Dry rot and other conditions detrimental to buildings (particularly wood and timber structures) can develop in enclosed spaces. Providing adequate ventilation can reduce the occurrence of these problems. Crawl space vents are openings in the wall which allow air movement. Such vents are usually fitted with metal grills, mesh, or louvres which block the movement of rodents and vermin. One common rule is to provide vents in cross sectional area equal to 1/150 of the floor area served.