Digging in: homeowners discover life underground

For the past few years Joyce Rinker has been ''happily dug in,'' as she puts it, among the forested sand hills of Michigan, paying a fraction of the heating and cooling costs of an aboveground home of similar size. In other words, she has chosen to live underground.

At present, of course, she still ranks among a minority of homeowners in the United States. But the ''underground club,'' as it is sometimes called, is rapidly losing its exclusivity. Some 10,000 fellow Americans have chosen to live this way in recent years, and the number is growing.

Modern, earth-sheltered homes are cheap to heat, low on upkeep costs, secure, and yet as bright and airy as most above-grade homes. Moreover, they are not necessarily expensive to build. In some instances they are even less expensive than conventional housing.

About 5,000 of these earth-tempered dwellings, ranging from all-wood structures such as Miss Rinker's to those made largely of reinforced concrete, have gone up in the United States in the past decade. And the trend seems likely to continue.

Those who live this way say there are valid reasons that this should be so. For one thing, living underground isn't the same as living in a basement, although there are some similarities. The natural lighting (and often an expansive view) available in a well-designed home remove any ''underground feeling.'' Once inside, you should barely know the difference.

Yet the earth-tempered house has several distinct advantages not available even in the superinsulated aboveground shelter. These include protection from high winds and from fires, other than those that start within the house itself.

Disadvantages, almost always the result of poor siting or indifferent design, include flooding and interior condensation on humid summer days.

Soil is not a great insulator, but earth-surrounded homes are energy-efficient nonetheless. Apparently the surrounding earth makes up in volume what it lacks in R-values. It shields the house from heat-sapping winter winds and plugs heat leaks more readily than the plastic film of a superinsulated aboveground house.

Admittedly, the home would lag behind air temperatures when a warm spring wind arrives, but on such occasions the occupant has merely to open the windows. Later in summer the soil has a valued cooling effect on the house.

Earth cover also muffles outside noise to a remarkable degree. Busy highways, nearby airports, and noisy neighbors don't bother the underground dweller.

Below-grade homes require less upkeep as well. There's little or no exterior paint that can peel, although, depending on the planting, the roof might need mowing on occasion.

Danger from grass fires, burning trees, or even nearby burning buildings is dramatically reduced (although internal furnishings are quite capable of burning). Violent winds, too, present little threat, and even burglary statistics are way down. Apparently climbing down into a house has an unnerving effect on the would-be intruder.

Insurance companies (at least the smart ones) recognize these benefits, and their rates reflect this confidence.

Obviously, the underground house should not be sited on a flood plain, alongside swamps, or where the water table is only a few inches below the surface. In humid climates condensation can present some problems during summer which might call for a dehumidifier. Even so, this is generally an indication of inadequate amounts of insulation in the walls.

The sloping hillside seems to be the preferred site for the underground or earth-sheltered home. The house is simply set partway into the hillside, leaving one or two sides open to the fresh air and the view. But flat sites don't present any obstacles.

Atriums or walk-down terraced gardens let in natural light and provide a private garden view. Some built partway or even totally above ground have earth trucked in and a protective berm (slope) built up against one or more sides.

Modern technology has made waterproofing underground homes a relatively simple matter. Even the all-wood structure is both waterproof and rotproof.

In the past 20 years some 15,000 basements in the United States have been built exclusively of wood. The all-weather wood foundation uses timbers that have been pressure-treated with a solution of chromated copper arsenate.

One such below-grade structure, disassembled after 40 years, showed no sign of deterioration. The beauty of this treatment is that none of the chemical leaches into the surrounding soil, always a problem with the creosote alternative.

Miss Rinker's 1,220-square-foot, all-wood home cost somewhat less to build than a similar above-grade structure. To meet existing codes in her region, 22 percent of the exterior walls had to be left exposed. These were dressed up with costly redwood siding.

Once, during below-zero weather and with no fire in the Ashley heater, the temperature in the Rinker house dropped at a rate of one degree F. every 21/2 hours.

Miss Rinker's story has been told in several magazines since the house was built a few years ago. It is also the lead story in the recently published ''Mother's Guide to Homebuilding and Shelter'' (Brick House Publishing, 34 Essex Street, Andover, Mass. 01810, $12.95).

Another invaluable guide is Herb Wade's Building Underground (Rodale Press, Emmaus, Pa. 18049, $14.95).