How Much Venting Is Needed for a Crawl Space?

Vents exchange crawl space air.

Proper venting of a construction crawl space provides a continuous flow of outside air throughout the area. Crawl space ventilation standards, however, have changed since World War II.

Traditional Practice

Building standards in 1958 stated that a crawl space should have four wall vents in the foundation. Vents were located at each corner of the crawl space. The four vents were required to have a combined venting area equaling 1/150 of the crawl space area. The bottoms of the vents were required to be placed no higher than 12 inches above ground grade.

FEMA Recommendations

Technical Bulletin 11-10 of the Federal emergency Management Agency sets slightly different standards. It states that net ventilation area of all openings should not be less than 1 square foot per 150 square feet of crawl space area. A proper vent opening must occur within 3 feet of each building corner.

Modern Recommendations

Conventional building technology in the past viewed foundation vents and outside air as the best way to control crawl space moisture. October 2005 research by Bruce Davis and Cyrus Dastur of Advanced Energy showed that crawl spaces should not be vented to provide better moisture control and energy savings.

References

• Builder Magazine: Closing the Crawl
• FEMA.gov: Technical Bulletin 11-10
• Basement Systems: Crawl Space Vent
• Advanced Energy: Designing Closed Crawl Spaces
• Rice Engineering: The Fallacies of Venting Crawl Spaces

Resources

• Clenson University: Two Special Cases of Crawl Space Ventilation
• Energy Design Systems: Crawlspace Design
• Forest Products Labs: Crawl Space Ventilation