Windows let in sunshine and allow you to see what’s going on outside. But sunlight coming through windows can also produce glare on computer screens and cause air-conditioning systems to work harder. The trick is to optimize windows to maximize energy performance and comfort while minimizing energy costs.
Many improvements have been made in window technology in the past few decades, with more on the way. New high-performance windows are much better at resisting heat gain and loss than older single-pane windows. But don’t assume you have to go out and buy new windows. Older windows can be improved by adding storm windows, shades, blinds, and shutters, inside and out.
How Windows Work
Windows and glass doors play an important role in the overall energy performance of your home. A single pane of clear glass is a pretty poor insulator, and most older windows let air in around their edges. Today, many technologies have improved window energy performance. But to admit light and keep your views clear, windows are usually much less insulating than most modern walls and ceilings. The most important difference between windows and the rest of your outer wall is that windows allow greater heat transfer and passage of light.
Window glass is often referred to as glazing. Thus, a window with a single layer of glass, or a single-pane window, may be called single glazed, and a window with two layers of glass, or a double-pane window, may be called double glazed.
Windows have three characteristics that you need to understand if you want to be satisfied with your window choices. These characteristics are
Visible transmittance (VT) indicates how much of the light that falls on a window is transmitted through it. A single-glazed clear window has a VT of about 0.6. A heavily tinted, multiglazed window may have a VT of 0.15 or lower.
Windows also transmit radiation we can’t see: shorter wavelengths in the ultraviolet (UV) part of the spectrum, and longer wavelengths in the infrared (IR) part. The sun’s visible and infrared radiation bring solar heat into your home. Ultraviolet radiation causes damage and fading, so you may want to control it as well.
The solar heat gain coefficient (SHGC) tells you the fraction of solar radiation that comes through a window to the indoors as heat gain, compared to how much solar radiation is striking that window from outside. An imagined window that transmits all of the available solar radiation has a SHGC of 1; one that transmits none has a SHGC of 0. Real windows fall somewhere in between. For example, in today’s marketplace, a single-glazed clear window with a relatively small frame may have a SHGC of 0.84, whereas a multiglazed window with special coatings and an inert gas between the panes may have a SHGC of 0.25 or less.
It’s crucial that you understand the implications of different VT and SHGC ratings if you want to select the best window for a given facade of your home.
During cold weather, windows with high insulation values are much warmer on their inside surface than are windows with low insulation values. This provides several benefits:
On hot summer days, well-insulated windows make your house more comfortable and reduce the need for air conditioning. This is particularly true of windows exposed to direct solar radiation that have a low SHGC. With well-insulated windows, you can feel comfortable at a higher thermostat setting—and if you’re in the market, you may be able to meet your reduced cooling needs with a smaller, less expensive cooling system.
The insulation value of walls and ceilings is often expressed as an R-value. (The R stands for resistance to heat flow.) Modern homes in much of North America have walls that are insulated to R-20 and attics that are insulated to at least R-40. By contrast, older single-glazed windows have an R-value of about 1. Double-glazed windows with special coatings may have an R-value of 3. Nowadays there are multiglazed superwindows with special coatings and inert-gas-filled cavities that have an R-value of 10 or more.
Window systems, however, are generally rated not by their resistance to heat flow but by the inverse: their thermal transmittance, known as U-factor. A material with an R-value of 10 has a thermal transmittance of 1/10, or 0.1. Thus, the lower the U-factor, the less heat transfer and the higher the thermal resistance.
You may also hear the term “U-value,” which represents the thermal transmittance at the center of the glass. You want to know the U-factor; this represents heat flow through the whole window, including the frame. The U-factor appears on window labels, so it provides the best basis for comparisons between units.
The core of a modern insulating window is the insulating glass unit (IGU)—a sealed assembly of two or more layers of glass with one or more coatings. These provide much better insulation than older single-pane windows, or even uncoated double-pane windows. Inert gas fills, such as argon or krypton, can be used to further improve insulation. Highly insulating windows, appropriate for very cold climates, may have three, and sometimes even four, layers of glass (or tightly suspended plastic film between layers of glass).
A low-e (for low emissivity) coating is another important component of the modern IGU. The coating consists of multiple layers of very thin metal or metal-oxide, deposited on the surface of one or more panes, which increase a window’s insulating properties. This film is so thin that it barely affects the view through the window.
One type of low-e coating admits the sun’s warmth for passive-solar heating while reducing heat loss from indoors to out. Compared to uncoated windows, windows with such a “high-gain” coating have a lower U-factor while maintaining a high SHGC and VT. This type of low-e coating is best suited for cooler climates or windows with good summer shading.
In hot climates where summer cooling is more valuable than passive-solar heating, a low-e coating designed to control solar heat gain may be more appropriate. By selectively blocking wavelengths the human eye can't see, the resulting window can reject more than 70% of the sun’s heat without sacrificing much of the daylight or view. Compared to a high–solar-gain low-e product, such a window will have a similar U-factor, a much lower SHGC, and a slightly lower VT.