The Double-Envelope House

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A Good Example

As we mentioned earlier in this article, many double-envelope homes have been built in the last couple of years. The bulk of them have been designed, and in some cases constructed, by either Lee Porter Butler and his firm, Ekose'a, or by Don Booth and Community Builders in New Hampshire. There are, however, regional double-envelope-house contractors springing up, and one of the more productive of them is a Virginia group called Alternative Builders. (Please see "Alternative Builders: An Alternative Business" below for an introduction to the numerous activities of this unique firm.) Alternative Builders has now erected six double envelopes, the most recently completed of which — shown in the accompanying Image Gallery — belongs to Tom, Claire and Tommy Whittle.

The family's home is located in the Blue Ridge Mountains of Virginia, and sees approximately 4,800 heating degree-days. The double-envelope design was particularly appealing to the Whittles because it combined energy efficiency with lots of glass — and they definitely didn't want to be shut away without a view of the lovely mountains surrounding the site.

Consequently, as they worked with Alternative Builders on the design of their home, the Whittles decided to incorporate two small windows on the north side of the structure and two more on the west end (to provide a view of the sunset). On the eastern exposure, however, there are only three tiny windows and a below grade door (for basement access from the outside). The main entry is an air-lock vestibule located at the west end of the sun space, and there are two other doors in the solarium.

The exterior wall is framed with 2-by-6s on 24-inch centers, and the cavities are filled with fiberglass insulation. All the windows in the outer shell are double glazed, including 420 square feet of the sun-space glass (it's angled at 55 degrees to take maximum advantage of the winter sun). An additional 160 square feet of vertical glass is set into the front of the solarium, and a short row of crank-open windows serves as the vent at the roof's peak.

The inner shell is built from 2-by-4s, on 16-inch centers, and is fitted with 3 1/2-inch fiberglass insulation. The wall is positioned so that the air space is no less than 12 inches, and — in fact — it's a great deal wider in the portion of the passage that doubles as an attic. Vapor barriers of 4-mil polyethylene line both sides of the air space, to keep moisture from penetrating the insulation and to prevent fiberglass shreds from getting into the circulating air. Since the temperature difference between the inner wall and the buffering zone is small, single glass was used throughout.

To comply with fire codes, Alternative Builders designed and built a damper — situated in the north wall — that's controlled by a fusible link. In the event of a fire, the device will close and prevent smoke from moving through the envelope.

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