Most insulated concrete block systems consist of lightweight concrete blocks whose voids are filled with rigid foam. But one of the most intriguing new systems uses no foam at all. The Hebel Wall System consists of solid aerated- concrete blocks (made from sand, cement, lime, and an expanding agent) that are as strong as concrete at onefifth the weight. And an 8-inch Hebel wall has an overall insulating value of R-30. Hebel blocks are laid up with thinset mortar and require no surface bonding. The lightweight blocks can be sawed, nailed, and routed with normal woodworking tools. Hebel built more than 70 homes in the Southeast last year at an average cost that was 2% higher than a