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.
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.
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