The term "earth home" is somewhat generic and can be applied to many different kinds of environmentally sound housing. For instance, underground homes, earth berm, sheltered or rammed houses fall under this category. A bermed house may be built above grade or partially below grade, with earth covering one or more walls. An “elevational” bermed design exposes one elevation or face of the house and covers the other sides—and sometimes the roof—with earth to 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 that common areas and bedrooms share light and heat from the southern exposure. This can be the least expensive and simplest way to build an earth-sheltered structure. Strategically placed skylights can ensure adequate ventilation and daylight in the northern portions of the house.
In a penetration bermed design, earth covers the entire house, except where there are 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 and access to natural light from more than one side of the house.
Best of all, they are just plain cheaper to build.
Everything's cheaper about these houses. Fewer doors, fewer windows, and less siding. ... In the exterior of a typical home you're maintaining four sides of the house plus the roof, in an earth-sheltered home you're only maintaining one side and a roof.