Controlling the heater via a humidity sensor

A project i have been wanting to design is to use hygrometer to turn on/off the heater i have in my room. The heater makes the room too dry. The plan is to keep the window open (to allow for humidity to enter the room) and switch off the heater when the humidity drops below a certain value. I am not very sure how to get this working, but googling around a bit led me to this:

http://marcusgun.awardspace.com/humidor.html