Passive Annual Heat Storage: Improving the Design of Earth Sheltered Homes

Image Gallery This geodesic dome is a new type of earth sheltered home. PHOTOS BY THE AUTHOR

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Are you pooped out from paying the power people to pump heat into your home all winter, only to pay them again to pump it back out all summer? If so, maybe it's time to open a special sort of back-to-the-land savings account—one that will let you make energy deposits all summer and withdrawals in the winter. And just where do you put six months of intense seasonal sunshine for safekeeping? To find the answer, you only have to look down, because you're standing on the bank!

As you know, the earth exchanges heat constantly, soaking it up from the sun all summer and giving it up to the atmosphere in the winter. In most areas, this annual flux doesn't level off until a depth of about 20 feet is reached—where the year-round temperature hovers near the average annual air temperature. A 20-foot depth of earth, then, can be a mighty big savings account, and it's dirt cheap. However, to open such an account, you've got to figure out how to make deposits and withdrawals, and you ought to find a way to keep the vault secure from robbers.

PASSIVE ANNUAL HEAT STORAGE (PAHS)

What I'm talking about, of course, is passive solar earth sheltering, but not just any old rendition of those now-familiar concepts of energy-efficient construction. Passive annual heat storage is a new approach to using the earth to store solar heat—one that treats dirt surrounding a dwelling as a part of the structure's thermal mass by insulating it from the elements . . . but not from the walls.

The technique calls for a specially designed cap, known as an insulation/watershed umbrella, that's placed a few feet above an underground building's roof (not against it), extending outward to isolate the earth around the structure from the temperature fluctuations of surface layers.

Windows on the south side of the dwelling let sunshine in to heat all the mass within the insulating umbrella. Slowly—ever so slowly over the whole year—a balance is achieved between the warmth of the summer sun and winter heat loss. Thus, an artificial average annual air temperature is established at the junction of the house's walls and the earth. Prevailing temperatures inside the building will be transmitted through the walls and into the earth, extending to a radius of at least 20 feet from the structure. By controlling the amount of sunshine let into the house and the amount of heat rejected (by shading and ventilation), it's possible to adjust the temperature of the surrounding soil with some precision.

Because of the tremendous mass of the building and surrounding soil—a volume of about 45,000 cubic feet (1,800 tons) for the 20 feet beside and below a 30-foot-diameter home—the interior temperature will vary only a few degrees throughout the year. And unless a major change is made in the annual heat-flow balance, it will typically float between about 76°F in the summer and about 70°F in the winter . . . without any additional form of heating or cooling required!

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