Frozen water in a wet crawl space (C) Daniel Friedman

Crawl Space Ground Cover for Moisture Control

  • CRAWL SPACE GROUND COVERS - CONTENTS: How, when, where, & why to install plastic on a crawl space floor to help control crawlspace moisture and mold. Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
  • POST a QUESTION or READ FAQs about using plastic or other ground covers in crawl spaces to control moisture, improve ease of entry, keep the area clean & dry, reduce risk of mold & insect damage

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This article discusses the use of ground covers to control crawl space moisture.

We explain how moisture moves from soil below the crawl space up into the building, how to select and install a plastic vapor barrier, where to put the plastic, and handling crawl space water, moisture, or humidity.

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Crawl Space Ground Covers to Help Control Crawl Space Moisture

Crawl space moisture control (C) Carson Dunlop Illustrated HomeAccompanying text is reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss. Our page top photo shows a wet, flooding crawl space in which a poly barrier and gravel had been placed on the crawl space floor.

[Click to enlarge any image]

Those wet concrete blocks in our page top photograph are diagnostic: water was still entering the crawl space through the foundation wall, ponding on to of the gravel-covered plastic "moisture barrier". Here we explain why the good idea of covering the floor of a dirt crawl space may not be enough to stop a building moisture and mold problem.

Readers dealing with damp or wet crawl spaces should start reading

The question-and-answer article below paraphrases, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

How Best to Control Crawl Space Moisture


I read your article "Controlling Moisture in Houses" (Solar Age 1/84), but it did not touch on this particular wet crawl space problem. I am having trouble controlling moisture in a crawl space. I believe that the house is over a wet-weather spring. -- Virginia Riffee, Georgetown KY


According to researchers Charles Jennings and Thomas Moody, who worked on TVA's weatherization program, installing a crawl space ground cover can reduce moisture from capillary rise by up to 90 percent.

Below we have updated the original 1984 article to add more effective steps besides just installing a ground cover -- DF.

Sketch (above left) showing the effects of covering a dirt floor in a crawl space is courtesy of Carson Dunlop Associates. Carson Dunlop's comment that a dirt crawl can contribute several gallons of moisture per day into a home is the best case. If the crawl area is actually wet from surface runoff, roof spillage, ground water, or plumbing leaks, the amount of water pumped into the home can be much larger and more harmful.

How to Place Polyethylene Ground Cover in a Crawl Space

Heavy polyethylene plastic sheeting works well since it resists deterioration by mold. Covering the poly with one or more inches of sand or smooth rounded gravel will protect it from occasional trampling.

In new construction, where the polyethylene overlaps on the ground it should be overlapped by at least two feet, or sealed using a caulk or sealant that will adhere to the poly, and the poly should be carried up the walls several inches or more, at least to grade line - a height equal to the height of soil outside.

Some installers use a sealant caulk or furring strips to secure the poly to the building foundation wall. We do not like to staple the poly to the sill plate on top of the foundation wall as doing so can in some areas provide a ready path for termite attack. For this reason we exercise similar care when insulating a crawl space foundation wall interior.-- DF

In building retrofit installations of crawl space moisture barriers, Jennings and Moody recommend leaving about 20 percent of the ground uncovered so that the structure is not subjected to undue shrinkage and movement. In particularly wet spaces, they suggest first covering 50 percent, then finishing up to 80 percent of the ground area in 10 percent increments every 4 to 6 weeks to reduce "moisture shock".

[This was 1984 vintage advice. Our building inspection and testing experience in the ensuing decades indicates that covering 100 % of a dirt crawl space floor with 6-mil poly, sealed as we described above, is the most effective practice and can substantially reduce unwanted building moisture and mold problems. -- DF]

Should Crawl Space Ventilation Be Included in a Crawl Space Dryout Scheme?

