A speed control for your exhaust fan will go a long way to controlling the environment in the grow space.Thank you Modest grower, the RH is at 52%, I would be able to bring that up a bit, but probably it is the (too low) temperature, which is at 18C / 65F right now. I think the (maybe too strong) exhaust fan is sucking cold air from the basement into the dresser that is my "grow room". I will see what I can do about that, if I can increase the temperature, the RH will go a bit up again as well.
Thanks again for your comment, I was too focused on the rabbit stuff that I forgot to look for anything else.