The Double-Envelope House

Probably no solar design has created so much discussion, and so many happy occupants.

| March/April 1982

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    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.
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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.
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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.
    MOTHER EARTH NEWS STAFF
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    Alternative Builders incorporates a vestibule on almost every house the firm builds.
    MOTHER EARTH NEWS STAFF
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    Whittle double envelope, cross section near east end.
    MOTHER EARTH NEWS STAFF
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    Part of Alternative Builders' crew.
    MOTHER EARTH NEWS STAFF

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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.

Chuck
3/15/2016 1:25:37 PM

Hello I worked in the 80's for a General Contractor that designed and built several custom envelope homes in the Eureka Ca. area. These had attached 2 story green houses on the south side with a double walled construction on the north. Insulated foundation walls and a moisture barrier and cobble rock in the crawl area. The concerns were the moisture especially the attic. The company did not build more than 5 or 6 but all seem successful.


Otis Gudlyfe
4/4/2011 4:29:33 AM

I've been interested in this house since the article ran back in the '70's. Are you aware of any adaptations for Log Construction? How well could this work with logs?


William Pearson_2
3/1/2011 5:06:38 PM

If any folks who have commented previously would like to discuss their experiences or concerns wtih me I would be happy to do so. I am an architect and co-founder along with Lee Porter Butler of Ekose'a in the late 70's. Mr. Butler created the double envelope design concept and I worked side by side with him on its development thru the early 80's. Mr. Butler is deceased, but I am advancing the work of Ekose'a through Ekose'a Homes, Inc. Our website is www.ekoseahomes.com and we can be contacted by email at info@ekoseahomes.com







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