Code of Federal Regulations Title 24. Housing and Urban Development § 24.3285.505 Crawlspace ventilation

(a) A crawlspace with skirting must be provided with ventilation openings. The minimum net area of ventilation openings must not be less than one square foot (ft.²) for every 150 square feet (ft.²) of the home's floor area. The total area of ventilation openings may be reduced to one square foot (ft.²) for every 1,500 square feet (ft.²) of the home's floor area, where a uniform 6–mil polyethylene sheet material or other acceptable vapor retarder is installed, according to § 3285.204, on the ground surface beneath the entire floor area of the home.

(b) Ventilation openings must be placed as high as practicable above the ground.

(c) Ventilation openings must be located on at least two opposite sides to provide cross-ventilation.

(d) Ventilation openings must be covered for their full height and width with a perforated corrosion and weather-resistant covering that is designed to prevent the entry of rodents. In areas subject to freezing, the coverings for the ventilation openings must also be of the adjustable type, permitting them to be in the open or closed position, depending on the climatic conditions.

(e) Access opening(s) not less than 18 inches in width and 24 inches in height and not less than three square feet (ft.²) in area must be provided and must be located so that any utility connections located under the home are accessible.

(f) Dryer vents and combustion air inlets must pass through the skirting to the outside. Any surface water runoff from the furnace, air conditioning, or water heater drains must be directed away from under the home or collected by other methods identified in § 3285.203.