An earth-sheltered building is either banked on one or more sides with earth, or built partially or entirely underground. A more precise definition of an earth-sheltered building is one that has at least 50 percent of its exterior surfaces (wall and roof area) sheltered by earth. Earth sheltering reduces the energy needed to heat or cool the building by (1) preventing the leakage of air out of and into the building and (2) placing an insulating barrier of earth between the walls of the buildings and extreme outdoor temperatures.
Earth sheltering does not need to result in a dark or damp environment. By exposing the south-facing wall to the outdoors, and making wise use of skylights or clerestory windows, an earth-sheltered building can be bright and airy and partially solar heated. These buildings also have other advantages, such as long life expectancy due to their heavy masonry construction, low maintenance and fire resistance as well as increased comfort because of minimal temperature swings and few drafts. Construction of earth-sheltered buildings is more demanding than for above-ground structures. They are generally more costly to build and the services of a structural engineer are required during design stages.
Soil and ground water conditions will determine structural and waterproofing requirements. For example, some soils are more susceptible than others to expansion when wet or frozen and will place more demands on the structure of the building. Permafrost should not be disturbed and therefore earth sheltered houses should not be built in permafrost.
The topography of the site, or lay of the land, will affect wind flow and drainage patterns, and will 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. Buildings on flat ground are bermed more easily on one or more sides. Berming is the practice of banking earth up against the walls of the building.
When a house is built almost entirely underground, the first consideration is to provide natural light and passive solar heat to the living and sleeping spaces.
Once again, an exposed, glazed south-facing wall is an excellent approach (see Figure 1). This can also be modified by building a greenhouse along the south wall. In either case, the floor plan is arranged so that the main spaces share light and heat from the southern exposure.
Another approach is the central atrium (see Figure 2). This allows for a floor plan that surrounds an outdoor space on three or four sides. The strategic use of clerestories and skylights will also allow more latitude in the arrangement of interior spaces. A properly executed design will leave the occupant with the feeling that there is very little difference from living above the ground.
With either approach, it is important to provide fire escape routes from bedrooms; building codes require this, and personal safety would demand this.
The materials used to build an earth-sheltered house must be able to withstand the stress imposed by the surrounding earth. When soil is wet or frozen, it exerts even greater pressure on the walls and floors of the building. Pressure also increases with depth. The common building materials (concrete and reinforced masonry, wood, and to some extent, steel) are all suitable.
Concrete is usually the first choice for construction of earth-sheltered buildings. Not only does it have the necessary strength, but it has the added advantages of durability and fire resistance. Unreinforced concrete, poured at the site, is used for footings and floor slabs and can be used for walls at shallow depths. Reinforced concrete, on the other hand, has the ability to resist loads at any reasonable depth and can be used for floors, walls and roofs.
Concrete will absorb and store solar heat as part of a passive heating system, and its heat absorbing qualities help to prevent large temperature swings.
Pre-cast concrete has all the advantages of poured concrete and more. It meets all structural requirements, and construction proceeds more quickly with pre-cast units. Special care must be taken, however, in making the joints between sections watertight.
Concrete block, surface bonded with glass fibre, can be used for walls up to two storeys but needs reinforcement at depths greater than six to eight feet. Cracks in mortar joints must be sealed carefully, and the porous quality of block demands extra care during the waterproofing process.
Wood has been used for walls and roofs of earth-sheltered buildings. It is less expensive than concrete but is not as strong. It needs to be pressure-treated with preservatives that will enable it to endure moist conditions. Conventional framing techniques can be used, and post and beam construction has also proven successful.
Steel can be used for beams and column supports, but it has been used in other less conventional ways as well. For example, large steel culvert sections have been used to form the shell for dwellings, finished on the interior and waterproofed on the exterior. They have more than adequate strength since they were designed originally for underground use. They are also impervious to water seepage but they must be treated to prevent corrosion.
Waterproofing is extremely important to an earth-sheltered house, and it must be done right the first time to avoid the major excavation that is necessary to locate and repair leaks. Underground structures must withstand prolonged periods of water pressure. The common waterproofing techniques used for basements, such as coating the exterior walls with asphalt, are not suitable for earth-sheltered dwellings.
The first step in waterproofing occurs during the site selection. The best way to avoid water pressure against underground walls is to choose a site where water will naturally drain away from the building. Survey the site for low spots and areas where water will collect. If at all possible, build above the water table. If the water table cannot be avoided, drainage systems can be designed to draw water away. Earth-bermed houses have minimal water problems because they are built on or just below ground level and moisture in the berms drains off by gravity.
The waterproofing materials for the building should meet three important criteria: (1) they should have a long life expectancy underground; (2) they should have resealing capacity at underground temperatures; and (3) they should have good crack-bridging capability.
At present, the most widely used products for waterproofing systems are asphalt and pitch-impregnated membranes, liquid polyurethane and bentonite.
Built-up asphalt or pitch membranes have been used successfully in underground structures for both roofs and walls. The felt products normally used in built-up roofs are not recommended because they tend to rot under prolonged exposure to moisture. Fiberglass fabric should be used in its place. The disadvantage to built-up membranes is that they lack good resealing and crack-bridging qualities.
In masonry construction, the insulation is usually placed on the exterior of the building. This allows the solar energy that is collected and other heat that is generated to be absorbed by the masonry and retained within the building interior. In most cases the waterproofing is applied to the building first, before the insulation. Because the insulation is exposed to the earth, it should be a closed-cell product such as extruded polystyrene. It must also be able to withstand the pressure of back-filling without being compressed. Air Quality and Ventilation
Because of its tight construction, the earth-sheltered home eliminates most of the causes of air-infiltration. A well designed home should include a ventilation system so that stale air can be exhausted and fresh air introduced to the living spaces. Some methods to provide ventilation include air-to-air heat exchangers, forced air furnaces with a fresh air supply to the return air plenum, and kitchen and bathroom exhaust fans used in conjunction with an open window to supply make-up air.
This factsheet is a condensed version of the fact sheet Earth-Sheltered Houses, published by the State of Alaska, Department of Commerce & Economic Development, Division of Energy & Power Development.
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