Successfully used to build houses in Europe for many years, "precast autoclaved, aerated concrete" (PAAC) blocks are now being manufactured in Georgia by Hebel Southeast. This system employs an oversized, lightweight solid block that is laid-up with very strong, thinset mortar.

During manufacture, the raw material is mixed from cement, fine aggregates, and a natural expansion agent. Once molded, it is given a moisture and heat treatment under pressure, called autoclaving. As a result, the material rises like bread dough, with thousands of tiny air pockets. The resulting block is durable, lightweight, and a relatively good insulator. Even more impressive is the fact that it can be cut and drilled like wood, using ordinary carpentry tools, and receive nails or screws.

Technically, these blocks have an insulation value of about R-10, but a wall built of these effectively offers about three times that because it blocks air infiltration and provides thermal storage mass.

The 8-by-8-by-24-inch-long blocks weigh about 28 pounds (conventional poured concrete weighs about 150 pounds per cubic foot). They're laid with a tiny, 3mm mortar joint between blocks; the latex mortar is trowelled on like a thick paint.

Cement-based stucco can be applied directly to the block surface. Or the block wall can be finished with any conventional siding on the exterior or drywall over furring strips inside. Channels for wiring are carved out with a router.

This system is currently available only in the Southeast. Finished inside and out, this system runs about $2.50 to $3 on average—about 1% to 5% more than conventional wood-stud or masonry construction with insulation but, as a Hebel spokesperson points out, "It's like buying a good HVAC system—you get a return on your investment over time."

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