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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

Great article! Should I close on a new home Thursday, May 29, 2014 with cupped floors all over the home? The builder does not want to replace the floors. He wants us to close and use dehumidifiers. We hesitate to move our furniture onto these floors. What would you suggest as a course of action? Thanks for your advice. The home is in Wake Forest, NC.
Richard Van Benthuysen  Physican Assistant  5/27/2014 6:44:59 PM

Richard, I would encourage you to delay the closing until you can have the floors professionally inspected by an independent certified inspector. Do not let the builder dictate the course of action here; he may not have your best interests in mind. You are in the driver's seat, not him. The moisture contents of the wood flooring and subfloor as well as the ambient conditions of relative humidity need to be measured and evaluated. The builder is trying to get you to move too quickly before the facts can be gathered and analyzed; you need to know absolute numbers and most importantly where the moisture is originating from. I would encourage you not to make any decisions without having this information. There are several scenarios that may have been going on here that need to be investigated. Please feel free to call me or email me and we can discuss further. Thanks.
David Paal  NWFACP Inspector  5/27/2014 8:20:15 PM

David: In a crawl space, should pressure treated lumber be used for floor joists, or regular lumber with a sealant? My crawl space is fully encapsulated - all outside vents sealed up - equipped with a dehumidifier and a sump pump. Also made sure flooring was properly acclimated. Used pressure treated wood for floor joists due to concern about non-treated joists possibly getting damaged over time from being in a crawl space. Now concerned pressure treated joists may have added moisture to my 3/4" x 3.5" hardwood flooring. 1/2 of hardwood flooring has been installed and has cupped. Plan to sand, but don't want cupping to happen again - either to the newly sanded or to the flooring that's yet to be completed. Thanks!
Mr. Dubbs  kitchen floor  6/30/2014 5:09:24 AM

Mr. Dubbs, Please email me offline where we can discuss this in further detail, or you can call me at the number listed below. There appears to be a lot going on in your situation and I have several questions to ask about your particular job before I craft any sort of a meaningful answer. Thanks, David....Email: or Cell: (317) 538-1469
David Paal  NWFACP Inspector  6/30/2014 2:05:00 PM

David, what does a system like you have written here cost? How much is the installation cost? What is the system's annual cost of running it?
David  Owner  7/1/2014 7:19:50 AM

The "system" is merely the material cost of a 6-mil polyethylene vapor retarder that covers your crawl space floor; if you want to totally encapsulate the crawl space, add the walls also. The cost would be quoted by a certified contractor that specializes in crawl space remediation. A good dehumidifier option to consider, and perhaps the best option currently available that I have experience with, would be the Therma-Stor Santa Fe Advance II, a unit built to specifically handle crawl space remediation. Since this article was written more than 3 years ago, building codes have been changing nationwide to eliminate perimeter vents on new home builds. Here in the Midwest, it is happening quickly at the local Builder Association levels. The reason is that there are simply better studies that have prompted the development of better technologies that are now available to control crawl space moisture than what we have been using the last 15-20 years. Energy Star efficiency, ASHRAE requirements and filtration standards have all improved significantly to better control crawl space moisture issues using dehumidifiers. It all starts, however, with the vapor retarder; if one is not properly installed it is a fruitless endeavor of wasted time and money. The system's annual cost would be best quoted by an HVAC contractor who would perform the work, based on the total square footage to be considered. Hope this helps.
David Paal  NWFACP Inspector  7/1/2014 9:05:02 AM

Hi, did the floors fully "relax" back to normal after reducing the moisture in the crawl space, or did they require sanding?
Keith  Home owner  7/9/2014 11:31:38 AM

Keith, They did fully relax back; no sanding was needed. Once the crawl space RH was brought under control, so was the underside moisture that the subfloor was experiencing due to the elevated RH in the crawl space beneath it. The moisture gradient from the bottom of the wood flooring board to the top of the wood flooring board also lessened; as a result the cupping relaxed. As a general rule, always deal with he source of the moisture first before the next steps are considered and/or taken. Patience will be needed. Sanding in many scenarios may not even be necessary, as it was in this case. If sanding is done prematurely, and the cupping continues to relax, the edges of the wood flooring will shrink downward and crowning will result. Crowning most times results from cupped floors that are sanded too soon; dealing with the source of the moisture first is required for this very reason.
David Paal  NWFACP Inspector  7/9/2014 12:18:40 PM

