Increasing amounts of insulation are used in buildings to conserve energy. In buildings of noncombustible construction, insulation is usually placed in the wall cavity with or without an air space. The use of foamed plastic insulation has been restricted due to its combustibility characteristics when tested by methods adopted for regulatory purposes. A series of tests on foamed plastics and con ventional insulation materials show that these methods do not realistically assess the fire performance of foamed plastic insulation when used in cavity walls.