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Crawl Space vents – To open or to close? That is the Question

April 14, 2015 3:22 pm Published by Leave your thoughts

Do you leave your crawlspace vents open all year?

Research indicates rather than removing crawl space moisture, venting makes the problem worse. Especially here in the Eastern United States.

Vented Crawl SpaceHere’s a snapshot of what happens to the air in a vented crawl space. When hot air rises, most of the air escapes into the upper areas of your home finding its way outside. As the hot air leaves, cool air rushes in through inefficient windows, leaky doors and open crawl space vents to replace it. Then the cycle is repeated. The air in the crawl space that gets sucked up into the home brings along moisture, dust, allergens, mold spores and possibly radon. Because of this cycle, any possibility of meaningful cross-ventilation, allowing moist air to leave through the crawl space vents, is negated.

Building Scientists have found consistently, that when warm, moist outside air enters a crawl space, through an open vent, it instantly cools and drastically increases the relative humidity of the crawl space. When the relative humidity goes over one hundred percent, the moisture is released into the crawl space atmosphere, resulting in condensation accumulation on the earth floor, walls and building components. This accumulation provides an environment conducive to support organic growths such as mold and mildew.

The 2003 International Residential Code (IRC) somewhat acknowledged the problem with – R408.2, Exception 1, stated, “Where warranted by climatic conditions, ventilation openings to the outdoors are not required if ventilation openings to the interior are provided.”

Then in the 2006 version of the International Residential Code Section R409 was added as an option – R409.1 “Closed crawl spaces shall be built to minimize the entry of the outdoor air into the crawl space. Specifically prohibited are foundation wall vents.”

The 2009 International Residential Code Edition carries the same theme but with more emphasis on energy efficiency such as air leakage controls.

Crawl space humidification by evaporation of soil moisture is related to the soil moisture content and temperature. Evaporation is generally greatest during summer, when the soil is warm, and least during winter, when it is cold. Vapor barrier ground covers help inhibit evaporation from crawl space floors, thereby lowering crawl space humidity levels.

Unless your home is relatively new, almost certainly your crawlspace has foundation vents. If this is the case for your home, at a minimum, I recommend completely covering your crawl floor with a 10 mil poly / plastic overlapped twelve inches at the seams. The seams should be taped with a vinyl/ waterproof tape. This covering will serve as a moisture vapor barrier over the earth / soil ground cover to inhibit evaporation into the crawl space. Running the vapor barrier up the foundation walls is recommended. If the vapor barrier is run up the crawl space wall to the level of the floor joists, termites could build mud tunnels on the foundation wall that would be hidden from view. So, a three inch inspection gap should be left, exposing the bottom of the wood floor framing and masonry wall.Vented Crawl Space

If the crawl space houses an appliance such as a furnace or hot water heater, the appliance, in most cases, needs make-up air for combustion. Consult your HVAC professional before sealing off any vents. Typically, where appliances are present an inexpensive “make up” air supply is easily installed.

To learn more about crawlspaces and if sealing your crawlspace is a better choice for your family’s home, visit: www.crawlspaces.org.


Chad D Collins 2014 NAHB Certified Green Home Professional of the Year (3) Chad D. Collins, Accredited Master Builder

An area native, NC Licensed General Contractor, National Association of Home Builders Master Certified Green ProfessionalTM, and Certified Aging-in-Place Specialist, Chad is one of less than 50 Accredited Master Builders in the state. He achieved this professional designations after years of experience and numerous hours of coursework via the NC Builder Institute. Chad has been an active member of the building industry since the early 1990’s. He joined the Home Builders Association (HBA) of Durham, Orange, and Chatham Counties in 2005. Chad was soon named HBA Recruiter of the Year and he subsequently chaired various HBA committees. Chad is a Licensed Home Inspector, as well as, a Licensed Lead Abatement professional.

In addition to industry awards in recognition of excellence in home building and remodeling, Collins Design-Build has twice been named Building Company of the Year via the HBA’s Triangle Sales and Marketing Council MAME (Major Achievements in Marketing Excellence) Awards. The company has also received a Best Green Built Home STARS Award from the NCHBA. Currently, Chad serves as 1st Vice President of the HBADOC, and recently completed one of Durham’s first LEED (Leadership in Energy and Environmental Design) residential projects. LEED is a green building certification program that recognizes best-in-class building strategies and practices. To receive LEED certification, projects must satisfy pre-requisites and earn points to achieve various levels of achievement.

In 2014 Chad was selected as the HBADOC 2014 Builder of the Year and recently honored at The International Builders Show in Las Vegas as the NAHB’s Certified Green Home Professional of the Year.

Contact Chad at 919-422-2818 or via email chad@collinsdesignbuild.com

To learn about homeownership opportunities, contact Emilee at emilee@collinsdesignbuild.com

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This post was written by Chad Collins


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