Anyone have any tips on preventing foamcore from warping? I can't seem to find foamcore at local stores here that isn't in poor condition or already warped, so I tend to buy it on Amazon as a box of 10. It arrives in great condition. I use the first 2 boards right away and they work fine. It's probably a few weeks before I then make use of the next 3 or 4 boards and those are already starting to show a tiny bit of warping. Then it's probably a few months before I put the rest of the boards out to use. The warping is very obvious on the remaining boards--to the point where I'm not sure I can actually use them.

Anyone have tips on how to prevent that? Here in Minnesota, the winters are very dry and the summers get surprisingly humid. I'm guessing it's those changes in humidity that cause the warping.

We tend to have damp here most of the year, so I stored my foamcore tiles in plastic boxes with one or two of those anti-damp sachets you get in vitamin containers and as a result haven't had any warping.

I 'inherited' a huge pile of foamcore from a bizarre company gift that included a lot of wasteful packaging. People were more than happy to give me the foamcore. Unfortunately, a lot of it has warped a bit, so I'm going to experiment with seeing if I can't cut it down to size, spritz it with little water, clamp it between two hard surfaces and let it dry for a while. Assuming that isn't too labor intensive, I'm hoping it'll alleviate the worst of the warping. I'd hate for 40 large sheets to go to waste!

p.s. The foamcore in your basement will survive much better if you can keep it clear of the walls and propped up off the floor (i.e. with airflow around it). Edge down is better, as moist air rises and will tend to move past the foamcore, rather than sitting trapped and slow moving under horizontal sheets.

Try lightly spritzing both sides at the same time before clamping or weighing down. That would balance out the moisture content on both sides so the forces working on the panel will be more even when it is drying.