Foamed plastics must be covered for fire protection when used in living areas such as the basement. A space is considered a basement when it is used for occupancy, exceeds 6' in height (1.8 m.), is used as a mechanical plenum for supply or return air, or contains flue pipes (except PVC pipes from high-efficiency appliances). If the space is not a basement it can be considered a crawlspace. Foamed plastics in a crawlspace are not required to be protected from fire and smoke spread. In single family homes, the sub floor above the crawlspace functions as the required protection of the living area from the adjacent concealed crawlspace and foamed plastics.