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 Photo courtesy Alpen Inc. This mountain home has three times the allowable glass square footage because the highly energy-efficient Alpen windowpanes enabled code compliance with much more glass.
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Home&Garden green builders corner
Windows play a big role in passive solar design and in a home’s energy efficiency. In summer, they can allow unwanted heat into the house; in winter, they can account for as much as 25 percent of a home’s total heat loss, equaling more than $20 billion in electricity costs nationwide. Although the cost of replacing faulty windows in an existing house is high, it’ll likely be well worth it and make a substantial difference in your home’s comfort level. Typically, upgraded windows also increase a home’s value and salability because of their energy efficiency and resistance to outdoor noise.
Windows can be constructed with one, two or three glass panes. Typically, the more panes, the better your windows will insulate. Good double-pane windows add to a home’s value because they insulate almost twice as well as single-pane windows. Better-quality double-pane windows have inert gases, such as argon or krypton, installed inside the air space to further slow heat transfer from the sun. In addition to lowering energy bills, multi-pane windows also make the whole house quieter and more comfortable in all seasons. Upgrading from single-pane windows to energy--efficient multi-pane windows can save up to 15 percent of your heating costs. Depending on where you live, this can add up to $100 per year and, over time, thousands of dollars in savings.
Virtually unnoticeable to the eye, low-emissivity (low-e) coatings are installed inside a double-pane’s air space. One coating helps prevent heat from escaping the glass in winter by reflecting it back inside. Another coating blocks summer heat from entering the home by reflecting sunlight out of the structure. In some areas, the 10- to 15-percent higher cost for low-e glass can pay for itself in a few years. Low-e coatings are now standard in many window manufacturers’ products. There are three types of low-e coatings to consider, depending upon your climate. High transmission, low-e windows are best suited for use in cold climates. Selective-transmission, low-e windows are ideal for homes in mixed climates that have both significant winter heating and summer cooling requirements. This window’s low-e qualities ensure winter performance by allowing sunlight to penetrate, while blocking most solar infrared to keep the house cooler in summer. Tinted, low-e glass provides glare control and high solar-heat rejection, which helps control solar gains in hot climates, especially on east- and west-facing windows.
SuperWindows (also known as Heat Mirror windows) incorporate super-insulating glass, with one or two suspended, coated plastic films between the inner and outer panes. This construction elevates the R-value performance to more than twice that of the best gas-filled, low-e window. The combination of suspended films, low-e glass and argon/krypton gas filling extends SuperWindows’ performance to R-15. SuperWindows also reduce glare by blocking 99.5 percent of the sun’s ultraviolet light and significantly reduce sound transmission.
Solar Heat-Gain Coefficient (SHGC) is another aspect to consider when shopping for energy-efficient windows. Depending upon a window’s orientation, you’ll want to choose a glass with either a high or low SHGC. On a home’s south side, where you want to enhance passive solar gain to naturally heat the house, choose windows with a high SHGC glass, which allows lots of sunlight to pass through the glass. On the west and east facades, use low SHGC windows to minimize sunlight and reduce heat gain on hot summer days. In most climates, the north-side solar heat gain is minimal, so SHGC values are not significant. Be sure to check a window’s visible light transmittance (often referred to as VLT), too, so that you don’t block solar heat at the expense of the light and view you want to achieve from a window. Work with your window supplier to choose the right combination for your climate. Finally, most window frames and sashes are made of wood, vinyl, fiberglass or aluminum. Wood, vinyl and fiberglass generally insulate better than aluminum or steel frames because they conduct less heat. Fortunately, one of the world’s leading manufacturers of energy-efficient windows, Alpen Inc., is based in Boulder. To learn more, visit their website at www.alpeninc.com or call 303-530-1150. With winter on the way, now is the time to consider energy-efficient windows for your home. Kim Master is senior associate at What’s Working Inc., a company that offers visionary solutions for green building, as well as co-author of Green Remodeling: Changing the World One Room at a Time. She’s also operations manager for The Boulder Green Building Guild (www.bgbg.org), an association of building professionals dedicated to promoting healthier, resource-efficient homes and workplaces. Go to top   
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