You wouldn't want the bedding to get wet because chicken poop plus water REEKS, but a little mistiness now and again isn't a major issue.
Generous roof overhangs are the best way to provide good ventilation while keeping rain out.
If they are fully-feathered and know that they *could* be in a dry place but *choose* to get wet it's unlikely that they'll do themselves any harm.
Chickens know how to be chickens better than we know how to be chickens. :)
If you put your hand into the damp feathers you'll probably find that...