We had new hardwood put down in our kitchen, fr, dr,and halls. First year I thought I choose the wrong floors because when I walked on them I thought I could feel each grove between each piece of wood. Then in 2013, the floors in the hall looked like they were cupping. I had Warren Flooring come out to give me their opinion, since they installed the floor. Pam said the floor was cupping for sure. But, not everywhere. We had inspection from one company, and our heating, and cooling people came out too. No standing water was found. We could not get a handle on what was going on though the inspections. So we did nothing. Now in summer of 2014, all the hardwood is cupping. The man that installed the floor came out, and put two moisture readers under the crawlspace for a couple days. Checkecd them two days later. One read 86 humidly, and the other one was 92. Now we know we have to fix this, but having trouble knowing what to do. We have our heating and cooling company coming out to investigate, to see if they can solve the problem. Plumber is coming too. It is so hard to know who to contact in cases like this so we are staying with people we know. I hope they can solve this, but if you have a good source to contact that we can trust, please let me know. We are located in Greenwood, IN, South of Indianapolis Indiana. 46142 area code. Thanks for your advise, and help! Dana Baker
Dana Baker  Home owner  7/25/2014 10:52:34 AM

Hi Dana, We spoke on the phone right after you emailed me a week or so ago. If you have additional questions, please call me at (317) 538-1469. I think we covered everything you just mentioned in your post.
David Paal  NWFACP Inspector  7/28/2014 2:29:38 PM

Hi David, thanks for your response earlier. Do you remember how long it took them to relax after getting the RH under control? We are dealing with a similar issue. When I checked the RH in the crawl space, it was 80-90% consistently. I closed all vents and added a dehumidifier, now the RH is generally between 45-55% and the temp is in the mid 70's. The only difference in our scenario is the builder installed and sanded the floors in the cold winter before the HVAC was installed. I assume there was low RH (it was really cold for sure). And now that the humid summer is in full swing... and... well. Because of this, I'm afraid we will have to re-sand, but not losing hope yet.
Keith  Home owner  7/29/2014 4:55:27 PM

Keith, If the wood floors are beginning to relax that would be a general sign that the crawl space RH is beginning to come under control. It will take time, however, for the wood flooring moisture content from the bottom of the board through its entire thickness to fully equilibrate. The moisture gradient differential throughout the thickness of the wood flooring must come under 1.0% before sanding is even attempted. If the sanding is done prematurely before the gradient has had a chance to equilibrate uniformly then the edges of the wood flooring will "crown" downward in a convex dimension over time, opposite of when they cupped upward to begin with. This may take several months. Patience is again the key word here. If crowning does occur, it means the wood flooring continued to relax after it was sanded. If left alone, the high edges may have returned to their original dimension on their own. Crawl space RH needs time to come under dehumidification control; you are being very proactive the way it sounds and that is the first necessary step. Hopefully, you have also correctly placed a polyethylene vapor retarder as well over the subsoil and/or gravel. The floor above should respond over time; keep in mind the subfloor moisture content has to also respond as it resides underneath the wood floor. Stay vigilant but also remain patient as well. Wood is hygroscopic and requires time to achieve an equilibrium moisture content (EMC) to the RH conditions it is placed within. With seasonal change it is important to maintain the RH in the crawl space below as well as your living areas above. You will eventually get there. Stay the course.
David Paal  NWFACP Inspector  7/30/2014 11:16:49 AM

This is how we handle crawl spaces in South Africa. It has proved to be very successful and since inception 10 years ago not one problem has arrisen. Click on the next link to view the process:-
Steven Suntup (South Africa)  Chairman SAWLFA  7/31/2014 4:08:11 AM

