Windows can be called a home’s “eyes on the world,” but it would be just as accurate to call them a home’s “holes into the world.” So crucial are natural light and outside views to making a home livable that people once put up with what would otherwise be unthinkable—dedicating large amounts of wall area to openings that allowed in unwanted cold, heat, rain, noise, neighbors’ stares, and bugs.
From literal holes in the wall, Romans began substituting glass to help keep out the unwanted. For the next two thousand years, window technology improved little. While advances were made in how to open and close windows—such as the development of casement and double-hung windows—the barrier between the inside and outside remained a single sheet of glass. Operable windows had their own issue—because they never closed perfectly, air could seep in.
Over the last hundred years, huge strides in building science have been made. Many advances have centered on improvements in insulation—and windows have not been an exception. Now, double-, triple-, and even quadruple-pane windows are available. Reflective coatings that minimize heat transfer are common, and sashes that seal well when the window is closed are standard.
The result is a bewildering array of window options that require explanation. So whether you’re settled in an older home that could use an energy-efficiency upgrade or building your passive solar dream home, here’s a guide to choosing the best windows for your situation.
Window energy efficiency is denoted by a U-factor, which indicates the rate of heat transfer. A U-factor is the inverse of R-value—for example, a U-factor of 0.25 equals an R-value of 4. The lower the U-factor, the better. Because windows are complicated and transfer heat by different methods (especially the radiation through the glazing and the draftiness of opening windows), U-factor is more comprehensive than R-value, which indicates resistance to heat flow.
A window’s U-factor accounts for the whole window system and how it performs to block the transfer of energy between the interior and exterior. It encompasses the glass’s insulating properties, as well as the frame material and leakiness. What it doesn’t take into account is how well a unit is installed: poor installation can be a big energy drain (see “Installation Makes a Difference” sidebar).
Higher quality double-pane windows commonly achieve U-factors in the 0.20 to 0.35 range, or about R-5 to R-3. Triple-pane windows typically have U-factors below 0.20, with some top-of-the-line models (typically German brands) coming in as low as 0.10. Not surprisingly, efforts to reduce heat flow often decrease the amount of sun coming through, something measured by the solar heat gain coefficient (SHGC), represented on a scale of 0 to 1—the higher the value, the more sun that comes through. If you’re interested in using your window for passive solar heat gain, it can be a challenge to find a highly insulating window that also allows for substantial heat gain, which would correspond to a SHGC of at least 0.50.
In quality windows, the glazing is sealed and the space between is filled with clear, inert low-conductive gas such as argon and/or krypton, the latter being more insulating, but pricier. The gas between the glazing acts like any trapped air, but these two gases are thicker and move more slowly than air, conducting heat less rapidly.
Window heat loss is most substantial through the glazing. In multi-pane windows, it occurs through convection inside the panes and by radiation, from a warmer pane to a cooler pane. Low-emissivity (low-e) coatings, microscopically thin and transparent layers of metal or a metallic oxide applied to one of the panes, are used to slow radiation between panes—reflecting or absorbing radiated energy. A pyrolytic low-e coating, applied while the glass is still hot, results in a baked-on, “hard” coating that is very scratch-resistant. It is also used in single-pane applications like storm windows. Sputtered low-e coatings are applied in a series of sprays after the glazing has cooled. It is considered a “soft” application and almost always must be on a side of the glazing that faces toward one of the other panes so it does not get scratched. Some companies use a “solar-selective” low-e coating that provides a low solar heat gain coefficient to enhance comfort in warm climates.