Bathroom mirror cupping due to shower steam. Will sealing it help?

I recently bought a 250 dollar mirror from Rooms To Go. It's a frameless mirror with a wood backing. I hung it in the bathroom and it has bowed away from the wall in the middle. Im assuming the steam from the shower has caused it. They are sending me a replacement so my question is how do i keep this from happening again? Can i seal the exposed surface of the wood backing? Will that be enough? What kind of sealant? Is there anything else i can do?