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Crawl Space Vapor Barriers: Facts and Fallacies

Homes with crawl space type foundations became very popular after the World War II, the so called baby boom era when there was a huge demand for new and more affordable homes. Crawl spaces can be built quicker and at a lower cost.

The only problem with crawl spaces is that they are extremely prone to moisture, which can lead to many related problems from pest and mold growth, to dry rot and structural decay.

In an attempt to control moisture in the crawl space, building codes all across the U.S. required that they be provided with a certain number of vents, and a vapor barrier over the floor.

The vapor barrier, usually a 6mil poly sheet lining the entire floor of the crawl, overlapped a few inches at the seams, and sealed with tape, was supposed to prevent ground moisture from evaporating into the crawl.

Moisture from condensation would then be dealt with by the air that would circulate through an adequate, code specified, number of vents. Actually, and unfortunately, that is still the case in some areas of the U.S.

After many years and millions of dollars spent annually in crawl space mold remediation and structural repairs, many builders began to realize that there was something very wrong with a building code requirements. Mold, dry rot and structural damages in vented, dirt crawl spaces are so prevalent in the U.S. that industry specialists call it a true housing epidemic.

What could have gone wrong?

As building scientists began to take a closer look and have a better understanding of how crawl spaces work, as well as their role on overall building performance, they realized that only one source of moisture was being addressed in these requirements: the ground moisture, which was being covered with the crawl space vapor barrier.

They forgot to account for water seeping through below grade concrete walls, and most importantly, their approach to control moisture from condensation, the crawl space vents, is actually a scientific fallacy. Not only the vents fail to address condensation, but they make the problem much worse.

During the warmer months, differences in temperature between the crawl space and the outside, cause the relative humidity levels that are present in the outside air to rise as it enters the cooler crawl space. For each degree it is cooled, there is a 2.2 percent increase in relative humidity levels. Thus, a 10 degree temperature difference between the outside and the crawl will mean over 20 percent rise in RH levels. The warmer it gets outside, the worse it gets.

Every time the RH levels rise above 60 percent in the crawl space, mold is likely to develop and grow, which, overtime can compromise your homes structural integrity. Not to mention indoor air quality and energy efficiency of your home.

Crawl Space Encapsulation

Several studies conducted by many reputable organizations, including Advanced, Energy, Habitat for Humanity, SouthFace.org and Building Science Corp. conclude that the best way to treat crawl spaces in new and existing buildings is through encapsulation.

Crawl Space Encapsulation is a treatment that combines three main procedures:

1 Proper placement of the crawl space vapor barrier, which is used to line both ground and walls to address moisture seepage from these sources

2 Air sealing of the vents and seams so that no outside air enters the crawl, Adequate conditioning or dehumidification, to address condensation.

3 Crawl space vapor barriers have evolved as well. From thin 6mil poly sheets, known to easily rip and tear, to multilayered, 10mil or 20mil liners as sturdy as a pool liner.

Crawl space encapsulation is not only efficient to control moisture, it is also proven to improve the homes overall energy efficiency an average of 18 percent, by curbing the infiltration of unconditioned air from the outside into the living areas.

That is the reason why this technique is now recommended by the U.S. Department of Energy and many green building organizations like the US Green Building Council.

Date Created: 22-May-2011
Last Updated: 2-Nov-2014
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