Durability and energy efficiency were advantages common to all of the concrete homes visited, but prices and architectural styles covered a range of tastes and pocketbooks. Three construction methods were seen in above ground homes: Shotcrete with a 4-inch shell of insulation, creating a dome; cast-in-place basement, first floor, and outer walls with insulation sandwiched in the center of the walls; and precast wall panels with a layer of insulation on the inner surface.
INFLATED BALLOON SHAPES DOME HOME
Although the dome home can be built with a basement, the Pleasantville, Iowa house is built on a 1450-square-foot slab. Heating ducts are under the slab, and a 42-inch deep footing encircles the perimeter. Polystyrene insulation was placed outside the footing to a depth of 2 feet, and dowels from the footing extend up into the base of the dome to tie the structure together. Once the foundation work was completed, a balloon form was anchored to the footing and inflated to the shape of the dome. Air pressure was maintained while all work proceeded in a controlled indoor environment. A 4-inch layer of urethane foam sprayed against the inside of the balloon provides an R-value of 32, significantly higher than most conventional residential construction requirements.
Once the urethane had cured sufficiently and necessary steel was set around the doors and windows, shotcreting of the shell began. The shotcrete is 2 to 3 inches thick at the base of the dome and only an inch thick at the top. There are no expansion joints, and therefore no cracks to permit heat loss. Because the dome is engineered to be entirely self-supporting, it requires no interior supporting walls, and floor layouts can be quite flexible.