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The thermal envelope house, also known as the double envelope house, is sometimes called a "house-within-a-house," which graphically describes the construction principles involved. The design employs a double envelope with a continuous airspace of at least 6 to 12 inches (152 to 305 millimeters) on the north wall, south wall, roof, and floor. You achieve this by building inner and outer walls, a crawl space or sub-basement below the floor, and a shallow attic space below the weather roof. The east and west walls are single, conventional walls. (A buffer zone of solar-heated, circulating air warms the inner envelope of the house.)
The south-facing airspace usually doubles as a sunspace or greenhouse. During the day, the sun's rays pass through the glazed, outer wall, heating the air in the sunspace. The warm air circulates around the airspaces in a "convective loop," giving up heat. The air can move from the sunspace, up and along the roof airspace, down into the north-wall airspace, into and along the crawl space, and back into the sunspace through slots in the floor. This completes the loop. Although there is generally no definite, continuous cycling of the air, there can be a shift in air movement patterns between day and night. During the day, heat is also stored in the crawl space, which uses a concrete slab floor, a bed of gravel, or simply the earth itself as "thermal mass" to hold the heat. At night, this stored heat escapes, circulating through the airspaces to keep the house warm.
Although thermal envelope houses were a popular design theory in the late 1970s, relatively few thermal envelope houses have actually been built. Brookhaven National Laboratory conducted tests on one house in the winter of 1980. Although the house performed well, the convection of heat by gravity alone did not occur evenly throughout the house. Temperatures on the three living levels were stratified. The house did have low energy needs, due to the high thermal insulation value of the double shell. Standard superinsulated houses perform about as well as thermal envelope houses, but thermal envelope houses cost more to build. Although they differ widely from the original thermal envelope concept, hybrid designs show an improved performance over the double envelope.
A drawback to the thermal envelope house is the potential for catastrophic fire damage. The connecting airways surrounding the inner shell provide a conduit through which flames may spread rapidly. The structurally protected chaseway also limits emergency exterior access. Under such conditions, firefighters may have difficulty containing a blaze in a thermal envelope house. Anyone seriously considering building a thermal envelope home should consider installing a protective fire sprinkler system.
Credits: US Department of Energy (http://www.eere.energy.gov/consumerinfo/factsheets/ca3.html)