Troubleshooting Cupped Flooring over a Heated Slab

Here's a long discussion about cupped wood flooring over a slab. aJune 12, 2014

I'm wrapping up a job now where there's a problem with the floors. Not my problem mind you, but the homeowner asked my advice and I'm not sure my advice is much good.

To the best of my knowledge, wood, once milled and installed only moves when the moisture content changes. Temperature change alone cannot (I believe) affect the wood. So here's the problem, his floors are cupping badly in one particular spot that just happens to be over where the bulk of the in-floor heating supply pipes run. So if the temperature cannot affect the wood, why would this spot be so bad? I'm thinking any humidity change should affect the floors throughout the unit, but the problem area is contained to about 12 square feet or so.

The floors are 4" or so wide walnut nailed onto plywood, laid on sleepers on a very old (50 years plus) concrete slab. The floor was laid and finished about two months ago. No idea at all of what the RH of the unit is, but the temperature averages between 75 and 80 degrees, so I have to think it's dropping. So any ideas? I'd like to be able to give him some worthwhile advice but I'm over my head on this one. One other flooring contractor says the floors were sanded improperly?

Forum Responses
(Cabinet and Millwork Installation Forum)
From Contributor W:
Do you have a piece of the off-cut? Is there any relief on the backside? How long was the material allowed to adjust to room temp before installation?

From Contributor M:
If the floor was installed and finished a few months ago in late summer/early fall, windows were probably open; other construction going on- sheetrock, tile, paint, etc. all these would add humidity to the air and surrounding site. Kick on the in-floor heat and as it warms up, the moisture leaves the wood flooring from the unfinished bottom causing it to shrink faster than the whole board which equals cupping! The wood was probably 6-8% MC with pieces higher or lower. The whole floor will shrink and move seasonally with extra movement over the in-floor heat. You should consider quartersawn material when installing solid wood over in-floor system.

From contributor A:

A few things could have happened. Like Contributor M said, it could be the moisture trapped when it was put down. Then the heating system is drying the wood from under, especially where the bulkiest part of that heating system is located. Also, could it be that the concrete slab does not have a moisture barrier? How bad is it cupping? Sometimes it is best to lay the floor down, wait a few weeks and then sand it so that it gives it time to settle. A floor man told me that once ago. Don't know how true that is.

From the original questioner:
The floor did have the bottom relief cuts in it. I saw it when it was piled up pre-installation. The material was on site for at least a week or two, maybe more, I'll have to ask one of the contractors to find out exactly how long. As Contributor M mentioned there was a lot of stuff going on, meaning some fluctuations in the temp and humidity. The whole unit was skim coated before the flooring went down, though there were a lot of touch-ups going on after as well as a lot of painting. Would the floor actually dry out faster on the bottom than the top, when the bottom is pretty much sealed in once the finish is on? I honestly don't know if they used a moisture barrier or not. I can't imagine much moisture coming out of concrete as old as this is?

This was supposedly the best floor guy in the state and charged a pretty good amount for his work, so you would think he would know if any of these things would be an issue ahead of time? I really do think it's tied in to the heat pipes concentrated in that area. The question now is how to fix it? The homeowner has plenty of extra wood so ripping out and replacing is an option. Of course waiting for it to settle and re-sanding would be preferable. I just don't want to give bad advice either way. If he was to sand it how long would you wait? I'm guessing it's probably going to get a bit worse given that we're just entering winter. Would you wait until spring, or even longer to see if it starts to ease out a bit in the summer humidity?

From contributor A:
I would sand the bad area down, seal it and see what happens. If it does it again then there's a problem. If not then it might just be a one-time thing.

From contributor J:
Temperature alone doesn't warp wood, but changing temperature usually changes RH, which in turn changes MC. In real-world situations heat can absolutely lead to movement even if it doesn't directly cause it. For the cupped boards, are the concave faces on the top, the bottom or a mixture of the two? Are gaps opening between boards? Are they pressing more tightly against each other and buckling upwards?

From contributor C:
Sounds like a moisture issue. If it is installed over a concrete slab without a vapor barrier or a poorly installed vapor barrier that is more than likely the cause.

From contributor K:
Sweating concrete plus no vapor barrier plus heat source equals a problem area.

