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, and would therefore
make them a better choice than batts or blankets if it were not for their flammability.
When installed inside a house, the boards require an additional covering of
a minimum of 1/2 in.-thick fireproof material such as gypsum wall board. Rigid
board has the advantages of structural strength, low weight and high R 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.
