The Double-Envelope House

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


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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.
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    Alternative Builders incorporates a vestibule on almost every house the firm builds.
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    Whittle double envelope, cross section near east end.
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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.

Mitkrats
2/16/2021 11:20:23 AM

Hi Roger, I think I may have purchased one of your S. WI envelope homes. We love it. Did you build in Walworth county? Thanks Tim


Roger
11/9/2020 3:12:35 AM

I was a General Contractor in Lake County Illinois in the 80's and was hired to build Solar Envelope Homes. I was fortunate to be educated during the Energy Crisis in Renewable Construction Methods and Modern Construction Design Materials and Application. The USG invested grant money to major university to improve home efficacy. They had appointed segmented tasks to Energy Physicists Professors to improve envelope construction. I had the opportunity to meet and discuss some of the conclusive methods they were preparing for the Energy Exhibit Home for the Word Fair. They were amazingly talented and came up with ground breaking methods and material applications. One of the Professors I meet, Dr. Kenneth Foster became friends. He would show many of the projects that they were working on. I was amazed and somewhat overwhelmed and captivated. They were the people that calculated resistance of materials and the best application. They introduced an easy to understand system (R=Resistance to heat and cold) and set the value standards in areas of the home construction with variations to accommodate the proper value to use in different geographic areas. The calculated vapor barriers, proper ventilation, ETC. (amazing work for guys with little construction experience and skills). It is unfortunate that probably less than 20% of the amazing technique and innovative materials "trickled down" to home building. Because I decided to continue my education on the energy sector, I had the opportunity to be chosen by developers from Wisconsin to build custom Envelope Homes. The smallest was just under 2000 SQ FT and the largest was just under 4000 SQ FT. I feel it was a wonderful and challenging experience to build these homes. They were well designed and very comfortable, quiet, and extremely efficient. The homeowners I got to know were very happy and I feel it was an honor and a privilege to build these wonderful homes. I continue to conduct holistic energy saving remodeling methods along with P/V Solar.


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.




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