Site-Specific Factors for Earth-Sheltered Home Design
Before deciding to design and build an earth-sheltered house, you’ll need to consider your building site’s climate, topography, soil, and groundwater level.
Studies show that earth-sheltered houses are more cost-effective in climates that have significant temperature extremes and low humidity, such as the Rocky Mountains and northern Great Plains. Earth temperatures vary much less than air temperatures in these areas, which means the earth can absorb extra heat from the house in hot weather or insulate the house to maintain warmth in cold weather.
Topography and Microclimate
The site’s topography and microclimate 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. A south-facing slope in a region with moderate to long winters is ideal for an earth-sheltered building. South-facing windows can let in sunlight for direct heating, while the rest of the house is set back into the slope. In regions with mild winters and hot summers, a north-facing slope might be ideal. Careful planning by a designer familiar with earth sheltering can take full advantage of the conditions on your particular site.
The type of soil at your site is another critical consideration. Granular soils such as sand and gravel are best for earth sheltering. These soils compact well for bearing the weight of the construction materials and are very permeable, allowing water to drain quickly. The poorest soils are cohesive, like clay, which may expand when wet and has poor permeability.
Professional soil tests can determine the load-bearing capability of soils at your site. Soil radon levels are another factor to consider, because high concentrations of radon can be hazardous. There are, however, methods for reducing radon buildup in both conventional and earth-sheltered dwellings.
The groundwater level at your building site is also important. Natural drainage away from the building is the best way to avoid water pressure against underground walls, but installed drainage systems can be used to draw water away from the structure.
Construction Materials and Considerations for Earth-Sheltered Homes
The construction materials for each earth-sheltered structure will vary depending on characteristics of the site and the type of design. Materials must, however, provide a good surface for waterproofing and insulation to withstand the pressure and moisture of the surrounding ground.
Concrete is the most common choice for constructing earth-sheltered buildings, because it strong, durable, and fire resistant. Concrete masonry units (also called concrete blocks) reinforced with steel bars placed in the core of the masonry can also be used, and generally cost less than cast-in-place concrete.
Wood can be used in earth-sheltered construction for both interior and light structural work. Steel can used for beams, bar joists, columns, and concrete reinforcement, but 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.