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It's hardly news that windows have an impact on a building's energy use. But even experienced facility managers may be surprised to learn how big that impact can be. But by paying attention to key energy use metrics and new window technology, windows can be used to help maximize energy savings.
According to the Department of Energy's 2010 Buildings Energy Data Book, windows are responsible for about 5 percent of all energy used by buildings in the U.S. That means that inefficient windows can quickly lead to a big financial hit.
"Especially in commercial buildings which are large and have lots of windows, it imposes a tremendous heating and cooling cost," says Ray McGowan, senior program manager with the National Fenestration Rating Council (NFRC).
However, determining the energy efficiency of a particular window system can get quite involved. For starters, it's not just the glass itself that must be considered. "It's the system we're looking at: the window, the wall, flashings, the gaskets holding the window. They all play a factor in the efficiency of the window," says Dave Griffin, senior project manager with Erland Construction.
With that in mind, the NFRC's window rating system breaks windows into three components: glazing, spacer, and frame. A change in any one component can affect the overall energy efficiency of a window, the Council says.
The metrics used to measure a window's energy performance also can get more complex than they might seem initially, notes Bradley Carmichael, a project engineer with Hoffman Architects.
A case in point is U-Factor. "This represents the thermal transmission of heat from the inside to the outside of a building," McGowan says. The lower the number, the higher the performance of the window, which is critical in cooler areas. "Northern climates demand the best U-Factors," he adds.
However, the type — say, whether it's operable or not — and size of a window can influence the U-Factor, points out Frances Hughes, senior associate with architecture firm ADD. "There are almost infinite ratios of thermal loss."
Even within the same window, the U-Factor can vary. "The U-value at the center of the glass is different from the edge of the glass," Carmichael says. That's one reason the actual U-Factor achieved by an installed window system can differ from the number used in marketing a window.
Another reason is because the actual U-Factor can be influenced by elements that aren't part of the window system itself, such as the integrity of the walls around the window, or the ratio of windows to overall wall area. The more a real-life application deviates from the test parameters, the more likely that the performance values may, as well.
Along with the U-Factor, several other metrics are key to determining the energy efficiency of a window system. One is the solar heat gain coefficient, or SHGC. This refers to the fraction of solar radiation admitted through a window, either transmitted directly or absorbed, and then released as heat inside a facility, according to the U.S. Department of Energy. The lower the SHGC, the less solar heat the window transmits. However, a high SHGC rating also means a product is more effective at collecting solar heat during the winter — which can be a positive in more northern climates.
Another critical measurement is the Visible Transmittance, or VT. This is "the amount of light in the visible portion of the spectrum that passes through a glazing material," according to www.commercialwindows.org. It's presented as a number between zero and one. "In commercial buildings, if you're using daylighting to turn off the electric lights, VT is important," Carmody says. To reduce the electric light load, the windows need to let some light through, even if they're designed to also limit the amount of heat that is transmitted.
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