Hello! I am new here and very glad I found such an excellent community of people and I sure I hope someone can offer me sage wisdom cuz I think I need it! Here's the story: we bought our first house last year and had 3 1/4 inch birch installed on our main floor and finished with a hard wax product. The installer and finisher used these super long nails and a glue when putting them down. The floors were perfect all fall, winter and spring until the end of the summer when I noticed that they were starting to cup. Our basement is a finished and was renovated last year before the flooring was laid (vapour barrier installed, R30 insulation, spray foam headers, etc. - should be nice and tight down there). What could have caused this?? It was a rainy and cool summer for us but that couldn't have been it..... could it?? Also perhaps relevant - the flooring had been milled 3-4 weeks before installation - could that be a factor? We also had pine installed a few weeks before the birch on the 2nd floor of our house - also milled 3-4 weeks before installation (installed by a different installer who i don't believe used glue) - and we have no problems up there. Did the installer not do something he should have? We spent a lot of money on this flooring and are quite devastated that they look they way they do. Any advice would be so appreciated!

The only thing out of the ordinary that I can tell was to have the flooring custom milled. I guess the use of birch as well. The mill operator should have given the buyer/you a report/tag the had the final moisture content of the wood. The installer should also have double checked the moisture content. It MAY have been too dry. I do not know. The subfloor should have also been measured for moisture content as well prior to wood delivery and installation. At this point, it is hard to determine WHY the floor expanded. But wood expands for only one reason; the absorption of moisture. The moisture can come from the air above the floor ( relative humidity ), moisture vapor below from humidity, plumbing leaks, and so on. At this point, contact the installer to examine the floor to determine the cause and source of the moisture. Ask the installer for the moisture meter readings he/she SHOULD have taken PRIOR to the installation. I suspect no measurements were taken, the floor was kiln dried too much and now, it is expanding. This may go down during the winter as you use your furnace IF it is from a higher relative humidity.

One last thing. Have you been damp mopping the floor? That is a no-no, especially for waxed floors. The wax finishes are not as moisture resistant as urethanes.

Where are you located? What is the current rh in your house? Could be the rh is to low, ie dry on top normal on bottom of planks.

The most probable cause of your problem is ELEVATED humidity. If your AC unit has not been used much this summer due to cool temperatures, and you dont have a dehumidifier in your home, the floor is just reacting to normal summer humidity levels; which would normally be handled by the above mentioned devices.
It will most likely flatten during the upcoming dry winter months and if preventative measures are taken next year you should not have a recurrance of the cupping.

The only thing i didnt see written, did you leave the windows open alot during the rainy summer months and let in alot of moisture. Have someone take a moisture reading on the 2nd floor and compare it to the floor in question.