What does U-value mean?
Did you know that the K-value is still used extensively, but is an outdated term? The European standard is now the U-value. The U-value is calculated differently. For the window you have to differentiate between three different U-values: The U-value for the window as a whole (window frames and glass), the U-value for the glazing and the U-value for the window frames (no glass). Again, the smaller the value, the better the insulation.