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By Ronald Kovach
Windows & Exterior Walls Article Use Policy
Window film , coatings, and window shades are among today's many glazing choices, and some key metrics can help assess them.
The most common energy-related use of window film is to reduce cooling costs by cutting solar gain. At the Houston Hyatt, daytime temperatures on the window glass could reach as high as 125 F during the summer, according to a measurement and verification study for window film installed at the property. To examine the effectiveness of window film, sensors were installed in three pairs of adjacent rooms, according to Bill Pettit, commercial marketing supervisor at 3M. Solar control window film was installed in one room in each pair. The result was that daytime temperatures were 30 to 40 degrees F cooler in the rooms where the film was installed, according to the study. When film was installed on entire floors, the filmed floors consumed 22.4 percent, 23.2 percent, and 27.8 percent less energy than the unfilmed floors for the summer, fall, and winter, respectively.
Other glazing tools? Facility managers may want to consider a low-emissivity coating or film. Newer low-e window films greatly reduce the radiation of absorbed heat into an occupied space, says Steve DeBusk, global energy solutions manager at Eastman Chemical Co. “The heat is held in the glass and only a tiny fraction of the heat absorbed by the window and film can radiate into the room. At an emissivity of 0.07, only 7 percent of this absorbed heat radiates into the room in summer on a hot day. In winter, this low-e coating helps reflect far-infrared back into the room to help lower heating costs.”
The glazing toolbox also includes automated window shades on the inside of windows, or perhaps Venetian blinds designed into double-skin walls. And electronically tintable, or electrochromic, glass, where introducing a little electrical voltage creates a chemical reaction to tint the glass a lot or a little as needed, to block (or allow) heat gain and block glare, while still permitting a view.
Amid the array of glazing choices, facility managers can use several measures to assess a window’s energy performance. Some key metrics:
Solar heat gain coefficient: This number, between 0 and 1, measures how much of the sun’s energy striking a window is transmitted through the glass as heat. The lower the coefficient, the less solar heat it transmits into an interior space.
“There’s a big range of coefficients you can get in glass today,” Sanders says. “And low is not always good, and high is not always good. Because if you happen to be in summer, you want to block as much of the solar heat as you can, but in the winter you might quite like to have that coming into your building.
“And the tradeoff between whether you want a high or low coefficient not only depends on where you are in the country, or what climate zone, but what kind of building you’re in. If you’re in a high-rise building in Minnesota, even in the winter they are cooling 11 months of the year. In those situations you’d want a different kind of glass than if you were in a one- or two-story building in Minnesota, where in the winter you’re actually heating for many months.”
U-value: This metric, generally between 0.20 and 1.20, measures heat loss and how well a window insulates. The lower the number, the more a window restricts heat transfer.
Visible light transmittance: This number, between 0 and 1, measures how much visible light is transmitted through glass. The higher the number, the more light is allowed through the window.
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