The thermal mass needed in passive solar homes such as this one in Naperville, Illinois, calls for concrete. Construction of homes like this one may lead to wider use of concrete for residential first floors, as contractors develop efficient forming methods and the buying public recognizes fire safety, strength and other advantages of concrete in addition to its good heat storage capacity.

The home in Naperville has a southern exposure primarily of glass so that passive solar energy can be fully utilized. Thermal mass is the heart of the system designed for this home and that mass is provided by a first floor of concrete. A plenum or air chamber is created by suspending the basement ceiling 2 inches below the concrete floor beams. The concrete serves as the primary collector and retainer of the heat being generated during daylight hours. The properties of the concrete make it the ideal medium for holding the heat. Warm air which rises from the floor is siphoned off at the top of the room and recirculated to the plenum.

The concrete first floor, a composite structure made of precast prestressed beams and a 3-inch-thick cast-in-place slab, was planned with simplicity of construction in mind. Pockets to support the prestressed beams were carefully blocked out when the basement walls were cast. Wall thickness was reduced at the top three inches, providing a bearing surface for slab edges and permitting the slab to be finished flush with the top of the wall. Loop-shaped rebars projecting from the top of the beams served as connection points for lifting cables, and also function as shear connectors to maintain structural continuity of the finished floor. The beams were 6 inches wide by 10 inches deep and 25 feet long and weighed in at about 1600 pounds each. The beams were strong enough to support slab forms without shoring from below.