From the original questioner:
Shouldn't the change in RH affect the entire floor then? That's the part where I'm confused as it's literally a 12 square foot or so section with the surrounding areas mostly un-affected. Concave faces are on top, and the joints are still tight, so far. My only problem with the concrete theory is the same problem as the RH, why isn't it affecting the entire floor? We're talking about something close to 4k square feet of flooring all installed the same way over the same concrete.

Would concrete still put out moisture after 50 plus years? It seems like it would have reached some sort of equilibrium by now? It may even be older, the building itself is much older, but it had significant structural renovations in the 50's I believe, so slabs could be from then. I'm going to ask one of the contractors today to see if they now about a vapor barrier being used.

From contributor K:
"Would concrete still put out moisture after 50 plus years? Seems like it would have reached some sort of equilibrium by now?"

Think of it this way: The 12 square foot area you are concerned with has a heating source below it. Both heat and moisture rise (as a by-product of heating and cooling) and it has nowhere to go but wicking through the concrete. With no non-permeable product to prevent it, it will continue its path upward leading to the cupping.

My guess is it constrained to the 12 square foot area as the bulk of the heating supply pipes are in that area, and the effect of the wicking in both the concrete and wood can only go so far before it continues its journey upward, in this case about 12 square feet.

Unfortunately, you cannot simply put a vapor barrier on the underside of the concrete to avoid tearing up the floor. You will need a sealer and vapor barrier for this to be a long-term solution. To test the area put in a dehumidifier in the room below for 36-48 hours, and you will be surprised at the amount of water you remove. Take a look at the Knowledge Base link below.

Installing Hardwood Floors

From contributor R:
Is the heating system new, or is it 50 years old? Moisture is being added from underneath (concave up, no gaps), so the solution will have to address how this is happening. Best case, it was in the underlayment/stringers. The subfloor will equilibrate with the walnut, then the whole lot will adapt to the ambient conditions. The area of greater heat causes this to happen faster (and with more visible damage).

There are other options, but they all deal with what's under the wood. Some quick ideas: leak in the hydronic system (new or old), the slab is only mostly old (new skim coat, or hydronic system set into slab), and the-very-probable-so-bears-repeating no vapor barrier on the 50 year old slab. Concrete breathes (takes on moisture in the summer, and releases it in the winter) too, this is why it needs expansion joints. The last two would be exasperated by locally more heat in the same way as the subfloor possibility. On another note, it's good in these situations to refrain from developing any opinion as to who's fault the whole thing is. I'm fairly convinced that the persistent opinion of tradesmen as crackpot shysters is mostly propagated by our own willingness to point out the shoddy work of others.

From the original questioner:

I talked to the contractor yesterday, who by the way got me the job, and he says there's no underlayment. He also thought that the slabs, (closer to 100 years old as I found out they were not changed in the renovation 50+/- years ago), would have no moisture left in them. I also talked with one of the finish carpenters and he went down a long list of reasons he thinks it's happening, Not enough time for the wood to acclimate, wood was wet when it came in as it was pouring rain out. Then once it got in the humidity level of the unit was way up as it was still late summer.

The plywood also came in while raining so it may have been wet, and on and on. So in the interest of not throwing myself into a fight I'm sticking with the truth. I don't know enough about flooring to offer a useful opinion. The flooring guy should be able to find a resolution to it. My feeling is still generally that the person putting the flooring in should have been involved enough to have known all the issues at hand, but there could be even more that I'm not aware of.

Now I have to focus on my own problems. It seems that two of my doors are now warping. They decided to shoehorn both the heating units and the stereo equipment into two closets with no ventilation! So when one side of the closets is at 100 degrees plus, and the other 80 degrees bad things are going to happen! The contractor finally convinced the owner to put a vent in, but too little too late. I told him we'll wait a few weeks and see if the doors move any more before we address that issue.

From contributor A:
It doesn't matter if the slab is 50 or 100 years old it can still let moisture through it if there's not a moisture barrier.

From contributor T:
Concrete is porous no matter its age. Unless it's been sealed with an epoxy paint or some other barrier, it is always wicking moisture from the ground it is on.

