Crawl Space Ventilation

The most effective crawl space ventilating system has open, 8-inch x 16-inch foundation vents spaced 6 to 8 feet around structures up to about 32 feet wide. Wider buildings occasionally require more ventilation supplied by adding fans in the center of the crawl space. When vents remain open and unblocked by landscape plants, and the crawl space is open and clear for cross-ventilation, few, if any, moisture-related problems exist.

Construction practices often interfere with adequate crawl space ventilation. For example, an attached carport blocks ventilation on an entire side of the house. An earth-filled porch blocks ventilation for its full length, and duct work for heating and air-conditioning systems can completely block cross-ventilation. The minimum standards of some building codes have followed similar restrictions by adding polyethylene sheeting to cover crawl spaces and then reducing vent areas.

The unfortunate result of less foundation ventilation has been the increase of wood rot in crawl spaces and moisture problems in the living areas of homes. Moist climates require you to provide and maintain adequately ventilated crawl spaces, not use techniques to reduce the vent area.

The best corrective procedure is to cover the crawl space surface with polyethylene. It is best to use polyethylene in addition to vents placed 6 to 8 feet around the perimeter of the building. It is important to vent through earth-filled porches and along carport foundation walls, since these are the most likely places for wood rot due to air stagnation from lack of continuous ventilation. Two easy methods provide vents in these areas.

1. Provide vents in wells in carports if there is not enough space to provide standard vents. 
2. Vent the crawl space behind earth-filled porches by using an 8- x 16-inch sheet metal duct to ventilate the crawl space.

Ventilation Area Needed

Clear Vent Area (CVA) means the area available on the surface of a ventilating material through which air may pass. The effectiveness of materials used in venting air and moisture vapor from crawl spaces is measured and stamped with a rating such as "CVA 50," which means that 50 square inches of clear ventilating area are available. Product literature may also be the source of information of the CVA's available.

In a crawl space, one square foot of clear vent area for each 150 square feet of floor area is needed. This means about one foundation vent spaced evenly, each 6 to 8 feet around the entire foundation wall.

Do not be fooled by the appearance of a vent cover. Most foundation vents are 8 inches x 16 inches to fit into the space of an absent concrete block.

8 inches x 16 inches = 128 square inches in area -- but this area is not clear vent area.

Insect screen wire reduces the area by about 50 percent, or to about 65 to 70 square inches.

Reinforcement or decoration reduces the area further, so that CVA's of foundation vents vary from about 30 square inches to 70 square inches, depending on how much area is covered by patterned decoration or reinforcement.

Example:

Assume you are planning a 1,500-square-foot house

1,500 sq ft house = 10 sq ft CVA needed
150 sq ft per vent

Convert 10 sq ft CVA to square inches 10 (12" x 12" ) = 1,440 sq in CVA needed

Vent Cover A = CVA 30 rating
1,440 = 48 vents with CVA 30
30

Or

Vent Cover B = CVA 70 rating
1,440 = 20.57 or 21 vents with CVA 70
70

The crawl space ventilating system, in addition to venting moisture vapor, serves to vent out other gases, such as those formed from residues in termite treatment, treated wood, and radon gases. Thus, the crawl space ventilation system protects the structure of the house and the health of its occupants.

By Dr. Frances Graham, Extension Housing Specialist.

Mississippi State University does not discriminate on the basis of race, color, religion, national origin, sex, age, disability, or veteran status.

Information Sheet 1488
Extension Service of Mississippi State University, cooperating with U.S. Department of Agriculture. Published in furtherance of Acts of Congress, May 8 and June 30, 1914. Ronald A. Brown, Director

Copyright by Mississippi State University. All rights reserved.

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