Insulated shell structures made by shotcreting a construction process originally used to build such structures as church buildings, homes and warehouses is now being used for earth-sheltered buildings as well. Before being covered or bermed with earth, the structures have two principal components: polyurethane foam and thin shell concrete. Construction begins by placing a circular concrete foundation, in which steel reinforcing bars are embedded for anchoring the dome. Next, a plastic balloon shaped to the size of the exterior is attached to the foundation and inflated. Urethane foam is then sprayed onto the interior surface of the air form, usually to a depth of 4 inches. Expanding to about 30 times its original volume, the cellular foam adheres to the balloon and assumes its hemispherical shape.
Spaces for windows, doors and skylights are blocked out, and any electrical or plumbing work required in the dome walls is embedded in the polyurethane. Steel-fiber-reinforced shotcrete is then applied in layers on the inside surface of the urethane foam, usually to a depth of about 2 « inches, though sometimes less. An alternative is regular shotcrete with conventional steel reinforcement (normally welded wire fabric). Finally, after shotcreting, the plastic balloon is removed and an elastomeric urethane coating is applied to the exterior of the shell to protect the polyurethane insulation.
The thermal shell provides substantial savings in energy costs because of several factors, one of which is that the sprayed-on urethane insulation provides a good thermal resistance (a 4-inch urethane foam thickness has an R-value of 28) and produces a monolithic exterior that reduces unwanted air infiltration in portions of the structure that are not earth sheltered.