Window glass: which thermal transmittance values are there and which are useful?

If you want to replace your old window, the U-value of the new window glass naturally plays an important role. In this article you can read what you have to pay attention to and what you should pay attention to in addition to the U-value of the window glass, and how lower U-values are calculated.

Energy saving windows and passive house windows

According to EnEV, only windows with a maximum U-value of 1.3 W / (m²K) are allowed when replacing windows. Windows with values above 1.3 W / (m²K) may no longer be installed.

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Default values

Usual energy-saving windows have values between 0.9 W / (m²K) and 1.1 W / (m²K). This is not enough for passive houses. Windows with less than 0.8 W / (m²K) are required here, which are correspondingly more expensive.

Minimum values

The minimum values are reached at around 0.4 - 0.5 W / (m²K). A window cannot (currently) be made more heat-insulating. In order to achieve such values, special triple glazing is required, and the space between the glass is filled with krypton to achieve such a high level of thermal insulation.

Improve the U-value

One way of reducing the heat transfer coefficient in windows is with so-called heat protection films. They are simply glued to the window.

In most cases, thermal insulation films last around 7-10 years before they have to be replaced. The degrees of efficiency vary depending on the manufacturer, but individual films certainly manage to reduce the heat transfer value in 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 the window glazing.

Some of these films can also serve as sun protection films at the same time. They reduce the heating of the rooms by the sun. Since this is definitely desirable in winter in order to save heating costs, the use of combination foils should be carefully considered and calculated.

U-value and g-value

In addition to the U-value of the glazing, the so-called g-value, the total energy permeability, is also an important parameter for window glass. It indicates how much of the sun's radiation energy actually gets into the room.

Shading solutions such as roller blinds or blinds prevent the sun from shining in, but the heat from the sun often penetrates the room when the glass is heated up. A high g-value is advantageous in winter because the sun can effectively heat the rooms; in summer it is rather undesirable.

For passive houses, windows with a high g-value and low U-value are usually most beneficial. This makes the best use of the sun's rays to save heating energy or even make it superfluous.

Consider total value

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 total energy losses in the window.