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Then, when the heated air reaches the upper limits of the greenhouse, it's drawn through the air passageway between the roofs of the two structures, down through the same channel at the north wall of the house, and back into the crawl space ... where it heats two feet of backfill spread atop the structure's well-insulated foundation.

Hence we have an "envelope" of warm air—which is kept constantly circulating by convection—surrounding the inner shell of the house. This envelope, then, not only tempers the inner structure, but also warms the thermal mass beneath the house (which can maintain its warmth for up to three days). Additionally, the air space between the two shells serves as a "buffer zone" to the cold outside air.

At night, the cycle is reversed. As the air within the greenhouse loses its heat (mainly because of the large amount of glass on the structure's surface), it begins to fall, forcing the warm air in the crawl space to move up the passageway in the north wall and through the channel in the roof... where it eventually spills into the greenhouse to start the convective cycle all over again.

In the warmer months, the envelope works in a slightly different manner. The summer sun—now a good deal higher in the sky—can't feed as much heat into the house because of the angle at which it strikes the three rows of windows. Whatever heat does pass through the glass, however, rises and escapes out the clerestory transoms, which can be easily opened. In addition, there's an outside vent pipe—routed underground—which feeds into the crawl space at the north side of the house. A controllable damper on this pipe allows the desired amount of fresh outside air to be drawn through both the crawl space and the air channel between the shells ... cooling the structure as it goes and finally exhausting from the clerestory windows with the heated greenhouse air.

Of course, in hot climates, extra steps might have to be taken to help cool the structure (window shades, a roof overhang, or additional vents), but these solutions are still passive ... and comparatively inexpensive.

THE THERMAL ENVELOPE

The house is designed according to a principle Tom calls the "envelope idea" . . . that is, the living quarters are surrounded by an envelope of tempered air. In place of a collector, a 300-square-foot greenhouse—faced with 390 square feet of double-pane, tempered glass—is incorporated into the south-facing side of the house. Behind this solarium, the structure's three bedrooms and two living areas are isolated from the outer walls and roof of the house by an inner "shell" . . . which is separated from the exterior by at least 12 inches of air space all around (with the exception of the east and west walls, which are common to both frameworks). This air space provides a passageway for the flow of warm air . . . not only creating a convective loop, but also maintaining the inner shell at a comfortable temperature (while still providing a "buffer zone" to the cooler outside shell).

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