Abstract: A building panel comprises a rectangular substantially flat block of foam material having a plurality of parallel grooves in the inner face thereof. A plurality of similar panels are assembled in edge-to-edge relationship to form a wall and a spacer is received within the aligned grooves of a plurality of building panels. A second wall spaced from the first wall is formed of a similar plurality of panels and the spacer member is received within the opposed grooves of the inner faces of both walls so that a space is formed between the walls. Opposed blocks in the walls are clamped together to align the walls.