In this article, you can find out what the U-values of windows are and which insulation values are common. In addition, how lowering the U-value affects heating costs and which insulation values are used for passive house windows.
Definition of the U-value
The U-value is the heat transfer coefficient of the window. The higher the U-value, the greater the heat loss. If the U-value is lower, the window is better insulated.
Why replacing windows makes sense to save on heating costs is explained further down in the article.
Other U-values for the window
For windows, the U-value of the glass (Ug) is not the only important factor. In addition to the glass, the U-value of the frame is also important.
In order to calculate the overall U-value (Uw) of a window, other values are also required:
Linear heat transfer coefficient of edge bonding
Size of the glazing area and size of the frame area
The perimeter of the inner edge of the frame profile (ratio of frame area to glass area)
All of these variables have an impact on the overall U-value of the window and must be taken into account.
In particular, the edge bond is an important variable that can have a significant impact on the overall U-value of the window. For this reason, spacers are also used. Their purpose is to improve the thermal insulation of the edge seal.
Typical U-values of different glasses
Depending on the thermal insulation quality of the window, the U-value can vary. The number and thickness of the glass panes and the filling determine the U-value.
Double insulating glass typically has a U-value of 1.1 W/(m²K)
Triple insulating glass has values up to 0.6 W/(m²K) (Passive House standard)
With krypton filler in the glass, U-values of up to 0.4 W/(m²K)
Old double-pane windows typically have U-values of 1.6 – 2.0 W/(m²K)
Old single-pane glass (without casement windows) even exceeds 2.4 W/(m²K)
The U-value of a well-insulated wall is 0.2 W/(m²K) and the U-value of a passive house wall is usually less than 0.15 W/(m²K). Even in the best case, much more heat is emitted through the windows than through the wall surfaces.
The use of thermal films can further improve the U-value of your windows. These films can also be used as sun protection for windows.
U-value of the windows and heating costs
In general, if the U-value is reduced by 0.1 W/(m²K) it means that approximately 1 liter of heating oil per square meter of window area can be saved per year. However, this is only a rough approximation.
For other types of heating, the savings vary accordingly. Replacing old windows with new ones can result in significant savings. With a window area of 40 m², replacing windows with a U-value of 1.8 W/(m²K) with energy-efficient windows of 0.85 W/(m²K) can save a lot of money per year on heating costs.