USDA – Crawl Space & Attic Space


The space between the bottom of the floor joists and the earth under any building shall be provided with a sufficient number of ventilating openings through foundation walls or exterior walls to ensure ample ventilation. Such openings shall be covered with a corrosion-resistant wire mesh with a mesh size not greater than half inch nor less than quarter inch in any dimension. The minimum net area of ventilating openings shall not be less than one square foot for each 150 square feet of crawl space area.

One ventilating opening shall be within three feet of each corner of each building where such opening are required.

Some exceptions are where ventilation openings may be vented to the interior of building where warranted by climatic conditions and the total area of ventilation openings may be reduced to 1/1500 of the under floor area where the ground surface is treated with an acceptable vapor retarder material, and one such ventilation opening is within three feet of each corner of building. The vents may have operable louvers.

The under floor grade shall be cleaned of all vegetation and organic material.


Cross ventilation shall be provided for each separate space. Ventilation openings shall be protected against the entrance of rain and snow.

The ratio of the total net free ventilation area to the area of ceiling shall not be less than 1/150. An exception to this, where the ratio may be 1/300 is if a vapor retarder having a transmission rate not exceeding one perm is installed on the warm side of the ceiling; between 25 to 50 percent of the required ventilating area is provided by vents located in the eaves or cornices with the balance of the required ventilation provided by ventilators located at least three feet above the vents in the eaves or cornices; or the attic space is accessible and suitable for future habitable rooms or walled-off storage spaces have at least 50 percent of the required ventilation area located in the upper part of the ventilated spaces as near the high point of the roof as practicable and above the probably level of any future ceiling.