EnergyWise House: Using Windows to Beat the Heat

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 northern climate home, this approach has a secondary benefit in that south-facing windows will draw direct, home-warming rays during the winter months. 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. Design Solutions The stair tower on this desert home brings snapshot views of the mountains and extra ventilation without large expanses of glass or unwanted heat buildup. 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," builder 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 façade, 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. Smart Windows 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. Auto Awnings The sun always rises in the east and sets in the west, but it sits higher in the sky in the summer. This means that its angle is more direct when it hits your windows. Days are longer in the summer, too. A hotter, more intense sun beats on your windows for more hours in the summer than in the winter. If you have windows facing west or east, the only solution is shade. Awnings that roll out to block the sun, cantilevers that cast shade from above, and trees that cast shade are the smart solutions for hot summer windows. With programmable settings, awnings can open or close automatically to meet your shading needs. Keeping windows cool means energy savings for you and your home. 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 NFRC (National Fenestrations Rating Council) label. These 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. Finally, consider coatings on window glazing to block heat gain and loss, and to reduce fading. "Low-e is now standard," says builder Cunningham of the California regulations. The next move is to buy spectrally selective low-e coatings, and argon or krypton gas fill to reduce condensation in northern climes. Energy-efficient windows work to keep unwanted heat out and to keep precious heat in, depending on the season. Heat energy always moves from high concentration to low, which is why windows draw heat from a room just like an ice cube draws heat out of a warm drink. Heat will always try to enter a cool house in the summer and escape from a warm house in the winter. Smart windows can block heat loss just as they block heat gain. The coatings on these specially treated glass units bounce heat back rather than letting it pass through. In cold weather spectrally coated glass works to radiate heat back into the room. So, a double-pane spectrally coated glass surface maintains a more constant temperature than single-pane glazing. As a result, it stays warmer and resists the heat transfer that is common to all window glazing. To maximize heating and cooling dollars, consider specially coated glazing in the most efficient window unit you can find. "Basically," Easley says, "buy the most energy-efficient product you can afford." It will provide savings in the long run. The EnergyWise House in an ongoing project that examines energy efficient building techniques, materials, and methods. For more information on the materials and companies participating in the project, see the EnergyWise House Video Resource Library.

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