FOUR REASONS
TO CHANGE WINDOWS RIGHT NOW

The U-value indicates how well a product insulates (keeps heat and cold out). The lower the U-value, the better the product insulates. What defines a good U-value depends entirely on where the window should be located. In old houses where walls and roofs leak a lot of energy, the U-value around 1.0 - 1.3 is enough to increase comfort indoors. If, on the other hand, you build a passive house that is to be self-sufficient, the U-value should fall below 0.80.

For windows, Swedish P-marking requires that the value be declared, thus no specific requirement for thermal insulation. SFDK requires ≤ 2.0W/m²K.

The G-value shows how much energy is transferred through direct solar radiation. The higher the value, the more energy and thus heating it generates (windows account for about 30% of heating in houses).

To think about here is how much heating you want through windows to the house. If you have large glass surfaces in dense newly built houses, it can be good to look at a lower G-value so that you do not have to cool down the house on hot sunny days. Also think about the location of the window in the latitude, and how much solar energy reaches the window.

The daylight transmittance is stated as a percentage and is a measure of the amount of daylight that enters through the window. A high daylight transmission is preferable, then it is experienced brighter.