The Uf value of the window frame of different materials

The Uf value of a window frame depends heavily on the material and the nature of the profiles. Often neglected is the influence of the glazing and the corresponding Ug value on the window frame. The frame is less influenced by the type of glass than by the storage, the embedding and the profile shape in the frame.

Initial values ​​of the materials

There are five common material variants from which window frames are made. If no specific U-values ​​are determined, the following calculation values ​​are used as reference values:

  • Wood approx. 1.8 W / (m²K)
  • Wood with metal approx. 1.8 W / (m²K)
  • Aluminum approx. 1.3 W / (m²K)
  • Plastic approx. 2.2 W / (m²K)
  • Composite materials approx. 2.8 W / (m²K)

The U-value of a window frame is enhanced by many structural designs. Modern aluminum frames achieve Uf values ​​of up to 1.0 W / (m²K) thanks to a non-conductive insulation core, because this prevents heat and cold bridges. This corresponds roughly to plastic windows, which achieve better Uf values ​​through insulating profiles and cavity techniques.

Energetically, the question of whether an aluminum window or a plastic window is more advantageous, balanced. In terms of price, aluminum windows with the corresponding quality features are higher with the same thermal insulation of the window.

Wood or plastic

When asked between wooden or plastic windows, the wooden window has advantages. The low-conductive massive material has energy advantages. The frugality of wood to temperature differences is not given in any other material.

Some plastic window versions have a special profile structure, which can influence the Uf value in a mechanical way. These include system windows with integrated ventilation for heat recovery and built-in spacers.


Among all window types composite products offer the worst heat protection on windows. They often do not comply with the statutory provisions of the German Energy Saving Ordinance (EnEV) and are not eligible for support through the Kreditanstalt für Wiederaufbau (KfW).

Tips & Tricks

When buying windows, pay attention to the way the U-values ​​are calculated. Important is the fixed Ug value of the glazing and two difference values ​​of the Uf value, which is averaged to definitely assess the frame.

Video Board: Window UG calculation