U value for windows and aluminium frame

U value for windows and aluminium frame

U-value calculation for windows

In windows there are always three different U-values:

  • Uw (w = window) – overall value of the window
  • Ug (g = glazing) - U-value of the glazing
  • U(f = frame) – U-value of the frame

The heat transfer coefficient Uw relates to the entire window. This value also incorporates the U-values for the glazing and the frame Uf. The overall value Uw is also influenced by the linear heat transfer coefficient (g = glazing) and the size of the window.

U-Wert Fenster


U-value of window glazing: Ug

The Ug-value is a function of the type of gas filling of the intermediate space between the glass sheets, the distance between the sheets and the number of sheets.
Typical U-values for thermally insulated windows are:

  • Double insulated glazing 24 mm with argon filling: 1.1 W/m2K
  • Triple insulated glazing 36 mm with argon filling: 0.7 W/m2K
  • Triple insulated glazing 44 mm with argon filling: 0.6 W/m2K
  • Triple insulated glazing 36 mm with krypton filling: 0.5 W/m2K

U-value of window frame: Uf

The Uf-value for the frame-sash combination is defined by means of measurement or calculation. The area for the calculation of the Uw-value is the cross-section of the profile.

For details, please refer to How to calculate Uf value


Linear heat transfer coefficient Ψg

The value Ψg for the edge seal of the glazing is first and foremost a function of the used material for the insulated glazing spacer. The standard material with the worst thermal properties is Aluminium. Spacers with improved thermal insulation are referred to as “warm-edge” spacers. These spacers are made of stainless steel or plastic. A larger edge cover of the insulated glazing in the sash profile further enhances the Y-value of the edge seal.
Examples of Ψ-values:

  • Aluminium spacer: approx. 0.08 W/mK
  • „Warm edge" spacer: approx. 0,04 W/mK

U-value of window: Uw

The heat transfer coefficient for windows and window doors Uw is usually calculated in the standard window size 1.23 m x 1.48 m.
Important: the U-value worsens as the size decreases, larger windows feature better values. This is because U-values achieved in glazing are better than in the frame material and therefore a larger glass area is able to produce a better thermal insulation value.
Acc. to the EnEV 2009, with standard glazing the Uw-value is not to exceed 1.3 W/m2K. Windows with a U-value of 0.8 W/m2K or better meet the passive house standard.
Uw-value calculation 
The following formula is used to determine the heat transfer coefficient: 

Ug = heat transfer coefficient of the glazing 
Uf = heat transfer coefficient of the frame  
Ψg = linear heat transfer coefficient of the insulated glazing edge seal 
Ag = glass area
Af = frame area
Aw = Ag + Af
lg = length of inside edge of frame profile 
(or visible periphery of the glass sheet)