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Earth-Sheltered
Houses
Let's assume you're in the market for a new
home. Let's further assume that, like many other people, you're concerned about
the limited supplies of domestic oil and gas, the unpredictable cost, and the
environmental price tag attached to the continued use of these fuels.
Is there any way to reconcile your
concerns and still build your dream home? The solution might lie in
earth-sheltered housing. These structures offer many energy-efficient features
and are often designed to use solar energy for heating and cooling. Some designs
make use of recycled materials in their construction.
If you do your homework, earth-sheltered
construction can be an attractive and rewarding choice in housing. The following
sections—Advantages, Disadvantages, Beginning Your Project,
and Cost—will answer some questions you may have. Once you've decided
on an earth-sheltered house, the sections on Design, Construction
Considerations, including location, climate, site, and soils, and Construction
Materials can help you make the house a reality.
Advantages
There are many advantages to
earth-sheltered construction. An earth-sheltered home is less susceptible to
the impact of extreme outdoor air temperatures, so you won't feel the effects
of adverse weather as much as in a conventional house. Temperatures inside the
house are more stable than in conventional homes, and with less temperature
variability, interior rooms seem more comfortable.
Because earth covers part or all of
their exterior, earth-sheltered houses require less outside maintenance, such
as painting and cleaning gutters. Constructing a house that is dug into the
earth or surrounded by earth builds in some natural soundproofing. Plans for
most earth-sheltered houses "blend" the building into the landscape
more harmoniously than a conventional home. Finally, earth-sheltered houses
can cost less to insure because their design offers extra protection against
high winds, hailstorms, and natural disasters such as tornados and hurricanes.
Disadvantages
As with any type of unusual
construction, there are some disadvantages associated with earth-sheltered
housing. Principal downsides are the initial cost of construction, which may
be up to 20% higher, and the level of care required to avoid moisture
problems, during both the construction and the life of the house. It can take
more diligence to resell an earth-sheltered home, and buyers may have a few
more hurdles to clear in the mortgage application process.
Beginning Your Project
You may want to start your project by
contacting the organizations that can help you locate architects, builders,
and contractors with earth-sheltered building experience (a few are listed in
the Source List). The experienced builders in
your location can also ensure that construction plans meet area building code
requirements, which can depend on a local official's interpretation.
Earth-sheltered designs can present some potential difficulty, such as
conformance to minimum window sizes in each room prescribed by fire codes or
building codes. Other problematic areas are roofing specifications and
insulation requirements specified by codes; earth may or may not be considered
an acceptable substitute for materials specified. Builders can inform you of
the local guidelines, and may also provide cost comparisons between
earth-sheltered housing and conventional housing in your area.
Using experienced professionals will
reduce your personal time investment in researching details of earth-sheltered
construction. Securing financing may also be easier if you work with those who
have firsthand knowledge of earth-sheltered construction and its benefits.
Cost
Comparing cost factors for building an
earth-sheltered house to those for building a conventional house can be like
comparing apples and oranges. Costs vary by region, depending on the area's
construction activity and cost of materials. Many earth-sheltered houses are
built by their owners and are customized in different ways, adding amenities
that can dramatically increase final costs.
However, when you factor costs, remember
to consider important expenses that occur over the life of a house, such as
exterior maintenance and utility costs. Greater initial investment can
actually mean your ongoing costs, such as heating, cooling, and maintenance,
will be reduced. Making the best use possible of natural resources may be one
of an earth-sheltered house's greatest advantages. Therefore, in comparison to
standard houses, while earth shelters tend to be slightly more expensive to
build, these higher initial costs may be offset by the lower energy costs
after completion. And while energy savings have been documented by families
living in earth shelters, attributing that savings solely to the
earth-sheltered design would be difficult, because most earth-shelters
incorporate a wide variety of energy-efficient features.
