Unit One: A Solar Adobe Home

Douglas and Sara Balcomb spend their winter evenings walking barefoot in a masterpiece solar adobe home.


The south and west walls of the Balcomb's solar adobe home. A full 80% of Unit One's heating and cooling comes directly from the sun.


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The city of Santa Fe, New Mexico—nestled against the treeline in the cool, dry foothills of the Sangre de Cristo Mountains—just has to be one of the best places in this country to make use of solar energy. Blessed with abundant sunshine—and a relatively low latitude of 36 degrees—the capital of the "Land of Enchantment" also has a real need for heating.

You see, although most people think of New Mexico as warm desert country, Santa Fe—which is perched just at the 7,000-foot line—is actually slightly cooler (on an average) than Denver, Colorado! While Santa Feans seldom have to suffer through more than a few consecutive days of subfreezing daytime highs, the mercury does plummet below zero on some winter evenings, and the town's average annual snowfall is over three feet.

In fact, in heating energy terminology, Santa Fe has almost 6,000 heating degree days. (This figure represents the total number of degrees that the mean temperature falls below 65°F during the year.) For comparison, Denver averages about 5,700 heating degree days, and New York nets approximately 5,250. And—just as an extreme example—Edmonton, Alberta rings in at almost 11,700 degree days.

It's not too surprising, then, that the hillsides above the quaint old historical New Mexican community (it was established around 1610) are dotted with adobe homes equipped with a wide range of solar heating equipment. And one of the most attractive (and successful) examples of this combination of new-age technology and age-old earthen home construction methods belongs to Douglas and Sara Balcomb.

We are fortunate enough to have detailed performance records for their solar adobe home, because both Balcombs are intensely involved in solar energy. In fact, Dr. Balcomb—who currently works in passive solar energy research at Sandia Laboratories—has published information on his family home in a Department of Energy pamphlet entitled Passive Solar Buildings: A Compilation of Data and Results (SAND 77-1204 [Revised]). And Sara—who is vice-chairperson of the New Mexico Solar Energy Association—takes part in the preparation of a regular solar newsletter and various sun-energy teaching aids (such as slide shows).

The Balcomb home is known as Unit One, because it was the original dwelling in a planned environmental community called First Village. Architect William Lumpkins—with solar engineers/ designers/builders Susan and Wayne Nichols—chose to blend a selection of solar techniques (many of which were pioneered by solar innovator Hal Miguel and used in his own Tusuque, New Mexico residence) rather than invest all their capital and energy in one system.

The resulting hybrid solar design consists of a greenhouse, a thermal mass wall, and two rock heat storage beds equipped with fans. This solar collecting system is not only fully integrated into the building's design (and thus made attractive), it really works: In the high-energy-demand New Mexico highlands environment, Unit One is 80% solar heated to a minimum temperature of 65°F. So, with 4/5 of the warmup" duties handled by ol' Sol, the Balcombs' backup electric heaters consume an average of only 850 KWH per year or roughly $48 (at the current power rate in New Mexico). Many folks would be overjoyed to spend that little on heat in one month!

The Greenhouse Effect

Over 80% of Unit One's solar-collected warmth enters passively through the 409 square feet of double-pane thermal glass which forms the greenhouse. The L-shaped house is entirely faced—along its southern exposure—by this solarium. Two-thirds of the glass (that is, 16 of the standard patio-size 32" X 76" glass panels) is angled at 60 degrees to throw sunlight on an adobe wall which separates the greenhouse from the living area. The other eight double-glazed panes stand vertically at ground level, and the center two can be opened to provide ventilation.

The earthen wall, which tapers from a 14" thickness at its base to 10" at its peak some 20 feet above the floor level, is the primary storage and transfer system for Unit One's solar heating. By noon on a chilly winter day, the surface temperature of the wall will climb to as high as 110°F. Then slowly over the course of the next 10 hours, that heat works its way through the earth-brick barrier. By 10:00 p.m. the inside of the wall reaches about 80°F, which will keep the living area warm through the remainder of the night. By morning, however, most of the heat is dissipated, leaving the adobe ready to temper the living area through the day (when—even in the winter—the home's interior would otherwise warm rapidly). This slow transfer of the heat of the sun is what solar engineers call indirect gain.

But there is a direct gain element in the passive system, too. The air in the greenhouse is also sun-heated. This warm, moist (and oxygenated by the plants) air can be introduced into the upstairs and downstairs by simply opening doors into those areas. Thus the Balcomb home is not only solar heated, but also has its own air filtration and humidification system. As Sara says, "It's a natural warmth. Many of the standard discomforts usually associated with winter are now things of the past: Cold feet, static electricity, dry skin, morning sore throat—they've all disappeared."

Storing the Excess

The majority of Unit One's solar-heating methods are totally passive, requiring no controls, no fuel, and no maintenance. But had the homes planners stopped at that point, much of the incoming energy would have been wasted. The fact is that—even on winter days—the greenhouse frequently draws in more energy than can be transmitted through the wall. That's where Unit One's active solar system comes into play. Rather than just vent this excess heat to the outside, the modem adobe's designers added a storage system.

Beneath the building's living room, dining room, and kitchen lie two rock storage beds—one 2' X 10' X 19', and the other 2' X 10' X 15' — containing a total of 50 tons of three- to five-inch-diameter cobblestones. These heat storage areas are connected, by ducts, to the greenhouse. Whenever the solarium's air temperature rises more than five degrees above the temperature of the stones, air is forced down from the top of the greenhouse and into the rock beds by two fans located in the backs of the bedroom closets. Thus the Balcombs can enjoy 70°F floors—with the heat being transmitted through the six-inch slab by means of convection—as well as 80°F walls.

