Building the Sun Cottage

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Earth tempering and thermal inertia are very important to direct-gain passive solar homes (buildings in which the living areas themselves act as solar collectors). However, though direct-gain homes embody the simplest form of passive solar construction (and are considered cost-effective by agencies such as the Farm Home Administration, making them eligible for government low-cost mortgages), they can get too warm on sunny winter days. Fortunately, it's possible to control daytime overheating through the use of thermal mass and earth tempering. Windows can be opened to lower the indoor air temperature—making the interior more comfortable—without preventing the exposed masonry areas from absorbing radiant solar energy and storing it for nighttime heating. Thus, the cycles of heat movement in the walls and in the surrounding earth stabilize the direct-gain solar building's tendency not only to cool off at night but also to heat up during the day! (These principles are illustrated in Fig. 1.)

And since those masonry and concrete surfaces act as collection, storage, and distribution areas for wintertime solar input and as earth-tempering conductors, it's important not to cover them with paneling or wall-to-wall carpeting. Doing so would insulate the living areas from the home's thermal mass. Rugs should be used sparingly — for comfort and accent — in direct-gain areas (though non-solar floor spaces, in baths and halls, for example, may be carpeted).

Whenever possible, select conductive interior finishes such as paint, plaster, tile, or textured stucco for walls . . . and use vinyl tile (without cushioned backing), quarry tile, or epoxy glaze for floors. Horizontal surfaces should be finished in a dark color to hide dirt and to readily absorb radiant energy. The dark red of tile or brick pavers, for example, is an excellent color for absorbing infrared radiation . . . the part of the spectrum that provides warmth. What's more, dark floor colors form an attractive contrast to beige or white walls.

All exposed (unbermed) exterior walls must be insulated around the outside of the structure to protect its thermal mass from outdoor temperature variation. As a rule of thumb for most parts of the country, insulation should extend four feet below grade . . . but the real goal is to extend the thermal break to a depth at which there's only a small temperature fluctuation. Then, for the roof and aboveground walls, the thickness of the insulative layer should be doubled.

Of course, an earth-sheltered building can get by with less insulation than could an aboveground building because its walls and roof are protected from direct contact with the atmosphere. You see, heat loss through any medium, including insulation, occurs by way of transfer of energy from rapidly vibrating warm molecules to slowly vibrating cool ones. In most construction, when energy reaches the building's exterior surface, it's drawn off by moving air molecules. Meanwhile, as currents move across those outside walls — and over the roof — carrying away heat, energy transfer continues through the construction. But with the Sun Cottage, earth sheltering protects much of the wall and roof from moving air molecules, so heat transfer is dramatically reduced. As a result, the energy absorbed by the earth-sheltered masonry walls tends to be reflected back into the building. Thus, the temperature gradient in the mass fluctuates back and forth throughout the daily cycle. In fact, the 12" rear walls are so massive that eight hours is needed for a temperature change to be completely transmitted . . . which means that warmth absorbed during the day will be returned to the interior at night.

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