Design
There are two basic types of
earth-sheltered housing—underground and "bermed" (or banked with
earth). Certain characteristics such as the location and soils of your site,
the regional climate, and design preferences are central to which type will
work best for you.
Underground housing means an entire
structure built below grade or completely underground. A bermed structure may
be above grade or partially below grade, with outside earth surrounding one or
more walls. Both types usually have earth-covered roofs, and some of the roofs
may have a vegetation cover to reduce erosion.
From these two basic types, three
general designs have been developed. They are the:
- atrium (or courtyard)
plan—an underground structure where an atrium serves as the focus of the
house and the entry into the dwelling;
- elevational plan, a bermed
structure that may have a glass south-facing entry; and the
- penetrational plan, which is
built above or partially above grade and is bermed to shelter the exterior
walls that are not facing south.
An earth-covered dwelling may have as
little as 6 to 8 inches (0.2 meters) of sod or as much as 9 feet (2.7 meters)
of earth covering the structure. An atrium design offers an open feeling
because it has four walls that give exposure to daylight. This design uses a
subgrade open area as the entry and focal point of the house. It is built
completely below ground on a flat site, and the major living spaces surround a
central outdoor courtyard. The windows and glass doors that are on the exposed
walls facing the atrium provide light, solar heat, outside views, and access
via a stairway from the ground level. Atrium/courtyard homes are usually
covered with less than 3 feet (0.9 meters) of earth primarily because there is
no benefit in energy efficiency from greater depths. This style also offers
the potential for natural ventilation.
The atrium design is hardly visible from
ground level and barely interrupts the landscape. It also provides good
protection from winter winds and offers a private outdoor space. This design
is ideal for an area without scenic exterior views, in dense developments, and
on sites in noisy areas. Passive solar gain—heat obtained through windows—might
be more limited, due to the window position in an atrium plan. Courtyard
drainage and snow removal are important items to consider in design.
The elevational and penetrational plans
are more conventional earth-sheltered housing designs. Elevational plans
expose one whole face of the house and cover the other sides—and perhaps the
roof—with earth. The covered sides protect and insulate the house. The
exposed front of the house, usually facing south, allows the sun to light and
heat the interior. The floor plan is arranged so common areas and bedrooms
share light and heat from the southern exposure. This type of house may be
placed at varying depths below ground level and is usually set into the side
of a hill. The view provided will be one of landscape, rather than open sky,
as in the atrium design. A structure designed in this way can be the least
expensive and simplest to build of all earth-sheltered structures.
However, the elevational design may have
limited internal air circulation and reduced daylight in the northern portions
of the house, although there are ways to alleviate these problems by using
skylights. The wide design of the house can be offset by close attention to
architectural details, landscaping, and exterior materials.
In the penetrational plan, earth covers
the entire house, except where it is retained for windows and doors. The house
is usually built at ground level, and earth is built up (or bermed) around and
on top of it. This design allows cross-ventilation opportunities and access to
natural light from more than one side of the house.
Construction
Considerations
The climate in your geographical area
will determine whether an earth-sheltered house can be a practical housing
solution. Studies show that earth-sheltered houses are more cost-effective in
climates that have significant temperature extremes and low humidity, such as
the Rocky Mountains and northern Great Plains. Earth temperatures vary much
less than air temperatures in these areas, which means the earth can absorb
extra heat from the house in hot weather or insulate the house to maintain
warmth in cold weather.
The site's topography and microclimate
determine how easily the building can be surrounded with earth. A modest slope
requires more excavation than a steep one, and a flat site is the most
demanding, needing extensive excavation. A south-facing slope in a region with
moderate to long winters is ideal for an earth-sheltered building.
South-facing windows can let in sunlight for direct heating, while the rest of
the house is set back into the slope. In regions with mild winters and
predominantly hot summers, a north-facing slope might be ideal. Every site
differs, but generally southern exposures offer more sun and daylight
throughout the year than north-facing slopes. Most designs can be built to
take advantage of each site.
