In windows there are always three different U-values:

• U_w (w = window) – overall value of the window
• U_g (g = glazing) - U-value of the glazing
• U_f (f = frame) – U-value of the frame

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

The U_g-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

The value Ψ_g 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

The heat transfer coefficient for windows and window doors U_w 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 U_w-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.
U_w-value calculation 
The following formula is used to determine the heat transfer coefficient: 

U_g = heat transfer coefficient of the glazing 
U_f = heat transfer coefficient of the frame  
Ψ_g = linear heat transfer coefficient of the insulated glazing edge seal 
A_g = glass area
A_f = frame area
A_w = A_g + A_f
l_g = length of inside edge of frame profile 
(or visible periphery of the glass sheet)