72 Y O U R G R E E N H O M EFIGURE 6.7outh-facing windows transmit much more solar heat in the winter than they do in the summer because the summer sun is much higher in the sky, so more of the sunlight reflects off of it. East- and west-facing windows transmit much more solar heat in the summer than do south-facing windows. Shown here is data for 40 North latitude (Philadelphia, Denver.)energy. On the north, maximizing the insulation performance of windows is most important, since there is little direct solar gain to worry about. Some, but not all, window manufacturers offer different glazing options for their windows, although there may be an additional charge to order a non-standard glazing. Figure 6.7 shows the relative amount of sunlight transmitted through vertical, unshaded glass at different orientations throughout the year.SELECTING HEATING EQUIPMENTHow good a job you do with energy design largely determines how much energy your house is going to use for heating. If you go far enough with insulation and airtightness (often referred to as superinsulation), and if you incorporate some solar features (see Chapter 7), it may be possible to actually eliminate central heating systems, even in quite cold climates. It will certainly be possible to dramatically downsize heating equipment.