From Gene Wengert, forum technical advisor:
If the wood is warmer than the air, then the microclimate at that hot spot actually has a lower RH that the cooler spots. This means lower MC. It also means that any moisture in the porous concrete will move out into the wood when the heat first comes on.

From Contributor S:
It sure sounds like there is a leak in the heating system. I think it takes a lot of moisture to cup the wood so bad. I do not know if this has been resolved yet, but this post started at the beginning of the month so by now you should be seeing some sort of change in conditions if this is from moisture from condensation relating to the startup of the heating system. If it is from condensation the wood should have started drying out by now, did the appearance change at all since?

From Gene Wengert, forum technical advisor:
When wood cups rapidly due to different MCs on top and bottom it is very difficult to flatten the wood even when any moisture gradient is removed. So, once a floor cups it can be difficult to flatten it again even though conditions are uniform throughout. Technically, it is due to the plastic properties of wood versus the elastic properties. This is why we encourage the same finish top and bottom so that a gradient of moisture is less likely to form within the wood.

From the original questioner:
If anything the cupping has worsened a bit and spread out from the area originally affected. Several others have looked at the floor and after pressing the subs and flooring contractor more information has surfaced. It seems that in this particular area they used some of that self-leveling floor product. On top of which was placed the sleepers, 3/4" ply and then the flooring. No vapor barrier was installed. So the belief now is that the combination of the bulk heating lines in one area combined with no vapor barrier, are pulling moisture from the self-leveling compound through the plywood and into the flooring. I'm not sure what the resolution will be yet, but am interested in the end result.

So what is usually used as an acceptable vapor barrier? I've seen many floors installed in a similar fashion and not had problems with the only barrier between the concrete and floor being plywood and rosin paper. I would have thought the 3/4" plywood itself would normally be enough of a barrier to keep moisture transfer pretty low. There is no moisture being added from the ground, these are all multi-story buildings with no contact to the ground, the only thing under the concrete is the unit below.

From Contributor M:
Is there any chance that the radiant heat tubes got punctured by a flooring nail in that area? A small leak could cause problems without seeing water running out.

From the original questioner:
That's a good question and one I thought of myself. I don't know how they would be able to find out if there was to be honest? I really don't have much knowledge as to how these systems work and whether or not you would see a drop in pressure or some other signs, especially if it was a small leak. I do know that there were a couple punctures in other parts of the unit that required the subfloor to be pulled and repairs done.

From Contributor H:
I have some firsthand experience that will explain what is happening here. I know of an indoor pool installation on a concrete floor that has a moisture problem on the floor around the pool. Turns out the heated pool (7' x 14') is drawing moisture up through the concrete floor. At first we were convinced the pool was leaking but over a period of months the water level did not drop. This meant the water had to be coming from somewhere else.

The pool is installed in the basement of a 15 year old house that never had any water problems in the basement. After talking to a number of pool professionals it was determined that it was the warm pool water that was sucking the ground moisture upward. Note that there are no water problems in the surrounding basement area. Also the house is located in a sandy, well drained area and so it's not a running river of water under the floor that is the cause. It is just the warm pool water warming the concrete which is then wicking the moisture up under the pool where it spread out to the floor around the pool. Based upon this experience I have to say that your heat pipes are warming the concrete slab which is then wicking moisture upward. This moisture is passing through the plywood subfloor and hitting the bottom of the walnut floor boards, causing them to swell. This forces the boards to cup with the concave side up.

From Contributor V:
The concrete age is irrelevant. It is sitting on the ground and the ground will always contain moisture and this is being wicked through the concrete into the subfloor to the hardwood due to no vapor barrier. Vapor barrier is always required between wood and concrete, no exceptions. This can be a layer of poly sealed at the seams, Dricore or I have also seen a product similar to platon system designed for under floors.

In the flooring world around my area at least, all warranty on solid plank flooring is void (quarter sawn or otherwise) when installed over concrete (even if there is a good V.B.). Most of the time they will recommend laminate or engineered product for concrete/radiant applications for slabs. I went through this recently on my own home as I was determined to find a way to install solid flooring on a slab with radiant heat but in the end I bowed to the manufacturers warnings and chose an engineered product with a good vapor barrier. I'd be almost willing to bet that this problem will get worse over time and likely require a gut and re-install with proper materials. Definitely one to stay out of since you have no vested interest.