The type of soil at your site is another
critical consideration. Some types of soil are more suitable than others for
earth-sheltered construction. For example, the best soils are granular, such
as sand and gravel. These soils compact well for bearing the weight of the
construction materials and are very permeable, which means they allow water to
drain quickly. The poorest soils are cohesive, like clay, which may expand
when wet and has poor permeability. Soil tests, offered through professional
testing services, can determine load-bearing capability of soils and possible
settlements that may occur.
Radon is an invisible and odorless
radioactive gas produced naturally when uranium in rock decomposes. It is
found in the soil and in outdoor air in harmless amounts, but can reach
dangerous levels when trapped. Radon levels are another factor to consider in
locating your home. Areas with high concentrations of radon can be hazardous,
although there are methods that can reduce radon buildup in both conventional
and earth-sheltered dwellings.
The groundwater level at your building
site is another important consideration. Building above the water table is
almost essential. Choosing a site where the water will naturally drain away
from the building is the best way to avoid water pressure against underground
walls. The site should be surveyed for low spots and areas where water will
collect. Seasonal or regular surface water flows should be channeled away from
the structure. Drainage systems must be designed to draw water away from the
structure to reduce the frequency and length of time the water remains in
contact with the building's exterior. Underground footing drains similar to or
greater than those required by a house with a basement are necessary in many
cases.
Construction Materials
The construction materials for each type
of structure will vary, depending on characteristics of the site, climate,
soils, and design. However, general guidelines show that houses more deeply
buried require stronger, more durable construction materials. Materials must
provide a good surface for waterproofing and insulation to withstand the
pressure and moisture of the surrounding ground. When soil is wet or frozen,
the pressure on the walls and floors increases. Pressure also increases with
depth, so materials such as concrete and reinforced masonry, wood, and steel
are all suitable. To reduce your transportation costs, it helps if appropriate
materials are locally available.
Concrete
Concrete is the most common choice for constructing earth-sheltered buildings.
Not only is it strong, it is also durable and fire resistant. Several forms of
concrete are used. Lightly reinforced concrete, which is poured and reinforced
at the site, is used for noncritical structural elements such as concrete
foundations, floor slabs, and exterior walls with less than 6 feet (1.83
meters) of earth cover. Precast reinforced concrete can resist loads at any
reasonable depth and can be used for floors, walls, and roofs. Concrete
absorbs and stores heat, helping to prevent temperature swings that can damage
some building material.
Precast concrete components are cured at
a plant or on-site location before they are used, thereby decreasing
construction time and cost in comparison to cast-in-place forms. The uses and
advantages of precast and cast-in-place concrete are similar, except that
precast concrete works best in simple or repeatable shapes. Special care must
be taken to make the joints between sections watertight.
Concrete can also provide supplemental
strength in other types of earthen construction. For example, a concrete
topping can be added to wooden roof planks, and cement "parging" (or
coating) can be added to walls with masonry construction before waterproofing.
Masonry
Masonry (i.e., brick or stone) can be used for walls that will receive
vertical or lateral pressure from earth cover. It is reinforced with steel
bars that are put in the core of the masonry in places of high stress, such as
weight-bearing walls or walls with earth against them. Masonry generally costs
less than cast-in-place concrete.
Wood
Wood can be used extensively in earth-sheltered construction for both interior
and structural work including floors, roofs, and exterior walls. Wood is
attractive for its color and warmth, and complements tile and masonry, as well
as concrete walls, floors, and ceilings. However, using wood as a structural
material requires wooden frame walls, which must withstand lateral pressure,
be restricted to a burial depth of one story. Beyond this depth, the rapidly
increasing cost of wood construction restricts most builders from using it as
a structural material. Although wood can cost less than other materials, it
does not offer the strength that a material such as steel does, so it may not
be the best choice for structural material in some houses. Wood must also be
treated with preservatives to prevent damage from moisture. If your structure
can make practical use of wood as a framing material, employing carpenters who
can rapidly construct a timber frame for an earth-sheltered house can decrease
labor costs.
