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Umbrella
Homes ...
from
Popular
Science, August 1986, pages 64-66.
This
simple underground house design uses a novel
insulating/water-shedding blanket that covers the structure
and surrounding soil. The umbrella creates a huge
subterranean thermal reservoir that soaks up the sun’s
energy during summertime and stores it for winter heating.
In many cases, the clever design makes a heating system
unnecessary. The
reverse can also be done for hotter climates.
By
JOHN HAIT My first
earth-sheltered house, an underground geodesic dome was
partially complete when the truckload of insulation my
colleagues and I had ordered arrived. Right away, we knew we
had a problem: How do you put flat, rigid polystyrene
insulation on a round house?
We called housing experts
all over the country, but no one had any ideas. Finally, Ray
Sterling at the University of Minnesota's Underground Space
Center suggested that we place a flat, insulating
"umbrella" in the earth above the building. This, he
said, would keep the domelike house warm by insulating the
soil around it.
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Figure
1 Geodome, the first umbrella home (in idealized form),
maintains a 66°to
74°temperature
year-round without heating equipment in western
Montana’s cold climate. In summer, solar hat
radiates in, falls on internal surfaces, and is
absorbed into the surrounding soil. The umbrella
traps heat in the dry soil until winter, when it
migrates back into the house. Convection-driven
earth tubes provides ventilating air.
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"What a marvelous
idea!" I thought when I heard his advice. After two weeks
of rigorous examination, I realized that the concept was even
more promising than I'd supposed. By then I was convinced that
the dry earth under an insulating/water-shedding umbrella
could store enough free solar heat from the summertime to warm
the house through the entire winter (see diagrams above). This
meant that a house could actually be constructed with an
unchanging built-in temperature, which would make heating and
cooling equipment unnecessary. Now, five years later, I still
think it's a marvelous idea. The Geodome, the house we built
in the cold and cloudy climate of western Montana, remains at
66 to 68 degrees F, even through the coldest winters.
The success of the Geodome
led to the establishment of the Rocky Mountain Research
Center, a nonprofit organization dedicated to the development
of what is now called the passive annual heat storage (PAHS)
approach to free year-round passive-solar heating. Four basic
points make PAHS different from techniques used in
conventional solar-heated earth-sheltered houses:
·
The
house's window shades are opened to collect solar heat in
summer.
·
The
umbrella's laminated sandwich of polystyrene insulation and
polyethylene sheeting (about R-20) insulates a huge mass of
surrounding dirt instead of just the house.
·
The
umbrella sheds water to keep the soil around the house dry.
·
The
natural-convection-driven ventilation tubes (see below)
provide very high heat retention efficiency by acting as
counter-flow heat exchangers.
Conventional
passive-solar theory tells us to exclude the abundant summer
sunshine by blocking it out with large window shades because
the typical (relatively small) thermal mass in a solar house
can store only a night's worth of heat. Yet we're also told to
make the windows large enough to capture what little solar
heat we can in winter. PAHS, on the other hand, uses the
summer's abundant sunshine to heat up a large body of earth
around the house to a comfortable 72 degrees F or so. That
warm thermal mass keeps the house and its occupants cozy all
winter. Simple thermal conduction transfers heat through the
walls, into the soil, and back.
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Twenty
feet underground, the natural soiltemperatureis nearly
constant (see diagram), and is equal to an average of the
entire year's worth of temperature changes on the surface. The
Geodome's inexpensive umbrella isolates the soil beneath it
from fluctuating outdoor air temperatures above. By
controlling the heat flow in and out,the blanket raises the
constant soil temperature around thestructure to reflect the
newly established average annualair temperature inside the
house. The result is a comfortable indoortemperature that
varies only six or eight degrees during an entire year,
whileoutdoor air temperatures may vary from minus 40 to more
than 100 degrees F.
Although the Geodome's
window area amounts to about six percent of its floor
area—less than most solar homes —the summer sunshine lasts
much longer, and so more solar heat is collected and stored
away than is available from any passive winter
thermal-collection system.
