Foundation Vent Open or Closed?

By: Handy Andy

There is a lot of confusion surrounding this matter and by following the wrong advice you can make problems worse.

Let's clarify something right from the beginning...  controlling moisture in the crawl space, optimzing foundation vents are proven not to work as you may have hoped for. Vents not only fail to control moisture, they actually contribute to aggravate the problem.

City-Data.com asked it's Raleigh-Durham-Chapel Hill, NC residence in 2009 what they do with their vents during the winter months.  The result was quite interesting - 60% said they closed the vents while 40% indicated they open the vents.

Greenville Utilities , in Greenville, NC, recommends to follow the North Carolina Uniform Residential Building Code on the number and placement of crawlspace foundation vents. The utility company recommends placing a polyethelene ground cover in the crawl space, which allows for the net free ventilation area to be reduced by 50%.

In past years, the home building industry pretty much always recommended to close the foundation vents in the winter to reduce the exposure of pipes freezing and to avoid cold air in the crawl spaces. During the Spring through the Fall months, the industry alway suggested to leave the vents open .

After much research the utility companies suggests that a change in this procedure is needed for most homes. It has shown that too much ventilation occurs during our hot and humid summers.During the hot and humid weather the warm, moist air migrates into the crawl spaces and through the vents and condenses on the metal ductwork, duct insulation, floor joists, floor insulation and curtain walls. This moisture can rust and damage ductwork and contribute to mold and mildew. The moisture also negatively effects the insulation of the duct and floor and can contribute to potential wood rot.

The final verdict may read like this... Let the crawlpsace dry with natural air movement in the Spring and Fall and leave the vents closed during the hot, humid Summers and throughout the Winters to prevent the moisture problems mentioned above.

You trusted handyman Handy Andy

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November  2012