Steel
Steel is used for beams, bar joists, columns, and concrete reinforcement. It
is particularly useful because of its high tensional and compressional
strength. The primary disadvantage of steel is that it must be protected
against corrosion if it is exposed to the elements or to groundwater. It is
also expensive, so it must be used efficiently to be economical as a
structural material.
| Building With Soil
There has been a recent surge in
homes that use soil as a primary building material. This type of
construction is literally "old as the hills." Some ancient
architecture, such as the Great Wall of China, shows the durability of
earthen construction. So building with earth and sand, always plentiful
and inexpensive, is not a new idea.
Although these houses do not typify
the earth-sheltered construction designs discussed in this publication,
earth is a major component in their construction material and many of the
same energy efficient ideas are used. Many buildings in the southwestern
United States use adobe—bricks constructed of tightly compacted earth,
clay, and straw—as their main construction material.
Another building material is
"rammed earth." This process entails mixing earth and cement,
and packing it into the wall forms with a pneumatic tamper. The result is
a rough approximation of sedimentary rock. In fact, this "stabilized
earth" achieves compressive strengths estimated to be about half that
of concrete. Walls can be made even thicker with little added cost, since
the labor and the formwork are the more costly items of a wall. Although a
steel-reinforced, eight-inch (20.32 centimeters) thick earthen wall is
strong enough for load-bearing walls, extra mass, coupled with good solar
design, can offer better insulation and increased heating and cooling
capacity. Compared to earth-sheltered houses, additional insulation may
not be necessary in rammed-earth houses, depending on the area's climate. |
Alternative Construction
Materials
A form of earth-sheltered house that has been receiving much attention is
referred to as an "Earthship." These houses are built to be
self-contained and independent; their design allows occupants to grow food
inside and to maintain their own water and solar electrical systems. Some
builders believe they have proven the design's ability to tap into the
constant temperature of the earth and store additional energy from the sun in
winter, although a back-up system, usually electric, may be recommended.
These Earthships carry out their
environmentally conscious theme by employing unusual building materials in the
form of recycled automobile tires filled with compacted earth for thermal mass
and structure. Aluminum or tin cans are also used for filling minor walls that
are not load-bearing. Foam insulation can be applied to exposed exterior or
interior walls and covered with stucco. Interior walls can also be drywalled
for a more conventional look.
Other Considerations
Waterproofing
Waterproofing can be a challenge in earth-sheltered construction. Keep in mind
these three ways to reduce the risk of water damage in your house: choose the
site carefully, plan the drainage both at and below the surface of the house,
and waterproof your house.
There are several waterproofing systems
currently in use, including rubberized asphalt, plastic and vulcanized sheets,
liquid polyurethanes, and bentonite. Each has its advantages and the one you
choose will depend on your site and house plan.
- Rubberized asphalt combines a
small amount of synthetic rubber with asphalt and is coated with a
polyethylene layer to form sheets. It can be applied directly to walls and
roofs and has a long life expectancy.
- Plastic and vulcanized sheets
are among the most common types of underground waterproofing. Plastic
sheets include high-density polyethylene, chlorinated polyethylene,
polyvinyl chloride, and chlorosulfonated polyethylene. Suitable vulcanized
membranes or synthetic rubbers include isobutylene isoprene, ethylene
propylene diene monomer, polychloroprene (neoprene), and polyisobutylene.
For all these materials, the seams must be sealed properly, or the
membranes will leak.
- Liquid polyurethanes are often
used in places where it is awkward to apply a membrane. Polyurethanes are
sometimes used as a coating over insulation on underground structures;
however, weather conditions must be dry and relatively warm during their
application. .
