If you have the right size of tub/tank and a nice sized water bowl humidity shouldn't be much of an issue. I notice that if my snakes get dehydrated they will run their body through their water bowls, or get a drink. If the humidity gets low their eyes may dent, in which case I'd say just pour some water in their tub, and right before shed day I pour a bit of water in their tub as well, this way their skin comes off cleanly every time.