My husband and I recently purchased a home and put in Eco strand bamboo with the QuietWalk underlayment over a concrete subfloor. The wood acclimated for about 24 hours, and the slab was never tested prior to installation. We've noticed some cupping in the floor. We had the installers out to check on it. They found that the moisture level of the wood was fine (8.4), but that the moisture level of the concrete slab was a little higher ("one or two notches" above normal, he said). The slab was tested the day of a heavy rain in our area, so I'm not sure if that made a difference. The installers noted that there was no evidence of moisture when they pulled the floor up. Also, the downstairs under the stairs closet is not exhibiting the same issue as the rest of the floor. Their suggestion was to tear up the floor and put down an additional layer of plastic, then the underlayment, then the floor back. He also thinks there may be some areas where the initial installation was done too tightly. I wanted to write to see if this sounds reasonable, or if there was another explanation. Also, the reason we chose the floor/underlayment was to abate our concerns about toxic substances in the floor, and I'm not sure if the plastic sheeting would fall in that category. HELP!

Photos would help... and lots of them.

cupping is generally caused by moisture, so it is very possible that moisture is migrating up from below the concrete. Poly might help, but it should be thick plastic (10-20 mil); the seams must be taped and it should be brought up above the floor (fastened to the wall behihd the baseboard around the edges. Difficult to do right, but can be done.

Thanks for your reply! That is exactly what they suggested. I wondered about the "greenness" of putting poly down, as our concern when we laid the floor was choosing something that wouldn't off-gas too much. They said they would off-gas it before they brought it over, but I just wanted to see if there might be another option that fits the "eco/green/no off-gas" thing. Pictures are difficult - we've taken a few for the manufacturer, but its hard to see the cupping. It's not major, and really only is seen in the light or if your dragging something across (like your foot).

24 hours doesn't seem like very long for flooring to acclimate- I always heard it should be weeks. Maybe bamboo is different? As said, cupping is caused by moisture- more on one side than the other. As for moisture in the slab- this can be condensation, as opposed to moisture coming from below. In our FL home, when we had cold weather followed by warm, humid weather, the floors would actually sweat, which is condensation, like a glass of iced tea. I think the conditions on the day it was installed could be a factor. Let everything acclimate, put down a heavy barrier, and install in dry weather.

Or put down foam first to stop the condensation.

abate our concerns about toxic substances in the floor,

What kind of toxic substances? In the concrete?

Bamboo is known for its stability and should be the perfect match for radiant floor heating. We in fact recommend many "Engineered" flooring products and especially "floating" flooring like the ECO. Unfortunately, though bamboo is a very stable product, engineering is not. I am embarrassed to admit my own recent remodel included Eco bamboo and I was quite surprised and more than a little annoyed when my own floor cupped. Since it was installed after more than two weeks of steady indoor conditioning and subjected to moderate reset temperatures from the sub-floor aluminum plated hydronic system with surface temperatures never exceeding 76°F, one can only assume a better, yes more expensive floor such as Yunckers, should have been specified. Never pays to get in a hurry and run down to the nearest big box for their latest sale.

By contrast the traditional, pre-finished 3/4" T&G red oak installed in the same week over exactly the same sub-floor, plates and tube, is perfect!

It is not the moisture, and a properly designed radiant floor heating system will not cause most properly installed quality floor covering to fail. When I get a minute to tear it up Junckers will go back down with no cupping, warping, splitting, separating or other such nonsense ever again.

So there.

You have to ask yourself; where is this bamboo flooring made? What is used to hold these strips of bamboo together? What type of quality controls are in place?

Bamboo floor quality varies widely, it is likely the most complicated floor to properly install/maintain, and it often has problems. Both slab moisture and the RH of the air in the building can cause bamboo floor cupping issues. We wouldn't recommend using it for a HR floor. Here are some informative NWFA articles:

Beyond Bamboo Basics

The Cause of Dry-Cupping

Worry about the toxic glue in the flooring, not a sheet of polyethylene. Consider avoiding the glue by using solid wood.

Cupping comes from a side-to-side moisture (%RH) differential - which is hard to avoid with room air on one side and concrete at a different temperature on the other.

Moisture can be an issue but if a vapor barrier lies between the slab and floor and/or the slab is heated, moisture is not possible. We have had situations where the slab never runs for lack of load in an underused or over lit basement but no flooring problems have been attributed to this.