I used to do framing for years while in college. Sadly, there isn't much you can do to keep moisture out of a frame. Even sealing the back doesn't prevent moisture from seeping in. My advice would be to avoid putting anything in the bathroom you're not able to replace. No originals, limited editions, etc.
But to prevent buckling, you can have your prints dry-mounted or wet-mounted. That will help keep them flat for longer.