DO NOT CLOSE ON THIS HOUSE ! ! ! CUPPED FLOOR IS JUST A SMALL PART OF THE FUTURE PROBLEMS LATER IN THE LIFE OF THIS BUIDING. I don't understand the remark dealing with the "main level" of this inspection report. Is this a to (2) story home? That's not moisture . . . that's just plain WATER on that pipe but when you write that there are HVAC ducts in that crawl space, that is another hidden problem. You can take all the flooring repair advice you choose but a cupped hardwood floor is a cupped hardwood floor - period. THAT FLOOR PICTURED NEEDS TO BE REPLACED! You did not show any of the ducting and that will be a constant source of condensation when located in a crawl space / should have been located in th e attic - - - moisture UGH. Then the author allows the consideration of SANDING a cupped hardwood finish floor. He should be sent back for training after that consideration. Have you ever seen a sanded cupped hardwood floor??? Figure it out for yourself. If you do sand that floor be sure that you have a design for the filler color between each plank line (?). Get as far away from that builder as you can, he does not understand that his work and materials are to be warranted ???
G.A.Custer  Construction Manager  8/1/2014 12:49:25 PM

Hi David, Great article. You answered all the questions I've been searching for the last few months with one, well crafted article. I am installing 4 1/4" maple hardwood across the first floor. Question - how does one install a proper vapor retarder? I have purchased a 12mil silverback from and am installing it myself. Its a great product and I've learned that most 6mil barriers will degrade over time. I don't want to encapsulate and I've laid out the plastic across all earth within 2-6inches of the wall/pillars. You said in your article to run the plastic up the wall/pillars and attach to the blocks. Is this absolutely necessary, and does it need to be done around the full perimeter and all interior piers? Also, im thinking about dehumidifiers...should I get one for the crawlspace, or one for the floor environment? Thanks in advance. -Rob
Rob  Web Developer  8/15/2014 6:06:29 PM

If you aren't going to totally encapsulate the floor and up the walls and stems, then you would simply run your polyethylene up 6"-8" all perimeters and vertical blocks and attach it with whatever will stick to hopefully dry perimeter block or concrete poured walls. I've seen foam sealant used around the block stems and that seems to work. A thick mastic of some sort might work as well. You might consult with a crawl space professional or your local HVAC contractor for their input on what they use. Overlap all the polyethylene sheets 8" at the seams and tape every seam you have; allow no penetrations for vapor to enter. Encapsulation all the way up is the better way to go; you simply adhere it at the top of the sill plate with foam or mastic, or, you can take a 2" x 2" wood strip and power screw it into the block or poured wall. The idea is not to leave any perpendicular penetrations at wall-floor-stem connections. Add a dehumidifier, preferably one that isn't a "disposable" make and model...but rather one that is designed for crawl spaces; the Therma-Stor Sante Fe Advance II is the one that perhaps is the "dehu of choice." It will outlast any other lesser model from the big boxes hands-down. The crawl can go with its own dehu unit. The living area should be OK with the existing HVAC if it is in good operating condition and is maintained properly. Hope this helps. Good luck.
David Paal  NWFACP Inspector  8/15/2014 11:39:56 PM

High relative humidity is a very common problem within crawlspaces. There are typically two causes to this issue: 1) a lack of vapor barrier installed over the soil or not properly installed. Vapor barriers should be installed so that the edges overlap 6-8 inches and sealed with a black caulking to prevent moisture from escaping from the seams and the edges where the foundation footings are. It's important to make sure that no vapor barrier wraps around any wood posts to support beams as this will cause wet rot to occur. Also, foundation walls below grade can also allow water to diffuse through the material, also increasing the RH of the crawlspace. 2) Most crawlspaces are ventilated to the outdoors. In hot and humid climates, a crawlspace that does not have the proper environment controls (dehumidification) in place, a high humidity ratio (Grains Per Pound) can lead to high relative humidity, especially if the crawlspace is 10-15 degrees cooler thus causing the RH to rise. Dri-Eaz products has designed a dehumidifier (PHD-200 that has a built in humidistat that can be set for the ideal condition to keep the RH at an acceptable condition to prevent cupping of hardwood floors, which is the result of higher MC of the sub-floor and thus the hardwood looking for equilibrium to the sub-floor. Installers should always pay attention to whats below the substrate such as a concrete slab or crawlspace. Using a thermo-hygrometer should also be an important tool to determine the moisture content of concrete using the ASTM 2170 testing protocol, and determining the RH of the crawlspace prior to installing hardwood materials. If the readings are to high, then consider using environmental controls such as dehumidification to bring those areas, or materials into an acceptable range for the installation of hardwood materials.
Darren Hudema  Sales Account Mgr/IICRC Instructor  8/18/2014 1:08:54 PM

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