What about thermal insulation for roof windows?
The glazing makes the difference
The current Thermal Insulation Ordinance (EnEV) prescribes a heat transfer coefficient (U-value) of at least 1.4 W/m²K for the entire residential roof window (U-w) for inhabited (heated) rooms. All roof windows approved for residential rooms meet or undercut this required value.
Depending on the type of glazing and the construction of the roof window, a U-w value (w=window) of up to 0.8 W/m²K is possible. Simply said, the lower the specified U-value, the better the thermal insulation properties.
U-value and more – Explanation of terms
The heat transfer coefficient measures heat transfer through a solid body (such as a wall) from one fluid (a gas or a liquid) to a second fluid due to a temperature difference between the fluids. In the case of a flat wall, it indicates the heat flow (heat energy per time) per area of the wall and Kelvin of the temperature difference between the two fluids. Therefore, its SI unit is W/(m²-K) (watts per square metre and Kelvin). As a rule, k (mainly in mechanical and process engineering) or U (mainly in construction) is used as the formula symbol.
U-Value = heat transfer coefficient
Ug (g=glazing) / value for glazing
Uw(w=window) / value for entire window
W/m²K = Watt per square metre and Kelvin