When Chris Senior was in college in the ’70s, he was convinced energy independence was just around the corner: windmills and solar panels would be everywhere, and we’d no longer all be plugged into the grid.
And from his anthropology and archaeology classes at Appalachian State University, he was learning that humans throughout history and prehistory had built to maximize the sun’s heat in the winter and ensure shade in the summer. “They knew it and we forgot it,” Senior says.
Today, his company AnchorageBuildingCorp.com builds exactly the type of house that would have excited his college-age self: passive houses. These super-green dwellings are designed with energy efficiency in mind. At his own passive residence in Chapel Hill, Senior explains, his water bill is higher than his power bill. A major element of this efficient and eco-friendly design philosophy is passive solar, which is a modern title for an idea as old as civilization.
“Using the energy in the sunlight to directly heat the building, it’s been done for millennia,” says Tommy Cleveland, a solar engineer with the NC Clean Energy Technology Center at N.C. State University. “A really nice example to see are the Native American cliff dwellings in the Southwest.” These were built in an extreme climate, Cleveland explains, that’s cold in the winter and hot in the summer. These ancient structures face south. If the windows faced any other direction, he says, they’d be colder in the winter and hotter in the summer.
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Building and window orientation are crucial to passive solar heating. To simplify, a south-facing window with an overhang receives sunlight and heat in the winter. The same window is shaded in the summer and cools the house. There’s more to it than that, naturally, but the core concept is that the house is designed to maximize the sun’s position in the sky, whatever the season. This approach doesn’t require active technology like photovoltaics — that is, solar panels — to work.
It can make a big difference. You can save between 10 and 75 percent on your heating bill, Cleveland says, depending on how aggressive a passive design your house has. Besides that, Senior says, about 40 percent of the electricity used in the U.S. is used by residential homes; uncompromisingly energy-efficient construction could make a big dent in that.
For all the financial and ecological reasons behind passive solar, Senior genuinely enjoys homes built this way. When a house faces the south – when it’s pointed at the equator, as he puts it – you can see the changes in the seasons in the position of the sun, and you can even track where on the floor its light will fall on the shortest day of the year. “It’s kind of neat to be square to the earth,” he says.
Senior and Cleveland shared some passive solar and efficient heating tips, ranging from the everyday to the ambitious.
Air sealing: “There’s a lot that can be done to homes,” says Cleveland. “Some of the simplest (changes) are air-sealing.” He suggests starting with the Department of Energy’s do-it-yourself air-sealing guide. Air-sealing, or reducing the amount of air that leaks in and out of your home by caulking and weatherstripping, is much easier than adding insulation, Cleveland points out. “That’s probably the lowest hanging fruit for improving energy efficiency in most homes,” he says.
Ductwork: About 20 years ago, scientists decided ductwork should be inside the house envelope, Senior says. In older designs, the ductwork is either in an attic or crawlspace—areas that are hot in the summer and cold in the winter. The heater and air conditioner work harder and waste a lot of energy in those temperatures. “Fighting is expensive,” Senior says. “If you can insulate the roof deck of your attic, it really helps improve the house.” He suggests getting a knowledgeable person to apply expanding foam insulation, and says this will make a significant difference.
Window placement: The sun is low in the sky and toward the south in the winter; in the summer, it’s higher in the sky and spends a lot of its time in the east or west. If you have south-facing windows with moderate overhang, this passively selects for letting the winter sun in and shading out the summer sun. “When designed correctly, you can get a large amount of sunshine coming in those windows in the wintertime,” Cleveland says.
The windows themselves: Changes in window technology have led from the familiar single or double-pane clear windows to triple-pane or thicker pane glasses with selective coatings. Some of these allow most wavelengths to come through, but block infrared wavelengths from leaving the warm spaces of the house.
“Most all of the windows sold around here have a low-E coating and it’s a type of coating that’s designed to minimize the amount of heat let in with the sunlight,” Cleveland says. These are fine for most windows, but if you have a true south-facing window (to within plus or minus 20 degrees) with good overhang, you may want to special order a window that lets in as much solar energy as possible. These aren’t necessarily more expensive, Cleveland notes, but are a specialty item.
Thermal mass: Sunlight comes in as radiation, and you want it to hit a dark surface where it will get absorbed. Simply painting a wall a dark color won’t work – it needs to be dense material that can store a lot of solar energy without dramatically changing its temperature. In the solar world, this is called thermal mass, and it typically has to be designed into the home.
“When sunlight hits a surface, it heats that surface, and the heat has to move back down into the material so that it can keep absorbing more energy,” Cleveland says. “It usually looks like concrete floors or brick floors – something thick and dense. That’s more important the more aggressive the passive solar design.”
Passive construction: Orientation is very important when it comes to actual passive construction, both the orientation of the windows and of the house itself. “You ideally would like the house oriented so its long axis runs east-west and not too much depth going south to north,” Cleveland says. This way, you maximize the benefit of your south-facing windows.
Ask a pro: There are professionals called home energy raters who will analyze a house and suggest cost-effective improvements, Cleveland says. Sometimes they can help you find a contractor, and sometimes they are contractors themselves.
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