The Double-Envelope House

By The Mother Earth News Editors
Published on March 1, 1982
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Custom kitchen cabinetry is a highlight of the structure's aesthetic side.
Custom kitchen cabinetry is a highlight of the structure's aesthetic side.
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The Whittle family enjoys one of their first evenings in their new energy-conserving double-envelope home.
The Whittle family enjoys one of their first evenings in their new energy-conserving double-envelope home.
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The double-envelope house is characterized by its large south-facing glass area and its clerestory windows at the roof peak.
The double-envelope house is characterized by its large south-facing glass area and its clerestory windows at the roof peak.
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This view from west to east in the solarium shows the second-story balcony, the floor slots for convective loop flow, and plenty of healthy greenery.
This view from west to east in the solarium shows the second-story balcony, the floor slots for convective loop flow, and plenty of healthy greenery.
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Alternative Builders incorporates a vestibule on almost every house the firm builds.
Alternative Builders incorporates a vestibule on almost every house the firm builds.
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Whittle double envelope, cross section near east end.
Whittle double envelope, cross section near east end.
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Part of Alternative Builders' crew.
Part of Alternative Builders' crew.

<p>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.</p>
<p>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.</p>
<p>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.</p>
<h3>A Review of the Theory</h3>
<p>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.</p>
<p>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</p>

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