Earth-sheltered design employs the earth as a major component of a building’s thermal control system. A house that is surrounded (completely or partially) by earth that stays at a steady 55-60° temperature year round requires less heating in the winter and less cooling in the summer. For comparison, a house that is exposed to 20° winter air needs to be heated 50° to be a comfortable 70°, while a house exposed to 55° earth only needs to be heated 15° to reach that same comfortable 70°.
In general, earth sheltered houses are less susceptible to changes in outside air temperature, so inside temperatures remain relatively constant. Earth sheltered design can also offer other benefits, including reduced noise, lower impact on the landscape, and less vulnerability to high wind, hailstorms, and other extreme weather.
One large challenge to constructing an earth sheltered building is water – all earth sheltered buildings must have extra precautions against moisture problems. Some sites are not suitable for earth sheltered design due to high water tables or low permeability soils. Earth sheltered buildings must also be designed to resist the pressure of placed on walls by the surrounding earth.
There are two basic types of earth-sheltered houses: underground and bermed (or banked with earth). The difference is in the degree to which the house is buried in or surrounded by earth. Bermed houses, which are partially surrounded by earth, are much more common than fully underground houses.