We recently redid ground floor with 4 inch red oak. It was nailed over plywood subfloor with tar paper used as a barrier. The wood ws acclimated for almost 2 weeks. The flooring was installed over a finished basement. Contracter says he tested moisture of wood before installing, unsure about sub flooring testing. He stated the level was 0. Now 2 months later we have cupping in certain areas not all. There was sufficient spacing along edges so that does not appear to be the problem. The contractor came back and is unsure of why the floor is cupping and says he will inquire with his finisher as to what can cause the problem. If it is humidity in basement will the floor come back a little bit if dehumidifier(which is used) dries it out. I dont want to resand and refinish without knowing cause.

If he tested the subfloor.... He would have gotten a reading, not 0. Also...solid hardwood MC should be within 2% of the subfloor reading, before installation. There's no way the hardwood tested within 2% of 0. What does he say the hardwood tested at?

I wouldn't sand a cupped floor until I found out the cause and remedy other than sanding if possible. Is your basement closed off or part of the HVAC system? I hear it is beneficial if the basement or crawlspace is sealed up and part of the system.

the answer will lie in testing the underside of the subflooring crawlspace and basement..

more than likely you will find high moisture content which transferred to the wood floor..

than determine the cause for the high MC in subfloor, correct this, and the floor may stabilize itself to an acceptable appearance..

Acclimation is not a time thing, it is a moisture meter thing.

I have had to let a wood acclimate for 2 months to get it where I needed it.

I shoot for 9-10%, that is our magic number here in Central Texas.

Amen to that. It's about matching moisture content. Also predicting what changes will happen in a home are important too.If I know a home will get damp in the next season change, I may opt to use some spacer rows in the installation.

I hear LL has a wood floors with built in spacer rows. It's all in the milling.