Regardless of the size or style of home being considered, there is no arguing that the typical American loves lots of windows. And as homeowners become better educated about the benefits of sustainable design, passive solar design often becomes a goal.
Regardless of whether you desire a passive solar or not, a key component to any energy-efficient home is the type of window frames and glazing you use. Windows today come in many types of thermal ratings, as well as various levels of light-transmittance. The type of glazing and frame you select can make or break your desire for an energy-efficient home.
For thermal comfort within your home, look for windows with the highest R-values you can find for the money. Window thermal performance is also referred to as “U-value”, which is the inverse of the R-value. Divide 1 by the R-value, and you have the U-value. An R-value of 3 would be a U-value of .33. Many window manufacturers provide both numbers, so the buyer should look for the highest R-value or the lowest U-value.
Typical double-glazed windows from the major U.S. manufacturers have R-values of anywhere from 2-3.5. Average triple-glazed windows garner ratings from 3.5 to 5, depending on the coatings used. Weathershield Windows, of Wisconsin, offer R-values of up to 7. Alpen Window Company, located in Colorado, has perhaps the highest thermal rating of American companies, with R-values up to 14. Some Canadian companies offer equally high thermal quality.
Window frames may be wood, metal, fiberglass, vinyl or combinations of these. Fiberglass tends to have the highest R-value, followed by vinyl, wood, and then metal. The selection of materials is, of course, determined by both their efficiency and the desired aesthetics for the home. In addition to R-value of the frame, however, a sustainably-minded homeowner should consider the sources of those materials to determine their impact on the environment. The lower R-values such as those found in vinyl windows can sometimes come at a significant cost to air quality and human and animal health.
Virge Temme
A passive solar home would typically have Low-E glass on the south side, and ideally would be equipped with spectrally selective glazing, which reduces heat gain in the summer and heat loss in the winter. If there are no awnings or other shading on the west wall of the house to reduce the heat gain and glare, the western windows could be ordered with a low solar heat gain coefficient (SHGC), perhaps combined with Low-E. Northern windows should be ordered with the highest R-value possible, as that is typically the coldest side of the house. The highest levels of coatings, however, can compromise the light transmittance and reduce the view somewhat; but you can balance the R-value with the percentage of visible light transmittance (VT), to provide you with the best possible scenario for your situation.
Different sides of the building usually are benefited by different types of glass. One size does not fit all. So before you order windows, sit down with a window expert or a passive solar consultant - or contact the Efficient Windows Collaborative in Minnesota — to determine the best combinations of windows for the size, location and site conditions of your home. Next to insulation, good windows are the second line of defense against high heating and cooling demands.