The Double-Envelope House

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A double envelope also taps its storage passively by reversing the convective loop. During the night the structure's greatest heat loss is through the expanse of glass in the sun space. That cooling causes air to fall to the floor of the greenhouse, while the (relatively) warmer air of the storage area rises and is forced up the north wall cavity. The continual imbalance in pressure then keeps the loop flowing.

In the summertime, however, the sun space is likely to gain far too much solar energy — despite the fact that the tilted glass is oriented to admit winter, but not summer, sun. To prevent overheating, vents are usually set into the roof peak of an envelope house, allowing the rising hot air to escape. And in some designs, "cool pipes" (air intake tubes buried in the ground, see the article on page 128 for a fuller explanation of this system) are linked to the crawl space so that earth temperature air can be drawn in and distributed through the envelope.

The energy-saving capabilities of the envelope design are numerous. For one thing, a great deal of solar heat is taken in through the greenhouse, and at least some excess warmth is stored in the crawl space for use during the night or on cloudy days. Consequently, most double envelope houses require very little backup heat. In fact, they often satisfy 80 percent (or more) of their thermal needs directly from the sun.

Now there's no question that a large part of the energy efficiency of such structures does result from their thick insulation. The two shells and large air gap produce a total R-value that typically exceeds 30! In addition, the double walls reduce infiltration (direct air leakage) to the living space and dramatically improve the thermal resistance of any north-facing windows, because of the roughly 12-inch-wide air space. (In fact, that gap can, in effect, increase window R-value by as much as 4 — without producing the condensation that tends to be a problem in conventional multipane windows.)

Another thermal benefit of the envelope concept shows up in the form of comfort. Because the air circulating inside the envelope is significantly warmer than that outdoors, the difference in temperature between the living area and the air passage is relatively small. Thus the heat loss for the inner wall is less than that of an equivalent insulative fraction of a single wall whose total R-value equals the double envelope's. As a result, the surfaces of the envelope's interior walls remain warmer than would equally insulated single-layer walls.

Envelope home residents also enjoy pleasantly stable humidity through the winter, since moist greenhouse atmosphere is continually circulated through the air space and can be admitted to the living quarters by cracking a door or window. (In the summer, however, excess humidity — and heat — is vented at the sun-space peak.) Furthermore, the constant but gentle and silent circulation of air prevents stagnation and lends a balmy feeling to the interior environment.

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