- Bentonite is a natural clay
formed into panels or applied as a liquid spray. The panels are simply
nailed to walls; the spray is mixed with a binding agent and applied to
underground walls. When the bentonite comes in contact with moisture, it
expands and seals out the moisture.
Humidity
Humidity levels may increase in earth-sheltered houses during the summer,
which can cause condensation on the interior walls. Installing insulation on
the outside of the walls will prevent the walls from cooling down to earth
temperature; however, it also reduces the summer cooling effect of the walls,
which may be viewed as an advantage in hot temperatures. Mechanical air
conditioning or a dehumidifier is often necessary to solve the humidity issue.
Proper ventilation of closets and other closed spaces should keep the humidity
from becoming a problem in those areas.
Insulation
Although insulation in an underground building does not need to be as thick as
that in a conventional house, it is necessary to make an earthen house
comfortable. Insulation is usually placed on the exterior of the house after
applying the waterproofing material, so the heat generated, collected, and
absorbed within the earth-sheltered envelope is retained inside the building's
interior. If insulating outside the wall, a protective layer of board should
be added to keep the insulation from contacting the earth. Depending on the
type of structure—wood, masonry, concrete, or steel—insulation may instead
be placed inside the walls before the waterproofing material is applied.
Air Exchange/Air Quality
Adequate air exchange must be carefully planned when building an
earth-sheltered dwelling. Generally, well-planned, natural ventilation or
ventilation by exhaust fans can dissipate ordinary odors. Any combustion
appliances that are installed should be "sealed combustion units,"
which have their own, direct source of outside air for combustion, and the
combustion gases are directly vented to the outside. In addition, indoor
pollutants emitted by formaldehyde foam insulation, plywood, and some fabrics
can accumulate and become an irritant if ventilation is not properly planned.
A Home for the Future
If you are looking for a home with many
energy efficient features that will provide a comfortable, tranquil,
weather-resistant atmosphere, an earth-sheltered home could be right for you.
With the general information in this publication and more details available
from the sources listed below, you are on your way to owning a home that can
protect you from the elements and the rising costs of energy and building
resources.
Source List
The following organizations can provide
you with more information on earth-sheltered construction.
American Underground-Construction
Association
511 11th Avenue South, Suite 248
Minneapolis, MN 55415
(612) 339-5403
Rocky Mountain Research Center
P.O. Box 4694
Missoula, MT 59806
(406) 728-5951
Solar Survival Architecture
P.O. Box 2009
El Prado, NM 87529
(505) 751-0462
Fax: (505) 751-1005
E-mail: earthshp@taos.newmex.com
Texas Energy Extension Service
Center for Energy and Mineral Resources
Texas A&M University
College Station, TX 77843-1243
For information about many kinds of
energy-efficient building ideas and renewable energy topics, contact:
The Energy Efficient and Renewable
Energy Clearinghouse (EREC)
P.O. Box 3048
Merrifield, VA 22116
(800) DOE-EREC (363-3732)
Fax: (703) 893-0400
E-mail: doe.erec@nciinc.com
EREC provides free general and technical
information to the public on the many topics and technologies pertaining to
energy efficiency and renewable energy.
Reading List
Complete Book of Underground Houses,
R. L. Roy, Sterling Publishing Co., 1994.
Earth Sheltered Housing, G. Klodt,
Reston Publishing Company, 1985.
Earth Sheltered Housing Design, J.
Carmody, Van Nostrand Reinhold, 1985.
Earth Sheltered Residential Design
Manual, R. Sterling, Van Nostrand Reinhold, 1982.
Earth Shelter Technology, Ll. Boyer
and W. Grondzik, Texas A&M University Press, College Station, TX, 1987.
Earthship Systems and Components,
M. Reynolds, solar Survival Press, 1991.
Passive Annual Heat Storage: Improving
the Design of Earth Shelters, J. Hait, Rocky Mountain Research Center, 1983.
- U.S. Department of Energy (DOE)
- National Renewable Energy Laboratory (NREL)
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