Down to Earth Homes
Earth-sheltered homes cut heating and cooling bills, create weather- and noise-proof shelter and blend in with the landscaping, including comparing options, designing with dirt.
February/March 2003
By Dan Chiras
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The soil bermed around this home in Tempe, Arizona, helps the house keep its cool.
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Earth-sheltered homes fit a wide range of climates and a variety of building sites — even flat ones. Paired with passive-solar design, an earth-sheltered home can save you tens of thousands of dollars in fuel bills during your lifetime. Earth-sheltered homes are comfortable, affordable and energy efficient. And, if thoughtfully designed, earth-sheltered homes admit an abundance of natural light and are far less of an imposition on the landscape than conventional aboveground houses.
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SUBTERRANEAN SECRETS
Earth-sheltered homes provide year-round comfort, but not because earth is a good insulator. It's not. Soil has an insulation value of about 0.25 per inch: 14 times less than wet-blown cellulose insulation and 20 times less than certain types of rigid foam insulation.
The secret of earth-sheltered homes actually lies in the constancy of the earth's temperature and its thermal mass. Below the frost line (usually 24 to 60 inches below the surface), the soil maintains a fairly constant 50 degrees, plus or minus a little, depending on the location. The baseline temperature in an unheated, earth-sheltered home in a cold climate (like mine in Colorado, for instance), hovers around 50 degrees. Besides taking advantage of the soil's temperature-moderating effects, earth-sheltered homes also are protected from heat-robbing winter winds and the scorching summer sun. Because conventional aboveground homes expose most of their surface area to the elements, they are more vulnerable to temperature swings, conduction, convection and air infiltration. If outdoor temperatures plummet to 20 degrees below zero, for example, an aboveground home will need a boost of nearly 90 degrees to be comfortable. Raising an earth-sheltered home's internal temperature to the same temperature (around 70 degrees) requires only a modest, 20-degree boost, which is easily provided by passive-solar gain from sunlight. The sod's constant temperature, transferred to the home, also means that you never have to worry about water lines freezing.
In summer, earth-sheltered homes use the sod to keep their cool. It may be a blistering 95 degrees outside, but earth-sheltered homes stay in the low 70s — as cool as any air-conditioned home — without use of a noisy, energy-guzzling air conditioner and without astronomical utility bills.
Passive-solar, earth-sheltered homes boast 80 percent to 90 percent lower heating and cooling bills than conventional homes. "Even when passive solar is not incorporated," says Jay Scafe, President of Terra-Dome, an earth-sheltered home construction company, "fuel bills are frequently 50 percent lower than a conventional, aboveground home."
Typically built from concrete, rammed-earth tires or cement blocks, most earth-sheltered homes resist fire, termites, rodents, rot, earthquakes, wind, hail, hurricanes and tornados. Because of the weather-proofing, many insurance companies offer reduced rates.
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