5.5.1 U value
The U value of a building component expresses the amount of energy transmitted from the warm side to the cold side. The lower the U value, the less energy is transmitted. It is often the aim to reduce the U value of building components in order to reduce the heat loss, and thereby the heating demand, of the building.
The U value is expressed in W/m2K. In glazing constructions, heat is transferred from the inside through the insulating glass unit to the outside by radiation, convection (warm air rises, cold air falls) and conduction. The U value for windows is denominated Uw and is a combination of the frame Uf value, the glazing Ug value and the cold bridge effect between glazing and frame, ψ. To reduce the convection loss inside the glazing cavity, the cavity can be filled with gas, e.g. argon or krypton. To reduce the radiation heat transfer, low emissivity coatings can be applied to the glass panes facing the cavity. Low emissivity coatings are thin layers of metal, invisible to the eye but with emissivity values down to almost 0. A standard glass pane has an emissivity of 0.84. By adding internal or external shading devices to the window, the U value can also be lowered by reducing the radiation to the sky and by improving the thermal resistance. The optimum cavity thickness is about 15 mm for argon and about 10 mm for krypton. VELUX roof windows are usually made with argon.
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.