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.




Get more information about the various U-values of Inoutic window systems!

U-Wert Fensterverglasung: 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.

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)


Get more information about the various U-values of Inoutic window systems!