Well you don't want high humidity or bone dry either. Get a guage and keep track of the humidity. You may be a candidate for a humidifier in the winter in Montana but get a guage and see what you have first. I forget the exact figures and someone can correct me but keep the humidity in the 35 to 40 percent range and you will be fine.
In regards to your original post, I may have read it too fast and glossed over on the crack in the wall. That may be from frost heave or settling and not a humidity issue as I suggested about the door. How deep is your foundation? I would point that out to the engineer who inspected it and he could tell you more than I can over the net. I'd have to ask a ton of questions about your house and even then it is hard to say without seeing. Call him up.