An earth-sheltered home is a house built partially or totally underground. Totally underground homes are in a minority and are usually built around a central atrium which is open to the sky, much like similar homes sometimes built by the ancient Romans. The majority of earth sheltered homes are built in the side of a hill or an artificial hill (usually called berming) is created around three of the sides and over the roof. The remaining side is entirely covered with windows to allow passive solar heating and a maximum of natural light from the sun. The hill is chosen or the berms are constructed so that the windowed side faces south, in temperate areas of the Northern hemisphere and north in temperate areas of the Southern hemisphere.
In theory, an earth-sheltered home can give better insulation against the elements, reducing heating and cooling costs and also reduce the noise levels coming from outdoors. In practice it is very difficult to design and implement such a house given the issues of water seepage, internal condensation, cave-like acoustics, and the lack of construction companies with experience in building residences underground.
In the 1970s and 1980s, Earth-sheltered homes were popularized by the back to the land movement as a form of self-sufficient homes.
The most famous and probably the largest earth-sheltered home belongs to Bill Gates, who had it built over a period of several years on a heavily wooded site on the shore of Lake Washington. It is an excellent example of the lack of obtrusiveness of this kind of home, since it appears much smaller than it actually is, when seen from the lake.
Earthsheltering is the architectural practice of using earth for external thermal mass against building walls.
The principle is that the earth, because of its high density, undergoes slow temperature changes and thus presents a fairly constant exterior temperature at the wall.
Thus, at the base of a deep earth berm, the house is heated against an exterior temperature gradient of perhaps ten to fifteen degrees, instead of against a steeper temperature grade where air is on the outside of the wall instead of earth.
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.
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.
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