Hardwood floors cupping even after replacement

Question

Let me start off by saying that I am NOT a flooring expert or installer, I am a homeowner looking for answers.

About 3 years ago, my home was built in southern Louisiana with reclaimed/antique pine hardwood flooring (7" planks) glued to the concrete slab. The house has total encapsulation foam insulation and a combination AC/dehumidifier. The house stays around 45% humidity.

Within a few weeks of moving in, we started having cupping problems. It got so severe that some of the planks came unglued from the slab due to the pressure. Because the contractor did not use a moisture barrier over the concrete (even though the glue he used had a moisture barrier in it), he assumed that the problem was due to moisture coming up through the slab into the wood. So after ripping out the old floor, he laid Bostik's MVP as a moisture barrier and then glued the new planks on top of that.

Fast forward 5-6 months later, and we are having more cupping problems. They are not as severe, but still very bad. The contractor had no idea what was happening, so he had a technical rep from Bostik inspect the floors. The report from Bostik said:

With the use of a Mini-Ligno DX/C Moisture Pin Meter, adjusted for species, readings were taken of the wood flooring. The results ranged from 5.00-5.40 %.

With the use of a Mini-Ligno DX/C Moisture Pin Meter, adjusted for species, a reading was taken with long probes of the wood flooring. Moisture Content measured at a depth of approximately 5/8” were within the expected range. This floor shows signs of “dry cupping” as the face was significantly dryer than readings taken at depth.

Cupping of solid wood flooring is a moisture differential within individual pieces of wood flooring. The moisture content was very low, dry on the surface. Readings taken within the flooring are consistent with what you would expect in your part of the country.

Can someone explain what this means? The contractor has no idea and will not install wood flooring in my home because of the liability. At this point, I would like to install engineered wood , but I am concerned we will continue to have this problem. What should I do?

edit: photos here: https://i.sstatic.net/a7CUG.jpg

Answer 1

@iLikeDirt covered a lot of important information, but I'd like to add something since the majority of the information assumes that there is a problem of excess moisture due to the concrete, while the expert assessment says it is actually dry cupping that is the problem.

Dry cupping implies that the interior humidity is dropping below the average frequently enough that the wood surface dries out. It sounds like they are suggesting that the vapor barrier between the slab and floor is probably doing its job correctly.

One way to help prevent this is to bring the wood into house and allow it to be exposed to the air within the home so the moisture content is similar to that within the home (acclimation). I like to assume this is standard practice and this was already done, but if you're using a special reclaimed antique pine wood, then maybe a longer period would have been desirable. Major seasonal humidity variation can still have an impact.

Do you know if there is a period during the year where it seems more prominent, or has it not been long enough? If I had to guess, it could be during the summer when the AC/dehumidifier is running more frequently. I'm not as familiar with the climate there, but in parts with cold winters you are more likely to encounter dry air issues during the cold months.

One thing that could be happening is that the glue on the underside of the wood is acting as a seal on just that side, inhibiting the wood from 'breathing' underneath. The only surface area for moisture to leave the wood is on top. A solution that allows the wood to better breath on the bottom, or simply avoids using glue or too much glue, might help. But then you need make sure the vapor barrier is doing its job or you will end up back where you started with too much moisture underneath, the expert seems to imply you no longer have that issue though.

You can still end up with this problem with engineered floors. It could still be a problem with an unknown source of moisture throwing things out of balance as well. I'd see what the neighbors are doing as a solution for your area and if they have similar problems before I spend more money.

Answer 2

• Solid wood flooring in a wet area is inherently risky due to the moisture everywhere.
• Pine flooring (a moisture-absorbent softwood) is inherently risky to install.
• Solid boards are inherently more prone to cupping than engineered boards.
• Gluing a wood floor to concrete is inherently risky because concrete is a big sponge that absorbs and releases moisture in an unpredictable and uncontrollable manner.
• Gluing a wood floor to anything is inherently risky because this install method allows air movement over the top of the wood, but not the bottom, encouraging moisture to congregate there.
• Laying anything moisture-sensitive on top of concrete after it's been newly-cast is inherently risky because of how it gives off moisture for a while.
• Very wide boards (6" and above) are inherently more risky to install as a floor, as they are inherently more prone to cupping if they were plain or flat sawn. This is especially true of solid wood, but it can affect engineered wood with a thick veneer layer. I recently examined a sample of a 7" wide engineered board with a 4mm oak veneer layer of Chinese origin and noticed it was already ever so slightly cupping. Stay away from the really wide stuff unless you know it's of extremely high quality.
• Finally, old "reclaimed" wood is usually marked by imperfections and "character." That's the whole point! It is likely that this wood was not of especially high quality to begin with when it was first installed however many tens or hundreds of years ago. "Rustic" or "Antique" wood is usually not noted for its level of precision or moisture resistance compared to modern materials. Back then, the solution to moisture problems was to make everything as leaky and air-permeable as possible to permit rapid drying. When you seal up an old building with vapor barriers and insulation without solving the underlying moisture problem, it often begins to rot. The same kind of thing seems to be happening here where one face of your boards are totally sealed up but the opposite face is exposed to moisture.

In isolation, a couple of these factors are probably no big deal, but when you combine all of them together, you have a recipe for a high chance of failure. If there is any way to install your floor properly, it would probably be as a nail-down install over a poorly-laid, air-permeable wood subfloor with an open space below it (e.g. solid boards with gaps between them and a basement underneath, not OSB sheeting over concrete), and with appropriate expectations of its quality and dimensional stability.

I would guess that in your slab on grade house, an engineered wood floor made of boards 5" wide or less with an oak veneer top layer installed as a floating floor above a waterproof underlayment would experience no problems whatsoever.

Answer 3

I won't go as far as @MichaelKaras but I would never lay 7 inch planks on concrete unless the concrete was 20 feet up in the air in an apartment building. Since yours is ground level you need to have a flooring system installed before you can do wood again. Meaning insulation then plywood subfloor and then your wood floors - probably engineered. You are looking at a ton of work getting this installed or finding an alternative flooring.