Shutters at night

The U-value for high-performance windows (UW ^ 0.85 W/m2K) is more than a factor five worse than for opaque walls (Uwan ^ 0.15 W/m2K). Some experience with insulated shutters exists, typically about 4 cm thick and made of material with X = 0.04 W/mK. If these shutters are closed and air tight during the night, the remaining heat losses may be reduced significantly (Feist, 1995). A detraction is the need for the occupants to open and close the shutters at night and in the morning. The promise of thermal shutters has been greatly diminished by today's highly insulating glazing.

9.5.3 The frames of high-performance windows

The heat losses of a conventional frame with Uf = 1.5-2 W/m2K are about double the losses of a typical triple glazing (Ug = 0.7 W/m2K); hence, these thermal losses are considerably high compared to other components of the building. The fraction of the frame area to the glazing area is typically about 25 per cent to 40 per cent. Obviously, the frame insulating qualities are also important! The frames can be improved in several ways.

First, the frame can be made thicker to accommodate an insulation material. A depth of 70 mm, which is typical for standard window frames, is too thin even if a highly insulating material is used. Insulated window frames available on the market typically are about 100 mm to 120 mm thick. The most important steps to thermally optimize window frames are described in Schnieders (2000) and Kaufmann et al (2002). Recent developments are available at www.passivehouse.com.

It is instructive to study the isothermal lines and heat flow lines of the thermally optimized frame construction (Schnieders, 2000). The insulating layer should lead through the frame section from top to bottom as straight as possible and should not be interrupted. Separate local insulating inserts are not very effective. In a thermally optimized frame, the isothermal lines are as 'short' as possible. The optimized frame on the right in Figure 9.5.4 is thicker (120 mm), has a deeper glazing rebate (20 mm to 30 mm) and the glazing separators are reinforced polycarbonate instead of aluminium.

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