Should my crawlspace vents be opened or closed?

Camel Cricket

I am sure that you have heard horror stores about what a home inspector may encounter inspecting a crawl space. Truth is, for the most part, the above is what we encounter by the thousands. It is not unusual to crawl through, what would appear to be, a carpet of Camel Crickets. Why are they there? They love dark damp places and eat the fungus which grows there. The darker and damper the more of these I crawl through. Therefore, should my crawl space vents be opened or closed?

This article below was published in the September 2012 issue of the Winston-Salem Regional Association of Realtors newsletter "The Professional Edge" in the section "From the Home Inspector". This article was written by High Point home inspector John Guy with input from Winston-Salem home inspectors Bill Dillon and John Woodmansee. This article was so "on the money" that I am sharing it here in its entirety.

The short answer is it depends. It depends on a myriad of factors including:

• Is the crawlspace typically dry?
• Is the grading around the house proper (minimum 6” of slope in 10’)?
• Are gutters and downspouts installed? Are they in good condition? Are the downspouts directed away from the foundation?
• Are gutters and downspouts cleaned religiously or are gutter guards installed?
• Are gutter guards clogged with debris (should be washed to clear debris annually)?
• Does the home have overhangs, awnings or porches to keep water away from the foundation?
• Is the a/c duct work installed in the attic or in the crawlspace? Is it insulated?
• Is a 90% or better moisture barrier installed on the crawlspace floor?
• Is the home surrounded with “mature” or overgrown vegetation?
• How cool do the occupants keep the home in summer?

If the answer to the above is no it may be wise to leave vents open at all times to aid in moisture control. Excessive moisture in the crawlspace can lead to a host of problems including termites and other insects, wood rot and fungal growth (mold). All crawlspaces should be inspected annually at a absolute minimum.

The old fashioned rule is that the vents should be open in summer for ventilation and closed in winter for improved energy efficiency and to prevent pipes from freezing in extreme weather. That said, during the “dog days of summer” during July and August, it may be wise to close foundation vents to prevent the muggy air from entering the crawlspace and condensing on cool surfaces, especially a/c ductwork and associated equipment. This “sweating” can be more severe when occupants maintain indoor temperatures less than 75 degrees F or so. Cool, dry weather during Fall and Winter provides excellent drying conditions. Beware that most automatic foundation vents are designed to open during warm weather and close during cold weather and may be contradictory to this.

Building codes have very specific requirements for foundation vents including the number, maximum distance from corners, etc. Generally vented crawlspaces must contain openings not less than 1 square foot for each 150 square feet of crawlspace ground area. This can be reduced if certain conditions are met. A 100% moisture barrier, now required, is always strongly recommended. It should be a minimum of 6-mil and cover all exposed earth with joints lapped not less than 12 inches. The purpose of the moisture/vapor barrier is to prevent moisture that naturally evaporates out of the ground from entering the crawlspace and migrating into the home, decreasing occupant comfort and/or increasing energy costs thru higher air conditioning bills. A mechanical means of drying the crawlspace i.e. a commercial dehumidifier may be wise, but only if a moisture barrier is installed and vents are closed. The dehumidifier should be elevated and drain to the exterior or a condensate pump. The condensate pump can be shared with the a/c system.

Recent codes have a provision for unvented crawlspaces but these must be properly designed and (of course) maintained. It is absolutely critical that these crawlspaces be installed properly, maintained and monitored. A remote hygrometer or continuous moisture monitor (available at retailers such as Radio Shack can be used) is critical to ensure that conditions do not worsen due to deferred maintenance, leaky pipes, changes in exterior grading, maturation of foundation vegetation, gutter system failures, etc. Information on sealed crawl spaces is available at www.advancedenergy.org. Some waterproofing and pest control companies specialize in closed crawlspaces.

For an unbiased, professional opinion and possible corrective recommendations on your home’s crawlspace and related moisture issues, a professional home inspector could be consulted.