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Crawl Space Moisture Leads to a Cupped Floor

By David Paal
April/May 2011
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The Problem

I was commissioned by a central Indiana homeowner to inspect a 1,500-square-foot main level hardwood floor showing generalized cupping two months after completion of the new home.

The Procedure

The ¾-by-3¼-inch solid red oak was nailed down over a full crawl space. Interviews with the dealer and wood flooring installation contractor revealed no examination of the crawl space had been made prior to wood flooring delivery. Acclimation took place within HVAC living conditions, but moisture readings were not documented.

The Cause

The main level revealed hardwood flooring moisture content (MC) averages of 9.7% to 11.3%, taken from depths of ¼, ½, and ¾ inch, in six separate areas throughout the main level. With indoor ambient readings in all rooms averaging 72°F and 44% RH, I knew the crawl space would provide critical information to explain why the cupping occurred.

As I entered the crawl space, my glasses fogged up immediately. While it was 34°F outside, the crawl space was a balmy 74.8°F, with RH readings topping out at 69.7%. A 6-mil polyethylene vapor retarder was in place over the pea gravel but fell short of the perimeter block walls and interior block stems by as much as 1-3 feet. No perimeter ventilation openings existed, but HVAC duct work did. There were small water puddles underneath various water return lines from the condensation dripping. The subfloor underside MC averaged 16.9%; the high reading was 18%. The evidence was overwhelming: Damp conditions with no cross ventilation allowed for unabated vapor absorption into the subfloor above. Cupping was an unavoidable result in this uncontrolled environment.

How to Fix the Floor

The MC differential from the top of the wood flooring board to the bottom needs to be within 1% and (in this area) between 6% and 9% overall. Only then should sanding be considered; full replacement may still be necessary.

The source of the moisture must be corrected before dealing with any cupped floor. Crawl spaces must be dry and should be a minimum of 18 inches from ground to underside of joists. Crawl space earth or gravel should be covered 100 percent by a 6-mil (minimum) vapor retarder of black polyethylene. Vapor retarder joints must overlap a minimum of 6 inches and be sealed or taped. It should also extend at least 6 inches up the block walls and stems and be attached and sealed to them. Where a proper vapor retarder is in place and when venting is required by local building codes, the crawl space should have open perimeter venting equal to a minimum of 1.5 square feet per 100 square feet of crawl space square footage, unless local building codes state otherwise.

Because this crawl space had no ventilation openings, continuously operated mechanical exhaust should be considered. Thermostatically controlled power ventilators mounted inside of multiple perimeter openings can control elevated temperature and RH conditions, and permanent high-capacity dehumidifiers are another option. Sensitive water lines and pipes should be insulation-wrapped as needed. Sump pumps are wise investments to handle heavy rains and snow melts.

In the Future

Because crawl spaces are considered part of the job site building envelope, they must be thoroughly inspected and moisture conditions within them stabilized before wood floor delivery or installation. This is not an option.

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See photos from this inspection:

Crawl space hygrometer readings were 74.8°F and 69.7% RH. The clear 6-mil polyethylene vapor retarder is visible.

The crawl space subfloor underside MC measured 18.0%.

Cupping resulted when the bottom of the wood floor had a higher MC than the top.

The cupping may relax some if the crawl RH can be lowered quickly.

The crawl space vapor retarder had large openings, permitting entrance of subsoil moisture.

Moisture condensation on cold water lines was observed dripping onto the vapor retarder.

David Paal is the owner of Greenfield, Ind.-based Premier Flooring Services LLC. He is an NWFACP residential and commercial inspector. He may be reached at 317/538-1469 or emailed at

Moisture barriers            Moisture testing wood floors    Wood floor acclimation    Wood floor inspections        Cupping    Moisture               


Maybe I missed something in your description of the problem. Was DK paper or 15# felt or any type of membrane laid over the OSB subflooring to protect the flooring when it was nailed-attached ?? The industry standards have been for over 100 years to use a construction grade double kraft asphalt center paper over any subfloor, Diagonal 1x6, Plywood, OSB before nailing down any hardwood flooring. Was that investigated?
Tom,  3/28/2011 6:09:32 PM

Tom, There was in fact a 15# black felt vapor retarder present; I just failed to mention it. It didn't stop anything. This is not new. Have seen it many times. They are not bullet proof as some may think. Always good to hear from you. Dave
David L Paal  NWFACP Inspector  3/28/2011 6:43:31 PM

