Abstract: A method for forming building walls by starting with blocks of expanded polystyrene, for example in typical 4'.times.16' lengths. Spaced apart vertical voids are formed in the polystyrene and a depression along the top of the block of polystyrene is provided for interconnection with the vertical voids to form concrete can be poured into the vertical voids to form vertical posts and concrete can be poured into the horizontal depression to form horizontal beams, thereby forming wall sections which are well-insulated because of the high R value of the expanded polystyrene and which have great rigidity because of the concrete posts and beams. An insulated roof structure is formed having air passageways formed over the insulation and leading to a roof top ventilator.