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Crawl Space Science

By Jesse Waltz, PE

Crawl Space Science

Crawl Space Science

The Sense of Crawl Space Encapsulation

~ By Jesse Waltz, Professional Engineer

Crawl space science. Let’s begin – there is a problem with a dirt crawl space floor. Additionally, if you have open crawl space vents, please keep reading.

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.

“Over half of the air we breathe on the first floor of our home comes from the crawl space.”

Properly sealing the crawl space from the earth and outside air is the solution to moisture and dampness in the home. Some contractors and homeowners will simply put down a thin, non-UV-resistant plastic liner, like a 6-mil plastic liner. This really has little effect as there’s still too much opportunity for air to enter from crawl space doors or hatches, crawl space vents, pipe penetrations, porous concrete walls, the hollow cores of blocks, and various other gaps and holes.

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.


Crawl Space Science – Proper Crawl Space Sealing

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. To avoid moisture’s negative affects, 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. 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 venting is adequate.

“To avoid moisture’s negative affects, a crawl space should be completely sealed and isolated from the ground and the humid outside air.”

Crawl Space Science – Crawl Space Research

North Carolina is a leader in crawl space encapsulation research and installation guidelines. The recently released 2009 North Carolina State Residential Building Code Book outlines how encapsulation needs to be installed for compliance to code. Proper installation requires leaving a 4″ space, or “reveal” between the top of the liner and the bottom of the sill plate so termite mud tunnels can still be detected. The pest control industry finally understands the benefits of a properly installed encapsulation system. Virginia and other states that have homes built on crawl spaces will no doubt follow North Carolina’s lead to accommodate encapsulation in their code books in the upcoming years. An informative study by the Department of Energy, Dominion Power and Advanced Energy on the benefits of crawl space encapsulation can be found at

“Proper installation requires leaving a 4” space, or “reveal” between the top of the liner and the bottom of the sill plate so termite mud tunnels can still be detected.”

Years ago, the theory was that crawl spaces would become dryer if they were vented. Now, years later, studies and experiences have proven that the most overlooked space in the house needs to have its air and humidity conditioned, just like the rest of the house.

Jesse Waltz

Jesse Waltz

Professional Engineer

Jesse Waltz, a Professional Engineer and a 1985 Virginia Military Institute graduate, is the founder of JES Foundation Repair. JES is a regional foundation repair, crawl space and basement waterproofing company with locations in Virginia Beach, Richmond and Northern Virginia.

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Crawl Space Science

Our Homeowner’s Guide to Crawl Space Repair is your resource for learning about why you have crawl space problems, and what to look for to see if you have a problem.

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