Rigid boards are plastic foams or fibrous materials pressed or extruded into board-like forms. Common materials include polystyrene, urethane, or glass fiber. Polystyrene and urethane have superior insulating qualities, but when used on interior wall, require a minimum 1/2 inch thick fireproof covering such as gypsum wall board. Rigid board has the advantages of structural strength, low weight and high R-value per inch. To achieve an R-30 using glass fiber blankets would require roughly 8.5 in. With rigid urethane boards, R-30 can be attained with only 5.5 in. Rigid boards come faced and unfaced. Some are faced with a reflective material that acts as a vapor barrier and reduces heat flow when facing a dead air space.