The Double-Envelope House

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The building's subfloor loop consists of a partial basement and a 40-inch-deep crawl space. The latter has four inches of earth lying atop a sheet of 4-mil polyethylene (for moisture protection). The earth itself is uninsulated, but the foundation and footings are lined with 1-inch extruded polystyrene. The backup heater for the dwelling, a large woodstove, is installed in the basement so that the heat it produces can circulate in the envelope.

Air returns to the greenhouse through its floorboards, which were made by ripping 2-by-6s, then spacing them on the joists in a pattern that allows one square foot of open area for each foot of the solarium's length.

The Whittle residence was completed in August of 1981 for a cost of $130,000. With 4,000 square feet of finished space, that somewhat awesome figure works out to a little less than a very reasonable $33 per square foot. It's likely that a smaller double envelope would be slightly more expensive per square foot than was the Whittles'. But the figure becomes particularly impressive when the quality of the construction is taken into consideration. Alternative Builders turns out custom homes and maintains high standards. For example, the roof is thoroughly vented, the kitchen cabinets were built to order locally, and the fireplace is handmade of rock.

Of course, the Whittles are just now riding out the last of the first winter in their new double envelope house, so performance figures haven't yet been compiled. But we did get a chance to talk to the owners of another Alternative Builders double envelope who've recorded its performance for almost two years.

The Test of Time

The Kimmels' home is at a lower altitude than the Whittles' and therefore sees only about 3,300 heating degree-days. It's also slightly smaller (3,500 square feet), has minor differences in envelope design, and is set 20 degrees east of south to capture the morning sun effectively. The expected sunshine for the two sites is about equal, though, so the major difference in performance should be in proportion to the difference in the number of degree days that each home is exposed to.

In the middle of their second winter, the Kimmels are still burning up the trees that were cut to clear the site for the house. There's a huge pile of wood yet to be used, and it probably won't be touched until the winter of 1982-83. The 1980-81 heating season consumed less than half a cord of wood for backup heat, and the lowest indoor temperature has been 62 degrees Fahrenheit. During the summer, deciduous trees provide the Kimmel house with lots of shade, and the glass in the solarium (there's a total of about 550 square feet, angled and vertical) is arranged to prevent sunlight from hitting the living area. Consequently, the warmest inside temperature was 82 degrees (in July 1981) and the family reports that the humidity level has always remained pleasant, as a result of the constant air circulation.

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