Photograph: typical mold on floor joists and subflooring over a wet crawl space -  © Daniel Friedman

In the original 1984 Solar Age article, the same experts were recommending what was conventional crawl space ventilation wisdom - specifications that were consistent with building codes:

1984 crawl space advice: "A ground cover should be used in conjunction with ventilation. The HUD standard [1984] typical of others, recommended four crawl space vents with a total minimum free vent equal to 1/150 of the crawl space floor area if there is a ground cover, 1/1500 with the ground uncovered. For best results, place two vents each on opposing walls."

Our photo (left) shows a severe and problematic mold contamination on the underside of the first floor of a building constructed over a wet crawl space. Ventilation had not helped one bit to avoid this problem.


Current Best Practices Crawl Space Moisture Control Advice

Conventional best practice crawl space moisture control has shifted from that 1984 view.

Crawl space poly and heat (C) Daniel FriedmanExperts observed that crawl space venting was not effective in many instances, for example depending on wind direction as well as the source and amount of crawl space water or moisture, crawl space vents were simply ineffective.

In some instances, such as blowing warm high-moisture laden air into a cool crawl space in summer months in some climates greatly increased the level of crawl space moisture and condensation, making crawl space moisture worse rather than better.

Our crawl space photo (left) shows that poly was placed on the dirt floor of the crawl area and a heat source was provided, salvaging an old radiator. We'd have preferred to see the poly extending up the crawl space walls a foot or so. But we notice that this crawl space looks dry: there are not mold nor moisture stains on the floor framing overhead, and no leak stains on the crawl space foundation wall.

Below we summarize the best way to avoid wet or damp crawl space problems under buildings. If your crawl area is already wet or damp, also

Current best practice in controlling crawl space moisture involves:

  • Seal the crawl space from outdoor air - close off those crawl space vents - and convert the crawl space to conditioned space, providing a small amount of heat where climate dictates, to help keep the area dry and above freezing
  • Identify and cure sources of crawl space moisture, such as roof drainage spilling around the foundation. In roughly 90 % of inspections performed by experts, we find that wet or moldy crawl spaces or basements that had been blamed on "high water table", "rising damp", or "built over a spring" were actually being caused by gutter and downspout defects, perhaps combined with in-slope grade that concentrated roof drainage right against the building foundation.

    Our page top photo shows that water was entering the building crawl area through the foundation wall - see those wet blocks along the bottom of the wall?
  • Use a moisture barrier such as 6-mil poly continuously over the crawl space floor and lower crawl space walls, up to grade level, sealed as we described above. In new construction the poly may be installed under a crawl space slab or gravel. In crawl spaces that are rarely entered, placing sand or gravel over the poly is probably not necessary, and its use can hide depressions in the poly that may actually be holding ponding water on top of the poly in some cases.
  • Inspect the crawl space periodically, at least once a year, to be sure that the poly moisture barrier is working as intended. As we just suggested, an outside water leak, such as roof spillage entering through a foundation wall, or an inside water source such as from a leaky plumbing supply or drain pipe, can place water on top of your crawl space poly moisture barrier, leading to a costly building moisture and mold problem.
    At CRAWL SPACE DRYOUT - home we show photos of just this problem. -- DF

Remember these are minimum values for average conditions. Your building may need special measures. If, after identifying and fixing outside sources of a wet or damp crawl space, you still find high water levels right under the crawl space floor, you may want to install a sump pump as well.

The question-and-answer article about use of a plastic barrier on crawl space floors to control crawl space moisture and mold, quotes-from, updates, and comments an original article from Solar Age Magazine and written by Steven Bliss.

The link to the original Q&A article in PDF form immediately below is preceded by an expanded/updated online version of this article.

  • Moist Crawl Space - Q&A on use of ground covers to control crawl space moisture - PDF version, use your browser's back button to return to this page

This article series describes the steps needed to get into, inspect, clean, and then dry out a building crawl space. We give a step by step crawl space entry, inspection, cleanout, dryout and keep dry guide explains how to get into or inspect a crawl space even if there is no ready access, how to assess crawl space conditions, how to stop water that is entering the crawl area, how to dry out the space, how to clean up and if necessary disinfect or sanitize the crawl space, and how to keep out crawl space water and moisture in the future.


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CRAWL SPACE GROUND COVERS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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