When Dan and Jill Mullendore decided to share their home with her parents, they knew more space would be needed. After they conferred with builder friend Herb Strack, the plan for a 24x28 foot trilevel home addition emerged. It featured a first-floor in-law apartment with a living room, bedroom, kitchen, and bath; a second-floor extension of the master bedroom; and a basement level with one exposed window wall and the other walls earth-sheltered. Concrete was Strack's structural material of choice. His previous experience with concrete placed in stay-in-place expanded polystyrene (EPS) forms had convinced him that the addition could be made easy to heat in Michigan's cold winters. STARTING THE WALL FORMS The first course of expanded polystyrene wall forms was set while the footing concrete was plastic, and rebar dowels were inserted into the concrete at 12-inch spacings. ERECTING FLOOR PANELS As soon as the grade slab had cured sufficiently, workers roughed in the interior partitions and erected shoring to support the first-floor panels. These 8x24-foot stressed-skin panels were 10 1/4 inches thick, with an insulating core and surfaces of 1/2-inch oriented strandboard. CONCRETING THE WALLS With the first-floor panels in place, the wall forms were erected below, and reinforcing steel was set into position within the form voids. The 3000-psi concrete was placed using a boom pump. The EPS form units measure 16x48 inches, which permits the core concrete to be 6 inches thick.