Q: I am planning to build a house using structural concrete insulated panels (SCIP)—that is, foam panels that have wire grids on each side and are faced with shotcrete, such as 3-D panels or the Solarcrete system. Is it possible to use foamed or cellular concrete as the facing for these systems? What are its structural limitations? What are its application limitations? Can it be shotcreted onto vertical surfaces? Can it be used with fiber reinforcement? Can this material be used as a roof with SCIP? Can you provide information on various foaming agents, their pros/cons, cost benefits, etc.?

A: A number of proprietary SCIP systems such as you describe are suitable for residential applications, but they depend on wire-reinforced concrete on both sides of the panels for their structural strength. This facing typically is shotcreted into place to a thickness of 2 inches or more. Depending on the system, the shotcrete mix requires a minimum compressive strength of 2500 to 4000 psi, or more.

Foamed or cellular concrete basically consists of water, cement, and preformed foam, with or without aggregate or chemical admixtures. It is a lightweight, low-density material of limited strength, with a maximum oven-dry density of 50 pounds per cubic foot and a maximum compressive strength of about 750 psi. When mixed, cellular concrete has a liquid consistency, and it usually is placed by pumping. It can't be shotcreted onto vertical surfaces.

Cast-in-place, low-density cellular concrete commonly is used on roof decks and in geotechnical applications. On roof decks, the material is used to provide a rooting base, thermal insulation, and drainage slope for flat-roofed industrial and commercial buildings. In geotechnical applications, cellular concrete with low-compressive strength is used to replace poor soils or as fill. You can learn more about cellular concrete from ACI 523.1R-06 “Guide for Cast-in-Place Low-Density Cellular Concrete.”

You may be confusing foamed concrete with autoclaved cellular concrete (also called autoclaved aerated cellular concrete), which is a manufactured building material produced in block or panel form and pressure steam cured. It has densities ranging from 19 to 63 pounds per cubic foot and compressive strengths between 300 and 1500 psi. More information is available in ACI 523.2 “Guide for Precast Cellular Concrete.”

When you're ready to proceed with your plans, consult the SCIP system manufacturers for system requirements and recommended details.