Building a Passive Solar Home: Part VI

Finish, insulation and backfill. Follow an architect as he leads us through the planning, pouring and pounding involved in the Sun Cottage.

| March/April 1984


Fig. 2, normal construction versus earth-sheltered construction.


All too many of us have found that building an energy-efficient home seems to be a dream . . . a fantasy that's kept just out of reach by escalating prices and high interest rates. Of course, MOTHER EARTH NEWS has long been exploring ways of breaking this vicious circle of waste, and one inventor of solutions—whose work we've shown you before (starting with Building the Sun Cottage Part I: Site Selection)—is architect Angus W. Macdonald. Angus developed a number of housing designs that apply low-cost building techniques to passive solar, earth-tempered homes . . . and agreed to relate, in a series of articles that has now spanned six issues of MOTHER EARTH NEWS, much of what he's learned about planning and building such structures. The series has followed the actual construction of one of the architect's standard designs . . . the Sun Cottage.  

Through the course of five articles, we've talked—in some detail—about what's involved in building an energy-efficient, earth-sheltered, passive solar home. Thus far, however, we've touched only lightly on the theories upon which such a structure's performance is based. Consequently, in this, the final installment of our series "Building the Sun Cottage", we're going to pay particular attention to the proper detailing that makes the various heat-exchange systems in an earth-sheltered, passive solar home work. Then we'll wrap up with that most crucial of all earth shelter components: waterproofing. The appearance, thermal efficiency, comfort, and longevity of the Sun Cottage are all quite dependent on how the structure is finished, so the final stages are perhaps the most important!

Heat Movement

The masonry walls and concrete floor slab of our building—those large surface areas surrounding the living space—provide great thermal inertia. . . a term that refers to how slowly a mass reacts to temperature change. Heat is stored in the mass and is later radiated gently and evenly into living areas, which is one reason why the Sun Cottage doesn't require a mechanical (or active) system to distribute stored energy.

Likewise, sub-grade (below ground) masonry areas conduct heat to and from the relatively thermally stable earth surrounding the walls. In fact, the ground around an earth shelter changes temperature so slowly that areas eight feet or more below grade are actually warmer during winter than they are in summer. What's more, the overall range of this variation is usually less than 10°F. We call the shielding effect of sub-grade construction earth tempering, because it tends to make the building's interior temperatures more constant.

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).   

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