THE DOUBLE-ENVELOPE HOUSE
Few new housing designs have drawn as much attention—or caused as much controversy—as has the double envelope. Pioneered in 1977 by Lee Porter Butler and Tom Smith in a house near Lake Tahoe, California (see
MOTHER NO. 56, page 120), the two-shell concept has gained an enthusiastic following. At the same time, however, the theory behind the thermal envelope has created a stir among solar designers.
When the Smith house was built, the dynamics of its performance were completely theoretical. No one had carefully instrumented such a building, and—accordingly—many architects and engineers reserved their acclaim . . . pending the availability of data on the efficiency of distribution and storage of the solar heat taken in through the home’s large south facing glass area.
Today there are hundreds of double envelope houses around the country, and the performance of the concept has been well documented. Very few experts now question the fact that thermal-envelope buildings are quite efficient … but the quibbling over why they work and about how well they compare with other passive designs, continues.
A REVIEW OF THE THEORY
The “collector” system for a thermal envelope house is a heat-producing sun space (which can, in many climates, double as a year-round greenhouse). It’s the method by which the sun space is incorporated into the structure’s heating system that sets this sort of dwelling apart from other solar-heated houses.
As the term “double envelope” implies, such a building is actually a house within a house. The exterior shell is load-bearing, and generally has a minimum of R-19 insulation. Between the outer and inner skins lies an air space (usually at least a foot wide) which extends from the east to the west end of the house along the roof line and the north wall. The inner wall is generally thinner—since the small temperature difference between the building’s interior and the air space requires less insulation—and supports only the structure of the living space. The passageway between the two walls is linked to the greenhouse by a crawl space or basement, which feeds air up through gaps in the boards of the solarium floor
The circulation of air through the envelope is entirely passive. The system takes advantage of the fact that warm air is less dense (and therefore more buoyant, since gravity’s influence is reduced) than is cold air. Sun-heated currents rise in the greenhouse and enter the envelope at the room’s peak … while the air between the shells—and particularly that along the north wall—loses heat and falls. The solar-heated air is then pulled through the passageway and the subfloor area, and returns to the sun space from below
Furthermore, as the air passes through the subfloor area, some of the heat it still holds is absorbed by the surrounding earth, rock, and/or masonry. These massive materials take in and store the warmth as long as they’re cooler than the circulating air. During the evening, however, the storage temperature may actually exceed that of the circulating air … which causes the thermal mass to give up heat.
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% (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″-wide air space. (In fact, that gap can, in effect, increase window Rvalue 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 (or At, in heating engineers’ lingo) 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 Rvalue 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 sunspace peak.) Furthermore, the constant but gentle and silent circulation of air prevents stagnation and lends a balmy feeling to the interior environment.