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

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THE NEED FOR BETTER DESIGNS FOR PASSIVE SOLAR HOMES

In the past, solar homes haven't been universally practical simply because in many areas the sun doesn't shine enough in the winter. In some areas, such as upstate New York, cloud cover blocks direct radiation on at least two-thirds of the winter days. And farther north—in much of Canada, for example—the few hours between dawn and dusk in January just don't have much heat to offer.

Nonetheless, fine homes have been built that capitalize on winter sunshine to offset a major portion of their heating bills. And some use can be made of solar gain even in the most frigid locales. However, in order to prevent overheating, these conventional active and passive solar homes are forced to discard (by shading) most of the summer's lavish supply of energy. And in a majority of climates, early attempts to use earth sheltering for storage have been thwarted by the need to insulate walls, thus crippling (or eliminating) a dwelling's thermal link with the earth's mass.

Still, these precursors to the passive annual heat-storage system have paved the way by demonstrating the principles of a more efficient form of construction. It's been obvious for years that the earth around an underground structure—even when it's separated by insulation—soaks up heat from the building when the interior temperature rises above that of the soil . . . and that, given the right circumstances, the earth will return heat to the building when the interior temperature drops below that of the soil. Earth-sheltered homes have long been known to have slowly changing temperatures that are largely controlled by the earth around them. The average of this annual flux is often referred to as the floating temperature by people who design and live in such buildings. If the auxiliary heat is kept off, the temperature will assume a certain level that is related to the climate of the area. In the late winter in Montana, for example, a conventional earth shelter might have a floating temperature of around 50-55°F. But oddly enough, the average annual air temperature (and thus the deep-earth temperature) in Montana is only 43-1/2°F!

Many designers at first assumed that an earth-sheltered house would take on the natural soil temperature, but experience has shown that this just isn't the case. Even an "old-fashioned" underground building modifies the temperature of the earth around its walls, because the owners add heat to the building (and therefore to the dirt around it) for comfort. The result is an adjusted floating temperature, and passive annual heat storage's trick is to get that temperature into the comfort zone.

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