We've learned several
lessons from the Geodome that have advanced our understanding
of integral year-round thermal systems. First, the design
temperature of 66 to 74 degrees is built in and is difficult
to change. This became apparent during its first winter. The
Geodome's tenant at that time, a salesman who was constantly
on the road, found that the house temperature was still at 66
degrees F in March—even with a few warm bodies or appliances
to add heat. We realized then that if you would like it a
little warmer or a little cooler in such a house, you would
have to enlarge the window area and install adjustable shades.
That way, the annual solar input could be altered to modify
the internal temperature as desired.
Second, thermometers
indicated that the umbrella altered the ground temperature
much farther out from the walls than we expected. I located
some National Bureau of Standards studies that showed that
air-temperature changes affect the soil temperature more than
20 feet down into the earth, so we concluded that the umbrella
should be extended to at least that distance beyond the walls.
Third, an examination
confirmed that the earth underneath the umbrella was bone-dry,
even though the soil on top was moist. The dry dirt below
makes waterproofing the structure easier, while the moist soil
above helps alleviate the desert-like conditions that often
occur on top of many earth-sheltered houses. Note that the
water table must not moisten the thermal mass.
PAHS seemed to offer a way
to build energy-efficient homes that require no commercial
energy supply for heating or cooling, but we realized that to
become truly practical, we needed to provide for ventilation,
heat retrieval, and moisture control.
No good solution presented
itself until one day when I was teaching a class of students
at the center about convective heat flow. After a time, the
discussion turned to-ward the use of earth tubes—pipes in
the ground that bring in outside air for ventilation. Then one
of the students asked about convective heat flow in earth
tubes. Before I knew it, the solution to our problem was
sketched on the chalkboard: an open-loop, convection-driven
earth-tube system (see diagram) that draws outdoor air into
the house to be heated by the summer sun, transfers it to the
buried earth tubes where it passes some of its warmth to the
relatively cool soil, and finally exhausts it outside. In
winter, the cycle would reverse itself.
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Essentially,
the earth tubes act as heat exchangers: If the air in the
tubes is warmer than the earth, the earth soaks up and stores
the heat. If the soil is warmer than the air, it gives up heat
to the air flowing through the tubes (see diagram). In this
way, the temperature of the outside air can be altered to
provide the house with warm fresh air in winter and cool fresh
air in summer.
The
tubes themselves must be very long (between 150 and 200 feet)
so they can snake their way back and forth> through the
soil under the umbrella. (For clarity, the tubes in the
diagrams are shown straight rather than bent.) Typically,
we use earth tubes that are between four and eight inches in
diameter. We lay each pair out under the umbrella so that they
slant downhill from the house to permit water runoff and so
they both exit the ground at the same elevation.
This type of earth-tube
arrangement differs considerably from the earlier
"cool-tube" installations, which have been in use
for some time. A single cool tube allows air to flow only one
way—into the house. The house can inhale, but it cannot
exhale. To exhaust stale air in winter, a window or vent must
be opened, which would dump large quantities of heat outside.
Also, lacking the insulating/ water-shedding umbrella, cool
tubes do not have the warm earth environment that allows the
air in the tubes to be heated as well as cooled. Properly
coupled, the open-loop, convective-heat-flow earth-tube system
and the PAHS sys-tem can provide free, year-round heating,
cooling, and ventilation for the earth-sheltered home.
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Figure
24 Second generation umbrella home in Missoula,
Montana was constructed by Tom Beaudette, the engineer
of Geodome.
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This still-experimental
housing technology is already being used to satisfy at least a
portion of the heating needs for several recently constructed
earth-sheltered homes. It is also being built into a number of
full-fledged PAHS earth shelters, such as the house depicted
above, which was constructed by Tom Beaudette, the engineer of
the Geodome.
Detailed explanations of
these concepts are provided in my 152-page book: Passive
Annual Heat Storage, Improving the Design of Earth Shelters.
It's available directly from the Rocky Mountain Research
Center
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the construction of the
house in the slide show are illustrated in this simple animation.
A multi-roomed structure can be designed and formed using
intersecting earthen domes and earth-formed doorways. They can be dug at different grade depths
and heights for varying purposes and levels of security ... from totally
above ground to partially below grade, and completely below grade. The
animation illustrates one half the dome above grade, the lower half
below, with the entire structure then bermed with earth.
