Unit One: A Solar Adobe Home

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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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This sun room/greenhouse provides most of the home's heating.
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A beautifully crafted balcony overlooks the sun room. 
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This attractive spiral staircase shows the attention of detail put into the house. 
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Diagram shows floor plan and convection patterns of the house.

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

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