U value for sloped windows (roof windows)
As roof windows are installed in sloped constructions, the Uw value will be higher than for windows installed vertically. The convection in the gas between the glass panes is minimum for a vertical glazing, increases when the glazing starts sloping, and is at maximum with horizontal glazing. Convection also depends on the type of gas and cavity thickness. In general, the cavity is independent slope when the cavity thickness is around 10 mm or less. This has an effect on the energy performance of a building, since the heat loss through the roof window is increased due to the larger Uw value. On the other hand, the solar gain and daylight are also increased. Roof windows are also exposed to a larger part of the sky than facade windows and are normally installed without any constructive shadows, thus increasing the amount of daylight and solar gain, as seen in section 1.5.3.
Traditionally, the U value is the single parameter used for evaluating the energy performance of windows. It is common practice to declare Uw for roof windows at 90°, i.e. as facade windows.
Even though heat transmittance increases with increased slope, passive solar gains increase even more. So the vertical value leads to fairer indication of the performance than the sloped value. VELUX is striving to have the U value of windows replaced by energy balance (see section 5.5.3).