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

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THE FIRST EXAMPLE: AN EARTH SHELTERED GEODESIC DOME

The world's first earth sheltered geodesic dome has achieved that goal! Built in 1981, the Geodome has a polystyrene/polyethylene (insulation/watershed) umbrella that's roughly half the size that we now know to be needed for optimum performance. Despite the minimal size of the protective cap, after its first summer of soaking up sunlight, the Geodome's late-winter floating temperature was 66°F!

Geodome needs to have only 6% of its 3,000 square feet of floor space in windows, which is a lower percentage than either passive solar or conventional construction employs, because the building obtains most of its solar heating during the summer months.

By the end of the first summer after Geodome's completion, an array of 48 sensors buried in the dirt indicated that temperatures 12 feet out from the north wall had risen to 64°F . . . 20°F higher than normal. In fact, the sensor array indicated that the entire ball of earth within the umbrella had very slowly been heated by the solar-heated home that sat at its core.

The following summer, shades were used on the most directly solar-exposed windows, and—naturally—the interior and earth temperatures dropped slightly, so that the late-winter floating temperature hit a low of about 63°F. But, like all good earth shelters, the home was still very energy-efficient: It actually used less energy for space heating than was consumed in warming water for domestic use! And the experiment has proved that the floating temperature of a passive annual heat-storage building is adjustable.

WHAT'S NEEDED FOR THIS PASSIVE SOLAR DESIGN?

An entire year's worth of heating and cooling (three to five million BTU) can be contained in an area that extends outward about 20 feet from the walls of a house. Furthermore, over this distance the accumulated resistance to heat flow (R-factor) is sufficient to block 90% of the loss.

An umbrella extending 20 feet from the walls is only sufficient, however, if the earth under the umbrella is dry. Though damp dirt has greater heat capacity than dry earth, the greater thermal conductivity of water allows too much heat to escape the confines of the insulated cap. It's inadvisable to build any earth-sheltered home where there's a high water table, and that same restriction applies to a passive annual heat-storage dwelling. But it's also important to protect the earth within the insulating umbrella from surface water; hence, the layers of insulation in the cap are interspersed with water barriers to shunt liquid down the upper surface of the umbrella to a drainage system.

The insulation/watershed umbrella we use in Montana consists of a four-inch-thick (at the center) sandwich of two layers of insulation and three sheets of plastic, which tapers down to one inch in thickness at the outer edge. In addition, we superinsulate (above R-30) the exposed surface of a PAHS building to reduce losses to the air. A good thermal connection between the house and the earth around it is important, so we don't insulate the backfilled portions of the building at all. Doing so would merely force us to overheat the house during summer to drive heat into the earth. As it is, Geodome varies only 6-10°F through the seasons. (Still, we have found that it's a good idea to have shades to adjust the summertime interior temperature.)

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