The Thermal Envelope Home

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So, in effect, the walls of this inner compartment act as both collector and storage medium. And this system—coupled with the fact that there are two feet of backfill in the insulated crawl space beneath the house—guarantees that heat gained during sunny periods (about 400,000 Btu's on a typical January day, as near as Tom can figure it) will be stored for up to 72 hours.

Naturally, Tom's envelope works just fine in the winter . . . but what about the summer? Well, during the warmer months, the "flow" cycle is reversed. When Smith opens the clerestory windows at the upper face of the house, the hot air gathered there is forced to evacuate, drawing behind it cooler air pulled from any open windows below. In addition, fresh outside air is drawn through an underground vent pipe at the north side of the building, and passes above and below the shell of the inner structure, cooling the area within.

MORE (OR LESS) HEAT AS IT'S EEDED

The basic design of the envelope house allows it to be about 80% energy self-sufficient in both winter and summer ... an impressive figure considering the simplicity of the passive system. However, additional heat can be supplied by a wood-burning stove, which more than makes up the remaining 20%. And, if an extended cold snap should happen along, the system is further backed up with baseboard electric heating . . . which— although it hasn't been needed yet—is required by local building codes and lending institutions.

On the other side of the coin, summer cooling has presented no problem at Tom's Olympic Valley location. However, he's quick to point out that—if the house were to be constructed in a hotter clime—simple adjustments (like the installation of window shades, extra vents, or a roof overhang) would provide ideal cost-effective solutions.

INSULATION IS THE KEY

Unlike many other energy-efficient houses, the Smith dwelling doesn't require an overabundance of insulation. A standard resistance value of R-19 is achieved throughout the exterior shell with rigid insulation board, and the inner shell is insulated—in the same manner— to R-13. It's important to note that—although both shells do require insulation for the system to work properly— excessive thermal protection not only defeats the purpose of the dual-surface design, but wastes money . . . which could be more wisely spent on heavier glass for the greenhouse wall.

Tom also took full advantage of insulation when he designed the structure's foundation. Because it serves as a thermal storage area, the "base" of the dwelling was insulated (with two inches of urethane foam) to a value of R-13 . . . preventing heat within the mass from escaping to the outside.

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