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Windows bring views, comfort, ventilation, and energy efficiency to a home. Windows also bring large solar heat gains and kick the cooling system into overdrive, not to mention the damaging UV rays that fade furniture, draperies, and rugs. Whether a home is designed to fit your dream site or you are working with a builder to select the best options for a neighborhood home, increased energy efficiency and reduced heating and cooling costs can be achieved by carefully planning window placement.
Protecting against the sun makes big dollar sense. According to the U.S. Department of Energy, an average household spends over 40 percent of its total annual energy budget on heating and cooling costs. In South Florida, consumers may spend that much on cooling costs alone. These costs can be reduced by 15 percent on average just by switching to energy-efficient windows and planning for window placement, shading, and ventilation.
Which Way to Turn
First and foremost, homeowners need to pay attention to which direction their home is facing. The angle of the sun in the summer and the length of direct exposure are extreme on eastern- and westward-facing sides of the home.
California-based energy expert Steve Easley explains how to avoid that beating sun, all while understanding that no one wants a blank wall with no windows. “Orient your glass so that it faces south,” Easley says. “The sun is higher in the summer, so it hits the glass at less of a direct angle. Also avoid large amounts of glazing on the east and west faces where the sun will heat it at a direct angle for long periods of time.”
For a hot-climate home, battling the sun and controlling cooling costs are the critical focus for design and energy calculations. In some areas of the country, design reviews and approvals focus on a building's ability to control solar heat gain. Progressive review boards want to know the impact of solar heat gain on cooling loads, or how much energy it takes to reduce a home's temperature. These boards require strategies for reducing heat gain without overly consumptive energy use. Controlling energy use in hot climates means controlling heat gain.
“When we created the Palm Springs EnergyWise House, we designed first for the views,” says Dennis Cunningham, a builder at Palm Springs Modern Homes, “then we did the calculations to see if we could make it work. We were lucky with this project that the views were north-northwest. As a result, we don't get any beating sun.”
Avoiding Heat Buildup
Solar heat gain is the biggest energy enemy in homes throughout the country. Better to place picture windows, sliding doors, and soaring Palladian windows facing north and south than suffer the extreme heat gain large that windows looking east and west will bring. For a home in a northern climate, this approach has a secondary benefit in that south-facing windows will draw direct, home-warming rays during the winter. No matter where a house is located, good window planning focuses on controlling solar heat gain during the summer months. If that same control can bring added warmth in the heating season, consider it a bonus.
Easley uses a single-pane sliding-glass door as an example of how glass selection and placement can impact a home's cooling load and energy use. Direct sun on glass will generate 250 Btu per square foot of glazing an hour, the same unit of measurement used to gauge a furnace's output.
For an average 6-foot by 7-foot sliding glass door, that makes 10,000 Btu an hour from the door alone. It would take nearly one ton of air conditioning power to compensate for this heat increase. Since an average home uses three tons of air conditioning per hour, the slider would be using one third of that cooling energy per hour. Solar heat gain happens everywhere, Easley points out. “Even in the Midwest you can still have air conditioning bills that cost as much as your heating bill, so never underestimate the impact of your glass,” he says.
Take measures to reduce solar heat gain in the summer. First, create shade wherever possible and provide overhangs for windows that receive great amounts of direct light in the summer. “Out here we need to avoid the western beating sun,” Cunningham says. “The western sun here is such a glaring, heating, candle-melting sun that it burns up the furniture inside, fades it.”
To compensate, Cunningham and architect Dan Thornbury faced the EnergyWise House to the north, with its side angled to the northwest, which allowed the home to cast its own shade on the east- southeast side of the home. Thornbury placed a stair tower on the west-facing facade, which is peppered with a mix of various sized windows to provide teaser views of the mountains without added glass and the accompanying heat gain. A number of the small stair-tower windows are also operable and function to provide ventilating cross breezes when the pocket doors are thrown open to the east.
Another way to cut the solar heat gain from windows is to install high-efficiency coated windows that are designed to block heat transfer in the winter and the summer. Selecting a double-paned, low-e coated glass unit can save hundreds of dollars on yearly utility bills. Moving to a spectrally selective low-e glass in a double-paned insulated glass unit (IGU) will bring the greatest savings and efficiency of all. These windows have a microthin layer of metal that reflects heat back toward the source. Spectrally selective coatings deflect the rays that cause heat buildup and UV degradation, while allowing visible light to penetrate.
“On average standard glass lets in 90 percent of the sun's heat. A standard low-e glass lets in about 70 percent. A spectrally coated glass lets in just 40 percent of the sun's heat,” Easley says.
Blocking heat gain in the summer is a smart strategy. Easley encourages building cantilevers above western and east-facing facades to shade windows below. Plant shade trees, add overhangs or awnings, and use the National Fenestrations Rating Council label. The NFRC labels indicate how much light a given window will let in (VLT), how much heat loss it is likely to incur (U-factor), and how much solar heat gain it permits (SHGC). The Energy Star label additionally matches windows to their climates.