The ability of window systems to reduce the amount of energy consumed by a facility has improved greatly over the past few decades. Among the reasons why are better understanding of how window technology such as low-e coatings, warm-edge spacers, electrochromic glass, blinds, overhangs, and building orientation can be applied to cut energy consumption.
The clear glass windows of decades past often transmitted more heat into the facilities. To minimize this, architects and designers considered the orientation of a building — that is, how it was situated with respect to the sun — when designing new buildings. They also turned to shading, using overhangs or blinds to limit the amount of heat transmitted into a facility, Carmichael notes.
Given the limits of early window technology to minimize potential heat gain, the thinking was "the smaller the better," when it came to window size, say John Carmody, director of the Center for Sustainable Building Research at the University of Minnesota, and co-author of the book, "Window Systems for High Performance Buildings." One example: during the energy shortages of the 1970s, some schools boarded portions of their classroom windows.
Of course, that's hardly an ideal solution. Most people, whether they're working, studying, or undergoing treatment in a hospital, enjoy a view outside. In addition, by letting light in, windows can cut the amount of energy required to run artificial lighting systems.
Fortunately, window technology has progressed. One example is "low-e" or low emissivity coatings. These metallic coatings reflect infrared light, which "helps hold heat in during the winter and reflect it back during the summer," McGowan says. The window becomes a more effective insulator. At the same time, the coatings allow a reasonable amount of light through.
Warm edge spacers are another means of boosting the thermal performance of a window, Carmichael says. In any double-glazed window, spacers are used to separate the panes of glass. However, the metal spacers used early on could conduct heat, reducing the benefits of the double panes in the first place. More advanced spacers typically contain a material, such as silicone foam, that can reduce conductivity as well as the likelihood of condensation, Carmichael says.
One new technology is electrochromic, or electronically tintable glass. The tint can change, either automatically or on demand, from nearly clear to darker, in order to cut off heat and light transmittance. Electronically tinted glass allows either occupants or the building system itself to control the light level and heat gain from a window. However, the glass typically can't be used for a complete blackout, says David Cook, principal architect, structural and architectural evaluation with engineering firm CTL Group.
Another emerging technology is vacuum-filled glazing in use with double-paned windows. Instead of filling the space between two panes of glass with air or an inert gas, the air is extracted, creating a vacuum. This not only can limit heat transmission, but the window systems then require less space than traditional double- or tripled-pane windows. "It's still early and being developed," Carmody says.
Even as glass and window technology advances, a number of other factors can influence the energy efficiency of a window system, as well as the most appropriate window for a specific building. One is the climate. If you're in a colder part of the world, a modest amount of heat gain can provide an advantage by reducing the amount of energy needed to heat the facility.
The orientation of a window still impacts energy efficiency. For that reason, some designers will choose slightly different glass tints for different sides of a building, while keeping the colors similar enough that the changes aren't noticeable, Hughes says. So southern exposures may be slightly darker than northern ones. "You're still getting the same thermal performance."
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