Frozen water in a wet crawl space (C) Daniel FriedmanCrawl Space Ground Cover for Moisture Control

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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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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:

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.

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.

Reader Question: Spraying foam insulation in a dirt floor crawl space - spray foam directly onto the dirt?

(July 4, 2012) Spray foam said:

I recently purchased a house with a crawl space, The crawl space had foam board on the wall poly on the floor tapped about 5-6" up the walls and bat insulation stuffed in the rim joust. I called around and found a spray foam company that says they have had a really good experiance spraying the rim joust and down the walls and across the dirt floor (yes I said dirt floor) he said it locks out any moisture pests and mold, he recommended filling in the vents with the foam that was cut out originally (basically putting back in the cut out pieces that were just laying down there, So I put all the foam cut outs back in and removed all the bat insulation and removed all the poly that was on the floor. His company came in and sprayed the rim joust all the way around and down the walls and across the floor.

(it looks like a big bath tube now, The first year Prior foam the house was so cold the furnace ran like 10 min shut off for 5 min and turned right back on, around 130.00 a month in the winter months, After foam the furnace runs like 5-8 min and is off for alot longer bills dropped to around 60.00 a month.

Do you have any comments on the foam being sprayed directly on the dirt, I have had no problems except for the flash floods we had 2 weeks ago (NORTHERN MINNESOTA) so much water collected under the foam it cracked in the low spot next to the sump pump and flooded a little but I am positive it was due to the sump pump crapping out. But I went to the local hardware store and bought spray foam in a can and fixed the crack and its all sealed up again.. I have looked every where for comments on spraying the dirt floor again just looking for comments on this.


Well that's one I've not heard before: direct foam application to a dirt floor. I suppose a closed cell foam might act like a poly vapor barrier but I wonder about the durabilty of a foamed dirt floor as well as access to run wires, fix a sump, or remove water below. The experts I've read never proposed such a measure; See Shipston's patent (2010).

Christian (2011), writing for the Oak Ridge National Lab certainly does not contemplate foaming dirt floors and instead agrees with my view on use of a vapor retarder (plastic):

Exposed earth in crawlspace is covered with Class I vapor retarder overlapped and taped at seams. - Christian (2011) p. 10.

The first priority is to solve the bulk water drainage flowing through the crawl space - Christian (2011) p. 34.

These citations discussing spraying foam as insulation or barrier material may be helpful:


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