Unit One's primordial "batteries" also provide the dwelling with a heat storage capacity which can see the Balcombs through extended spells of inclement weather. And—while the home's two- to three-day storage capability doesn't match the staying power of a more expensive, totally active collector setup—Dr. Balcomb points out that a passive system is (in one way, at least) more efficient: Even on the snowiest Santa Fe day, Unit One's solar features are still absorbing same energy because dispersed sunlight is better than none at all. The active collector unit that provides the Balcombs' domestic hot water supply on the other hand requires more intense light to operate. (For example, there have been a number of days since Unit One's completion—in August of 1976—when there was a passive solar homeheat gain, while the water heater didn't receive enough sunlight to operate at all.)

Hang on to What You've Got ...

It's impossible to build and maintain an energy efficient house without proper insulation, and the Balcomb home is no exception to this rule. The thermal mass wall — which serves as the dwelling's nighttime heater — is also (in effect) an insulator: The wall's heat transfer function prevents it from having any significant heat loss since the outer surface losses are turned into inner surface gains. The building's east and west walls, however—which are also constructed of adobe — are more directly insulative. Though their mass does tend to delay inside temperature changes — just as does the greenhouse wall—they don't transfer any significant amount of solar heat to the home's interior.

The north-facing wall is buried below ground level to a depth of four and a half feet. This subterranean portion is built of eight-inch concrete blocks (with the cells filled for extra insulation) and is sealed with plastic roofing cement and two inches of rigid polystyrene. Above ground level, the northern wall consists of a stucco-covered frame of 2" X 8" studs on 16" centers and is insulated by a 1 1/2" layer of fiberglass batt, a vapor space, and then six more inches of fiberglass batt. Three inches of urethane foam in the roof complete Unit One's insulation package.

...and Get Amazing Thermal Stability

There is a popular misconception that life in a passively heated solar home involves some compromises, such as donning and shedding clothing as the weather changes, lighting fires for extra warmth, etc.

Sara Balcomb, however, has a bone to pick with such "myths": "I believed all that myself," she says, "until I lived through a winter in my passive home." And Dr. Balcomb's careful records of year-round temperatures in Unit One certainly show that the supposed necessity for compromise is indeed a myth, at least in this particular solar abode.

The dwelling's thermal mass wall and insulation combined with its 20% active storage capability conspire to keep Unit One's inside temperature incredibly stable. During the month of December 1977, for instance, the Balcombs' living area's temperatures varied by a grand total of 4°: from 67°F to 71°F.

Sara claims that it takes about two days for the house to react to a change in weather, and that there is never more than a 5° swing in any 24-hour period. The actual recorded extremes for the house are a low of 65°F in February (limited by the electric baseboard units, which are thermostatically controlled at 65°F) and a high of 76°F on a 97°F summer afternoon. And, as Sara points out, "A variance of 21° is hard to achieve even with conventional air conditioning. We did it simply by natural means."

Temperatures do fluctuate in be greenhouse, naturally—on many days by as much as 35°F. This rise and fall is merely evidence of solar heating at work. Still, the coldest temperature the Balcomb Mans have ever had to tolerate was 45°F (on a -17°F winter night), while the hottest summertime high was 98°F Judging by the health of the resident greenery, the solarium's moderated outdoor environment is no hardship.

And, of course, the solar techniques that heat Unit One also function to keep the Balcombs cool in the summer. When the sun is at its higher "warm season" angle, a roof overhang (and the balcony) shade the adobe wall from any direct sunlight. The mass of the wall then serves as a stabilizer, thoroughly negating the radical temperature extremes of the desert day and night.

The storage system can also be used—by reversing its thermostatic control—to cool the rock beds at night and transfer the chill during the following day. To prevent overheating of the greenhouse during hot spells, the builders positioned vents at the base of the vertical windows and at the top of the staircase. Natural convection currents can thus introduce cool air at the bottom and usher heat out the top.

The Costs ... The Benefits

Unit One cost a total of $104,000, of which $24,000 went for the lot. Approximately $12,000 was spent on the solar heating system, but $8,000 of that expense was granted by the Department of Housing and Urban Development under the 1974 Solar Demonstration Act. (Of course, any current solar construction has a portion of its cost offset by the federal 10% tax allowance and various state tax breaks.)

Still, the per-foot price of about $42 (for the 1,900-square-foot home) may seem a bit steep to many readers. If so, bear in mind that housing costs in the Santa Fe area are higher than most regions by several dollars a square foot. Then consider the absence of economy measures in the dwelling's construction. To an adobe fancier—and the lure of an earthen home "growing" out of the land is very real to many folks—the Balcomb house falls somewhere between shelter and art.

Furthermore, Unit One is a thing of beauty that works ... and works with the environment. It is built of earth and draws most of its heat from the sun. Inside its walls and windows the Balcombs enjoy a lifestyle which dispels most of the myths about passive solar living in the kind of comfort that none of their previous homes could offer. 

More on Adobe 

If you're interested in finding out more about adobe construction—and how you can "do It yourself" for next to nothing—you may want to consult the following back issues of this magazine.

1. "The Owner-Built Home: Rammed Earth"
2. "The Owner-Built Home: Earth Block Construction"
3. "Modern Home From Mud"
4. "Modern Home From Mud Pt 2"
5. David Wright: Passive Solar Design