The Double-Envelope House

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The Controversy and the Latest Poop

Much of the quibbling about double-shell houses centers on the fact that the air cavity must lose heat (particularly in the north wall) in order to power the convective loop. And it's true that, given the relative temperature difference between the interior and the cavity and the cavity and the outside, most of the passageway's BTU will escape to the outdoors.

Technicians have calculated that the efficiency of the convective circulation system provided by the double envelope is far lower than that of a blower system powered by an electric motor. And there's a growing consensus among designers that the double envelope suffers a slight loss of thermal efficiency because of its air circulation system, as well. The trade-offs, of course, would be the noise of the blower and the need for electricity to run it.

Early doubts about whether or not the envelope flow pattern actually reverses at night have recently been shown to be misplaced in most cases. Though the flow rate is undoubtedly much slower at night (perhaps about two-thirds of the daytime air movement, according to a computer simulation developed by Joe Kohler and Dan Lewis of Total Environmental Action, Inc. in Harrisville, N. H.), the double-shell dwelling's greenhouse is warmed at night by air circulating from the crawl space through the north wall and roof cavities.

The question of how much heat is actually stored in the crawl space is much more difficult to answer, however. The rate of thermal transfer to the earth or gravel depends on a number of factors — including the moisture level in the storage medium (water stores much more heat, and transfers it more quickly, than does earth), the possibility of stratification of air flow (with the warmer air collecting near the floor and cooler air sinking down against the earth), the particular type of soil involved, and the temperatures of the air and the earth. Rough estimates of storage capability suggest that only between 10 and 25 percent of the available BTU can be stored, but even at that relatively low rate, it's now acknowledged that the tempering effect of the earth on the envelope air does play an important role in the double shell design's energy efficiency.

Though there's much more known today about the double-envelope concept than was the case four years ago when the Smith house was completed, it's obvious that the available data still don't answer all questions. Thermal-envelope houses are very popular among owners because they're both efficient and pleasant to live in. And when one considers that the raging controversy among experts really concerns only a few percentage points of efficiency one way or the other, the double envelope is likely to enjoy even greater popularity in the future.

Alternative Builders: An Alternative Business

The firm that built the house shown in the photos accompanying this article is one branch of a thriving "back to the country" conglomerate based in Independence, Va. Alternative Builders was conceived over five years ago as a result of three friends' common interest in alternative energy; but it wasn't until 1979 that the triumvirate officially founded the company (which included one member's woodstove business: Goosecreek Heating). The group has since built just about every sort of energy-efficient house you can imagine.

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