Some homeowners seal off crawl space vents year-round in order to stop termites in the summer, heat loss in the winter, and moisture buildup that can lead to mold growth; others open and close the crawlspace vents with the season. Is there such a thing as a right or wrong answer here? The experts have differing opinions.

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Vote: seal off crawl space vents

Jesse Waltz, a foundation engineer, votes to seal off crawl space vents altogether. "Properly sealing the crawl space and removing the moisture from the ground and air is part of the solution that helps provide a mold-free and insect-free environment, which leads to a more energy-efficient and healthier home," says Waltz. "To avoid moisture's negative effects, a crawlspace should be completely sealed and isolated from the ground and the humid outside air. An effective method to lower crawl space humidity, tested and perfected throughout the U.S., Canada, the U.K., and Ireland, is a proper crawlspace vapor barrier system," he continues.

"This involves installing a 20-mil 7-ply sandwich of high and low-density polyethylene with polyester-cord reinforcement on the dirt floor that is extended, and fastened to the walls." This extra-heavy reinforced lining is treated with an antimicrobial finish that protects against mold and mildew growth under the crawlspace liner. It is tough enough for service people to crawl on and safe enough for storage. In some cases, a high-performance dehumidifier air filtration unit is added in the crawl space to assure that humidity levels stay under 50%. Always consult your HVAC contractor to make sure crawlspace venting is adequate.

Vote: open and close crawlspace vents seasonally

In a recent article published by This Old House Magazine, co-owner of Silva Brothers' Construction Tom Silva votes to open and close crawlspace vents seasonally.

Building codes generally require working vents in the crawlspace to allow outside air to circulate under the floor in the summer to prevent moisture buildup that, among mildew and mold, encourages wood rot. In the winter, when the air is drier, the vents are closed to reduce the chance that the pipes in the crawlspace might freeze. So our answer is to open and close the vents in your crawlspace seasonally, instead of deciding to seal off crawl space vents altogether.

The science behind crawlspaces

As air rises in a home, it carries with it the air that was previously in the crawl space. That includes moisture and mold spores, as well as anything else that may be airborne down there. As this air rises in the home, replacement air is drawn through the vents. This replacement air is made up of unconditioned outside air that enters through vents and other leaks. This natural upward air movement is called the "stack effect" - similar to how a chimney works.

Consequently, whatever is in the air at the lowest point of your home eventually flows up into the living areas. Almost half of the air we breathe on the first floor of our home comes from the crawl space. A dirt crawl space with open crawl space vents is a never-ending source of moisture. Even if the dirt's surface seems dry, digging down a few inches reveals moist earth. This moisture is constantly released into the crawl space.

It's the moisture that causes mold growth, musty odors, and eventually structural damage, not to mention that insects and critters love moist environments. Additionally, energy costs are higher. Moisture ruins houses by providing a hospitable environment for mold and other fungi, and insects that destroy wood framing. Crawl space moisture, and the mold and mildew that thrive in this environment, affect not only the floor system directly above, but also the entire house. Three things destroy organic materials such as wood or insulation: water (moisture or high relative humidity), heat, and ultra-violet radiation. Of these, water is by far the most damaging.

How to open and close crawlspace vents

If you decide to not seal off crawl space vents year-round, the simplest way to close your crawlspace vents for the winter is to plug them from the outside with foam blocks made specifically for crawlspaces. Then remember to remove the plugs when the weather turns milder in the spring - and as you do so, check to make sure that your crawlspace vent screens are intact so that insects and rodents don't make nests under your home. You could also choose to go with automatic vents, which are designed to work without electricity, and open at approximately 70 degrees and close at around 40 degrees.

Already experiencing moisture or mold in the crawlspace?

If you are experiencing mold in your crawlspace, there are several solutions you can look into for dealing with the moisture issue that led to the mold growth. Some solutions for dealing with crawl space moisture issues are as simple as installing a crawl space vapor barrier, addressing the grade of your landscaping, or properly guttering rainwater away from the property; however, a crawl space service expert may be necessary to solve your issue.

AdvantaClean offers crawl space moisture control solutions:

  • Crawl space inspection
  • Moisture control products
  • Crawl space insulation
  • Crawl space vapor barrier
  • Crawl space encapsulation - AdvantaSeal
  • Controlling moisture and humidity in the home
  • Black mold removal
  • Drainage and sump pump installation - AdvantaDrain

More info? For more information about common places for moisture in your home, check out our related post: Crawlspace Moisture Problems.

Call 877-800-2382 to schedule an appointment with an AdvantaClean crawl space moisture control specialist today!

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