The Double-Envelope House

Image Gallery The double-envelope house is characterized by its large south-facing glass area and its clerestory windows at the roof peak. MOTHER EARTH NEWS STAFF

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, Calif., 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.