U-value at the window - what should be considered?

Since the EnEV in Germany is in force, the U-values ​​of the windows are known to almost everyone. What you should be aware of and what you should know about U-values ​​in windows, you can read in this article in detail.

Different U-values ​​at the window

Confusion often causes the conflicting information in windows, as there are more than one U-value at the window. Whichever is meant here, is usually indicated by subsequent, subscript letters. If such a letter is missing, you can never say for sure which U-value is actually meant.

By default they are used.

  • the Ug value of the window
  • the Uw value of the window and the
  • the Uf value of the window

In addition, the so-called linear heat transfer coefficient? G plays an important role
a roll, as well as the g-value of the window. Most important for the practice, however, is the total heat transfer coefficient window (Uw).

Explanation of the values

Ug value

That's the U-value of the glazing. "G" stands for "glazing" in this case. The U-value of the glazing is not the total U-value of the window. That should not be confused. The Ug value only indicates how much heat escapes through the glazing of the window - but not through the entire window.

The U-value of the glazing can be massively changed by the construction of the window and by design details. This can mean either an improvement or a deterioration in the total value.

Uf value

The Uf value of the window is the heat transfer coefficient of the frame. This indicates how much heat is lost through the window frame (up to the glass pane).

Window frames must be insulated to minimize this heat loss. The better the insulation, the better the overall thermal insulation capacity of the window.

Uw value

The Uw value is the total value of the window. It describes how much heat in total escapes through the window construction. In this case, the "w" stands for "window". However, this value does not simply correspond to the sum of Uf and Ug value.

For the total heat value of the window, other parameters also play a role:

  • the window area (Ag) and the frame area (Af)
  • the ratio of glass surface to frame surface
  • the circumference of the glass edge as well as the
  • linear heat transfer coefficient of the edge bond (at the border between the pane and the frame)

All these parameters are calculated according to a standardized calculation formula to obtain the total U value of the window.

Linear heat transfer coefficient

The frame edge has a special meaning. Between glass and frame, a lot of heat may be lost. The amount of heat that escapes in this sensitive area is indicated by the linear heat transfer coefficient (? G). It can be positively influenced by special constructions (spacers).

Standard specifications and typical values

According to the EnEv, U-values ​​above 1.3 W / (m²K) are no longer allowed for new windows. Typical, 15-20-year-old double windows have a U-value of around 1.4 W / (m²K).

Energy saving windows are usually at 0.9 W / (m²K) - 1.1 W / (m²K). Below 0.8 W / (m²K) are already passive house windows. Minimum values ​​of 0.4 W / (m²K) are possible.

Video Board: Window U-value calculation