition to the U-value of the window glass, and how to calculate lower U-values, read in this post.

Energy-saving windows and passive house windows

According to the EnEV, only windows with a maximum U value of 1.3 W / (m²K) are permitted during window replacement. Windows with values ​​above 1.3 W / (m²K) may no longer be installed.


Usual energy saving windows have values ​​between 0.9 W / (m²K) and 1.1 W / (m²K). This is not enough for Passive Houses. Here windows with less than 0.8 W / (m²K) are required, which are also more expensive.

minimum values

The minimum values ​​are reached at around 0.4 - 0.5 W / (m²K). Heat-insulating a window (currently) can not be performed. In order to achieve such values, special triple glazing is required, with the glass space filled with krypton to achieve such high thermal insulation.

Improve U-value

One way to reduce the heat transfer coefficient at the window, so-called heat protection films. They are simply glued to the window.

In most cases, heat protection foils last about 7 to 10 years before they have to be renewed. The efficiencies vary depending on the manufacturer, but individual films are able to reduce the heat transfer value of window glass by up to 30 percent.

With costs of around 50 - 60 euros per square meter and easy installation, this is an interesting alternative to noticeably reduce the U-value of window glazing.

Some of these films can also serve as a sunscreen at the same time. They reduce the heating of the rooms by the sun. Since this is definitely desirable in the winter, to save on heating costs, it is important to carefully consider and calculate the use of combination foils.

U-value and g-value

In addition to the U-value of the glazing, the so-called g-value, the total energy permeability, is an important factor in window glass. It indicates how much of the sun's insolation energy actually enters the room.

Although shading solutions such as roller blinds or blinds prevent solar radiation, the warmth of the sun often penetrates into the room due to the heating up of the glass. A high g-value is beneficial in winter, because the sun can warm the rooms effectively, in summer it is rather undesirable.

For passive houses, high g-value and low-U windows are usually the most advantageous. This makes the best use of the solar radiation to save heating energy or even make it superfluous.

Take total value into account

The U-value of the glazing (Ug) alone is not the decisive criterion for windows. It is always about the total U-value of the window (Uw), which indicates the energy losses at the window altogether.

Video Board: Physics - Thermodynamics: Conduction: Heat Transfer (5 of 20) Double -Pane Window