In buildings that use the sun as a heating source, the most important element of design is a southerly exposure, and windows are the most common form.

Window Position

Your windows must face south so the sun has maximum exposure into the house, although you will lose only about 10% of the potential solar radiation by placing solar windows up to 25 degrees to the east or west of true south.

Window Size

The size of your windows is very important: if they are too small your home will miss out on the full benefit of the sun, but windows can also be too big, leading to overheating, even in winter. The recommended size ranges for windows in passive solar homes vary according to climate and latitude. For Pittsburgh, whose average temperature in January is 27 degrees and whose latitude is 40.440 N, the recommended ratio is .25 square feet of window for every 1 square foot of space floor area. Therefore a 200-square-foot space will need 50 square feet of south-facing windows. This ratio will keep your space at an average temperature of 65-70 degrees during the day.

Window Alternatives

If your wall space is otherwise occupied, or if you’d like to mix things up a bit, try using skylights, Sola Tubes or clerestories throughout your home as well. The passive-solar-heating principles are similar. In fact, clerestories offer some benefits over windows in that, if they’re designed correctly, they can accept “low angle” winter sun while preventing “high angle” summer sun from entering and overheating your home.