The Double-Envelope House

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


| March/April 1982



074-154-01-im3

Custom kitchen cabinetry is a highlight of the structure's aesthetic side.


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.

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


mark rose
1/30/2009 12:04:32 PM

regarding the 01-05-09 comment about the envelope design I did the construction drawings and design for the envelope house in the '82 article in MEN please contact me for more information rosedesign579@earthlink.net Mark Rose


mc_2
1/5/2009 9:07:22 PM

I like what I'm reading about double envelope and passive solar. I like it so much that I'm getting over being upset over being told over and over and over that the climate here is simply too wet to be hospitable to straw bale construction. I think it would work for a total rebuild/remodel of the structure we're currently in. We have really excellent solar exposure on the roof (major unobstructed SSE face; gets the sun quite literally all day). It would even lend itself to the very strong probility that, if I want to salvage this badly jerrybuilt rathole, we're probably going to need at least one new loadbearing wall (and four wouldn't hurt). The existing walls could become the inner envelope; their existing insulation is about right for that. The main problem I see, right off, is that right now we're dealing with a split-level design, with the single-story level having primarily shaded northern exposure. I wonder if that could be remediated by going to a full two-story design (something my spouse wants to do anyway) with the new load-bearing walls??? That would take us up to about 1750 sqft, including solarium, which I would definitely hope to be able to use as a greenhouse... Can anyone tell me if I'm completely off-base in thinking this design is workable with what I've already got, and viable in the Northwest Arkansas area????


tdavenport
11/24/2007 8:45:20 PM

I'VE JUST REDISCOVERED THE ENVELOPE HOUSE. I AM CURRENTLY BUILDING TO THE PASSIVEHAUS STANDARD IN MONTANA. I WOULD BUILD A ENVELOPE HOME TO PASSIVEHAUS STANDARDS. BE SURE TO PUT FOAM UNDER THE BASEMENT SLAB OR CRAWL SPACE. I USE 5" OF EPS FOR AN R OF ABOUT 20. THE GROUND TEMPERATURE WILL BE CLOSE TO THE WELL WATER TEMPERATURE, WHICH IS AROUND 55 DEG. F. IN MONTANA. THE GROUND SOURCE HEAT PUMP PEOPLE FIGURE ABOUT 44 DEG. F. THE CLOSER TO THE SURFACE, THE CLOSER TO AIR TEMPERATURE. IF ONE KEEPS THE HOME AT 65 DEG. F. THEN THERE WILL BE A TEN DEGREE TEMPERATURE DIFFERANCE TO THE GROUND AT 55 DEG. F. IF THE GROUND HAS AN R VALUE OF 2 THEN 1 DIVIDED BY 2 IS 0.5 U VALUE BTU PER SQ. FT. PER DEG. F. 1000 SQ, FT, X TEN DEG. IS 10,000 X U VALUE 0.5 IS 5000 BTUS EVERY HOUR LOST TO THE GROUND OR AS TO GO TO THE DEPTH OF TEMPERATURE OF 65 DEG. HEAT FLOWS TO COLD. USE FOAM. THE R 20 FOAM U VALUE IS 0.05 BTU 500 BTUS WOULD BE LOST EVERY HOUR TO THE EARTH. 1 KW IS 3412 BTUS. I WILL STUDY THESE EFFICIENT ENVELOPE HOMES. THANKS! SOLAR IS AWSOME!


shvonna
8/10/2007 2:51:51 PM

We currently bought an envelope home and want to figure out how to use is to our benifit it has two levels and a basement and arouns the bottom floor there is a walk way all around the house with each level having wooden floors that allow air movement to circulate i need help to figure this house out can anyone email me and tell me how to work it


todd_10
8/2/2007 9:42:06 PM

In a double envelope house with a full basement, is it possible to finish the basement and still maintain the double envelope circulation patterns? Is leaving vents open in the walls thoughout the basement sufficient for the airflow?


ken_19
6/24/2007 7:18:18 PM

looking for infor;mation on plans for enevelope house


linda_82
3/4/2007 9:35:17 PM

We had build a double-envelope house in 1986. Since then we have sold our home four years ago and miss the house we once had. We would like to build this type of home again, but can not find anyone with the plans on how to go about building this house. Can you help us in any way. Thank you






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