For starters, determine the type of window you can use in the space. The most airtight (and therefore, most efficient) windows on the market are inoperable windows—that is, they can’t be opened. They are ideal for passive solar gain, and should be used if there are enough other operable windows in the building.
If you install operable windows, consider purchasing hinged windows—casement, hopper, and awning windows. These windows open with the aid of a crank mechanism and shut very tightly against weatherstripping—provided they are well-made. Slider windows—single-hung, double-hung, and horizontal sliders—tend to be less airtight and aren’t generally advised for passive solar applications, unless you buy high-quality units.
For optimum performance, new windows should be as energy-efficient as possible. Look for low-emissivity (low-e) double-pane windows. These windows have a special transparent, heat-reflective coating to prevent heat from escaping in the winter and entering in the summer.
Windows used for passive solar gain should also have a high solar heat gain coefficient (SHGC). A rating from 0 to 1, SHGC indicates the amount of solar energy that passes through a window. The higher the number, the more solar heat the window allows to enter. If you are in a cold climate, buy windows with SHGC of 0.5 or higher. In slightly warmer climates, windows from 0.4 to 0.5 SHGC will work.
Most low-e coatings reduce solar heat gain coefficient considerably. However, a couple of window manufacturers have devised low-e coatings that don’t interfere with solar heat gain. One option is PPG Industries, which manufactures two low-e/high SHGC products: Sungate 500 and Sungate 100 window glass with SHGC of about 0.7. This glass is excellent in colder climates—where lots of solar gain is desired. However, PPG only manufactures window glass—not the framing unit. To obtain windows with their glass, you will need to contact a local window manufacturer to see if they will order PPG Sungate glass and install it in their frames. Another U.S.-based option is Serious Windows, which sell a “high” SHGC window, with values ranging from 0.39 to 0.5. Several Canadian companies—Thermotech, Accurate Dorwin, and Inline, for example—manufacture low-e windows with coatings that allow several different solar heat gain coefficients.
For best performance, south-facing windows should be argon-filled, have a condensation resistance of 0.5 or higher, visual transmittance of 0.5 or higher, and a U-factor of 0.3 or lower, (the lower, the better). Argon insulates glass. Visual transmittance is a measure of visible light entering the window (below 0.5 and the window appears dark). U-factor is a measure of heat movement through a window. It’s the reciprocal of R-value, so a window with an R-value of 3 has a U-factor of 0.33. Air leakages should be under 0.3, preferably about 0.1 cubic feet per minute per square foot. Since you will be dealing directly with the window manufacturer, if you insert Sungate window panes, be sure to stipulate these parameters.
South-facing windows live a difficult life. They are exposed to lots of sunlight, and choosing frame materials that resist expansion and contraction is smart. Wood-framed windows with an exterior cladding of aluminum to protect against the weather are generally good performers, as well as all-fiberglass units. Avoid aluminum-framed windows unless the manufacturer has insulated the frame, since uninsulated metal frames conduct heat out of the building in the winter and into the building during the summer. Buy windows with warm edge spacers, which consist of foam inserted between the panes of glass along the perimeter of the window. They reduce heat loss around the perimeter of a window, greatly decreasing heat loss in the winter and heat gain in the summer. Warm edge-spacers also reduce condensation, which damages wood sills and sashes (the wood that holds the panes of glass in place).