Inkbird sells a humidistat for greenhouses that might do what you want.
For something more complex, a humidistat signal could trip a programmable relay to open a solenoid or motorized ball valve. Any good industrial tech capable guy could make that work for you as well. It's not clear to me what exactly you want to do, but programmable relays can do it, so can an Arduino, PicAxe, etc. There's 101 ways to do what you need.