Q: I have a few questions concerning earth sheltered building methods. First, is it more cost-effective to set such a dwelling back into the side of a hill, or merely to berm around the structure? Second, how should different soil types be dealt with? Finally, how much earth cover, if any, is desirable on the roof? (I live in an area that experiences between 4,000 and 6,000 heating degree days annually.)
A: In your climate, it'd be to your advantage to orient your house in a southerly direction in order to make use of solar gain. Southern builders, who on the other hand usually find that cooling is more of a problem than heating, may choose a different orientation to aid in dissipating heat. If your land has a south-facing slope, it'd be more economical to build into the hill because less earth moving would be required. If your site is flat, however, you may find that excavating just 30" down will yield enough material to berm around the house without having to haul any additional fill. And in terms of energy savings, an earth bermed dwelling can be 95% as efficient as a completely below-grade structure.
As far as soil types go, the most important consideration is not to backfill directly against the walls with earth having poor percolation, such as clay. Instead, coarse sand or gravel should be placed against the house to allow water to find its way to the footing drains, thereby eliminating the possibility that freeze-expansion pressure will stress the waterproofing membrane. (Of course, if appropriate backfill isn't available on the site, it'll have to be hauled in.)
The earth layer on the roof should be thick enough to support a living ground cover, but it's not the place for growing trees and shrubs! Heavy earth roofs (2 to 4 feet thick) are not cost-effective, since they require expensive structures to support their load. In terms of insulating value, an extra 2 inches of rigid foam would be a better thermal barrier (and less costly to install) than an additional foot of earth. As opposed to having no earth layer at all, the advantages of the natural covering are evaporative cooling in the summer, aesthetics, longevity, gradual drainage, and protection from the elements — all of which can be accomplished with no more than an 8-inch earthen roof here in northern New York (in drier areas, where water weight won't be a problem, 12 inches might be desirable).
Rob Roy is the director of the Earthwood Building School and author of several books on earth-sheltered housing and cordwood masonry construction.