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THERMAL INSULATION (U-VALUE)
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.
SOLAR ENERGY TRANSMITTANCE (G-VALUE)
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.
DAYLIGHT TRANSMITTANCE
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.
Safety against wind load
Safety against wind load is stated from A1 to C3. Wind load is tested in two steps. Frame deformation is stated in class A-C where C is best. Indicated in combination with pressure class 1-3 depending on how hard the product has been tested under, where 3 is highest in Sweden. For class 3 it is 1200 pascals. After that, you finish with a safety test with 1.5x higher pressure (1800 pascals for class 3).
Water tightness
Rain density measures how well the product seals against rain. Windows are listed in class 1A-7A where 7A is best.
A means that the product has been tested in unshielded condition, B shielded. At pressures above 600 Pa, class Exxx is specified, eg E900.
Air permeability
The air tightness indicates how well the product withstands air pressure. Listed in class 0-4 where 4 is best. Both class 3 and 4 are tested below 600 pascals, but in class 4 a third less leakage is allowed.
