I wish I had this problem. It's so dry here, your coca-cola will evaporate from the can if you don't drink it fast enough :D
Well, ok, that was an exaggeration.. But I have to use a humidifier in my sealed grow room just to keep the humidity above 20%, and I use a hydroponics method which naturally increases humidity.
70% isn't *too* bad.. Are you having any issues with mold or mildew? If not, I wouldn't worry about it, too much. Sounds like you need to figure out something for fresh air intake, though. Could you enlighten us as to what your exact setup is? Lights, fans, sealed/not sealed grow room, air intake, exhaust, co2? etc. All these bits of information will help us determine the best solution for ya. Welcome to the Farm, by the way!