Question? Just ask us!
Free Encyclopedia of Building & Environmental Inspection, Testing, Diagnosis, Repair
InspectAPedia ® Home
WATER ENTRY IN BUILDINGS
AGE of MOLD - Old is the Mold?
BASEMENT CEILING VAPOR BARRIER
BASEMENT MOLD WATER IMPACT
BRICK WALL DRAINAGE WEEP HOLES
BUCKLED FOUNDATIONS due to INSULATION?
BUILDING DAMAGE ASSESSMENT & REPAIR
CONDENSATION on WINDOWS & SKYLIGHTS
DEW POINT TABLE - CONDENSATION POINT GUIDE
EFFLORESCENCE, Salts & White / Brown Deposits
FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP
FLOOD DAMAGED FOUNDATIONS
FLOOD VENTS & FLOOD PORTS
FLOODS IN BUILDINGS-mold
FLOOR DAMAGE DIAGNOSIS
FOOTING & FOUNDATION DRAINS
FOUNDATION BULGE or LEAN MEASUREMENTS
FOUNDATION CRACKS & DAMAGE GUIDE
FREEZE-PROOF A BUILDING
FROST HEAVES, FOUNDATION, SLAB
HUMIDITY LEVEL TARGET
ICE DAM PREVENTION
MOISTURE CONTROL in BUILDINGS
MOLD INFORMATION CENTER
NOISE / SOUND DIAGNOSIS & CURE
ODORS GASES SMELLS, DIAGNOSIS & CURE
SEWAGE BACKUP, WHAT TO DO
SEWAGE BACKUP TEST & CLEANUP
SEWAGE BACKUP PREVENTION
SEWAGE PUMP CLOG DAMAGE
STAIN DIAGNOSIS on BUILDING EXTERIORS
STAIN DIAGNOSIS on BUILDING INTERIORS
SWEATING (CONDENSATION) on PIPES, TANKS
TOILETS, INSPECT, INSTALL, REPAIR
TRAPS on PLUMBING FIXTURES
VAPOR BARRIERS & CONDENSATION in BUILDINGS
VENTILATION in BUILDINGS
WATER ENTRY in BUILDINGS
WINTERIZE A BUILDING
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.
Green links show where you are. © Copyright 2014 InspectApedia.com, All Rights Reserved.
Accompanying 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.
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 at CRAWL SPACE DRYOUT PROCEDURES.
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?
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:
Current Best Practices Crawl Space Moisture Control Advice
Conventional best practice crawl space moisture control has shifted from that 1984 view.
Experts 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 see CRAWL SPACE DRYOUT - home.
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.
Green link shows where you are in this article series.
Frequently Asked Questions (FAQs)
No FAQs have been posted for this page. Try the search box below or CONTACT US by email if you cannot find the answer you need at InspectApedia.
Try the search box just below or if you prefer, post a question or a comment in the Comments box below and we will respond promptly.
Search the InspectApedia website
HTML Comment Box is loading comments...
Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.