No matter how you slice it, windows are giant holes in your building’s walls! But high performance windows mitigate energy loss through these glass-covered holes, without sacrificing the quality of light or aesthetics. There are three main design elements to high performance windows: insulation, coatings, and the frame.
A cut-away of a triple-glazed window with thin glass set in an insulated fiberglass frame (image courtesy of our partners at Alpen High Performance Products).
Insulated windows rely on old technology: the first insulated window was patented in 1865 by Thomas Stetson. An insulated window simply consists two or more panes of glass bound together by a seal around the edges. The seal keeps air between the panes, and the air slows the transfer of heat between the layers of glass. This insulation helps prevent heat transfer in winter, where the thermal energy naturally moves from the warm conditioned space to the colder exterior, and in the summer, where heat from the hotter environment moves to the cooler conditioned space through the window.
If there are two layers of glass with a single layer of insulating gas, the window is known as double-paned or double-glazed. If there are three layers of glass and two layers of insulating gas, it’s a triple-pane or triple-glazed window. The layers of glass may be different thicknesses.
Spacers are used between the glass layers to stabilize the glazing and keep the panes the correct distance apart. They also allow for thermal expansion and changes in pressure and help prevent moisture and gas leaks.
What’s new about insulated windows is what’s between the panes of glass. Instead of just air, most modern high performance windows contain argon. Argon is inert (unreactive) gas found in the atmosphere. It’s colorless, odorless, and non-toxic. Argon has a greater density than air, which makes it a better insulator. In fact, argon’s thermal conductivity is 67 percent less than air. A window’s thermal conductivity is described by its U-factor, or the rate at which a window (or door) conducts non-solar heat flow. The lower the U-factor, the better the insulator.
Argon isn’t the only gas available to insulate windows. Krypton and xenon, or blends of the two, provide an even lower U-factor than argon because they’re denser. But, the trade-off is cost: insulating with either of these gases cost more than argon.
In addition to insulated glass, high performance windows may have a low-e or low-emissivity coating. Emissivity is the measure of how much a surface emits thermal radiation. Low-e coatings are microscopically thin and minimize the amount of infrared and ultraviolet light transmitted through the glass. The amount of visible light is unchanged. The coating is typically a metal or metallic oxide that is applied directly to the surface of one or more of the layers of glass. Like insulating gas, low-e coatings reduce the U-factor of the window.
In the summer, low-e coatings keep interiors cooler by reflecting the heat off the exterior of the glass. And because low-e coatings also reflect ultraviolet light, furnishings fade less than with conventional window glass. In the winter, low-e windows reflect the interior radiant heat back into the building rather than transmitting it through the glass.
Low-e coatings at work in summer (image courtesy of our partners at Alpen High Performance Products).
Low-e coatings at work in winter (image courtesy of our partners at Alpen High Performance Products).
Spectrally selective coatings are a specialty low-e option. Spectrally selective coatings are optically designed to reflect specific wavelengths while allowing others to be transmitted. For example, a spectrally selective coating may reflect infrared heat from the solar spectrum. This gives the window a low solar heat gain coefficient (SHGC) as well as a low U-factor. The SHGC is the percentage of solar radiation emitted through the window.
The last piece of the high performance puzzle is the material that holds the glass in place—the window frame. The window frame provides structural stability to the window, resistance to rain and moisture, and like glass, have thermal properties. Window frames can be constructed of wood, fiberglass, vinyl, composites, or metal.
Wood frames are a traditional choice and provide good insulation.
Vinyl frames and fiberglass frames have air cavities that can be insulated to increase their energy efficiency.
Composite frames are made from a wood and polymer material. They offer a similar or better efficiency than wood frames.
Metal conducts heat very well, so metal frames are the least energy efficient frame material. Heat loss can be mitigated though through the addition of a thermal break—an insulating strip of plastic or other material placed between the frame and the sash.