Roof windows

Skylights must be positioned to the exterior of the roof section (that is, in the plane of roofing tiles). This contradicts the previous section on advice for positioning windows in walls; indeed, the thermal bridge effects at the skylight edges are rather high. This, combined with the direct exposure to the cold night sky, results in roof glazing losing much heat. Some skylight products use double glazing together with a third hard low-e coating on the exterior surface of the outer glazing. The resulting Ug-value is about 1 W/m2K. Rooms with tilted windows need more heating power in winter and tend to overheat more in summer. Nevertheless, the daylight benefit for such rooms can be substantial. The overheating problem can be reduced with an effective solar shading, which should be on the exterior.

Source: G. Lude, ebok

Figure 9.5.12 Bevelled wall openings to increase solar gains; the aspect ratio in this example is as good as in standard walls without any thermal insulation

Double-sash windows (an outer and inner window) may achieve Ug and g-value equivalent or superior to triple glazing, but also at a high price. Here, as was done in old farmhouses, the inner window should be more air tight than the outer window to minimize condensation. The gap between the windows is a sheltered place for sun shading. This construction also provides good sound isolation. Some constructions are commercially available (see www.passivehouse.com).

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