A composite building construction system comprising a frame, an expanded wallboard, and a concrete layer. The wallboard comprises an outer portion and inset portion wherein the wallboard is attached to the frame with the outer portion covering an exterior side of the frame and the inset portion is disposed within the frame. The wallboard thus provides exterior and interior insulation for the system. The concrete layer is sprayed on to the outer portion of said wallboard, forming a strong durable composite construction system. One or more rebar supports may be attached to the frame at the exterior side by key bolts and are spaced apart from the wallboard. A wire grid may be attached to the key bolts whereby the grid and the rebar supports provide a support structure for the concrete layer. The frame comprises steel and the wallboard comprises insulative expanded polystyrene.