Search Google for a "humidistat".

I wanted a humidistat hooked to a bathroom external-venting fan to help manage the humidity, especially condensation and ice on the windows in the Winter in Ottawa, Canada. I asked at Home Depot and they were able to provide me with something like a Broan DH100W Dehumidistat, 24V/120V (maybe a different model number 6 years ago), plus a ventilation fan. I had an electrician hook this up to the house circuit (enough though he though a simple timer was all that was needed. When the timer has a maximum of 30 minutes - I dont think so!). We can adjust the percentage humidity setting inside dependent in part of the outside temperature and corresponding humidity (there may be some way to automate that relationship). We no longer worry about condensation and ice on the insides of the windows. It can get a bit dry during the winter, but weve gotten accustomed to that.

A happy home owner, Jim Fraser