Great article. I have found it is good to exceed the minimum ventilation standards. Several builders have done that here in the Houston area and have far better performance of their floor and construction materials. You also must be very careful when applying ground cover here to be sure that you do not trap water under it. This may require a drainage system in the worst cases.
Edward Robinson  Consulting Engineer  3/29/2011 4:36:32 PM

Edward, Thanks. Perimeter venting is not even mentioned in the residential building codes for many HBAs here in the Midwest, which totally amazes me. The HVAC contractors I'm sure don't object to the added business they've been gaining as a result. It's a struggle just getting folks to kick down the crawl space door and take a few measurements before installing that floor. Preventive maintenance goes a long way. I'd rather know up front and do my diligence than have someone pay untold thousands in damages later on. Research from Sept 2010 points to moisture claims totalling $500M per year. Amazing. It's the little things like testing the concrete, checking the crawl space, checking the subfloor, know the guidelines, asking the tech departments, etc. We all could be the solution if we just took the time to do it. Thanks Ed. Dave
David L Paal  NWFACP Inspector  3/29/2011 7:10:58 PM

David, I think you might be disappointed using natural or mechanical ventilation to solve those floor problems, especially ventilators running on a thermostat. I do agree that 100% coverage of the ground is the first step, but even then clear really works as good as black poly. My very, very simple test is that if you can screw up the inside of the house by leaving windows open while the heat or AC is on, you can screw up a crawl space by venting it. This is one of those things you don't want to try at home, but if you don't believe me, just open a window or two while your AC is running and see what happens. If you see problems, try to solve them by opening more windows. I assume no liability if you do though. Simple solution: close vents, install 100% ground cover, install dehumidifier, go to next job.
Craig DeWitt  Indoor Environment Engineer  4/2/2011 6:51:41 AM

Craig, The mechanical ventilators have been used successfully here in many Midwest applications that I have have been involved in, as have open perimeter crawl space vents. Either one. There are a number of wood manufacturing Tech Directors who have made the recommendation for either that I have spoken with. The NWFA has openly endorsed both, with those I have spoken to. I disagree with your premise that venting a crawl space can screw it up when I have tested dozens of them that are performing fine that are properly ventilated (open vents) according to spec. I guess my experiences have been different than yours. We agree to disagree on this one. No offense taken. Dave
David L Paal  NWFACP Inspector  4/2/2011 10:35:56 PM

Dave, Nice article but I would have to agree with Craig here. FYI outside air drawn into crawlspace will cool and hold more moisture. The formula is for every drop in 1 degree, Rh goes up 2.2. Or if outside air cooled in crawl space by 10 degrees there will be an increase of additional Rh of 22%. Therefore here in MN encapsulated crawlspaces are common with dehumidification capabilities. This removes any conditionals from outside sources and provides homeowner with consistent climate conditions and our problems have been minimized drastically. One simply can not make this guarantee with open venting spaces. Also you mentioned earlier regarding 15# felt paper was used however I believe the perm rating is near 5 which would be the less preferred choice for conditions like this. Vapor retarder with less than 1 is available and would be the correct choice. Roy Reichow
Roy Reichow  NWFACP Inspector  4/9/2011 1:04:29 PM

Enclosing the crawl space is called "enveloping". In reading this article I would start looking for the source of heat. Why is the crawl space so warm in 30 degree heat?
Ray Darrah  NWFAcp  4/11/2011 12:20:50 PM

You are fighting a battle on two fronts. Moisture you can SEE and moisture you CANT. Visible moisture will almost always lead you back to its source. That makes fixing it a manageable task for correctly qualified individuals. When dealing with the unseen, the sooner you get your opinion out of the equation, the sooner you will have a listening audience. A listening audience is more inclined to give you the needed time to implement the necessary changes. In short, know the facts or learn the facts and apply the facts. Ground cover is key, one to two gallons of moisture can escape from every one hundred sq. ft of soil. Look in the crawlspace, dry dirt DOES NOT equal dry crawlspace. Cover the soil, end to end and side to side. Do not leave exposed soil! Think about it, firemen cut holes in roof tops everyday to let smoke and heat out. Ground cover is like that roof, if there is a hole or piece missing, that is where the invisible moisture is going to escape from the ground.
Pat McGrath  Owner McGrath Distributing  4/12/2011 12:37:43 PM

I take it your report found the Builder or remodel contractor to be at fault for not ventilating the crawl space.
Jim Jackson  Installer  4/13/2011 1:59:08 PM

Great Article Mr David Paal Member of NWFA
Jeovanni Pime  Flooring Installer  2/11/2012 11:16:30 AM

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