To solve the problem, you must first recognize were the moisture comes from. If you are noticing frost, condensation, or other evidence, the cause of the noisture is interior living spac air leaking into the atticc, contacting a cold surface, and condensing. Other evidence that this is happening uneven snow melting & ice building up on the eves; Difficulty maintaining a comfortable humidity level; lower sections of the house being noticable cooler; excessive drafts (as is drafty windows. If air goes out, it has to come in.). The solution is not increased ventilation, in fact at ties I have solved it, in part, by eliminating ventilation. Rather, the solution is finding where the air is leaking & plugging the leak. You may need the services of a professional with the experience to find the problem areas (Unless your in Northern Wisconsin, I'm unavailable :) ). I suggest checking with your local utility cmpany, as they many times have a list of qualified individuals.
If you have more questions regarding cold climate issues, you might check out: http://www.weccusa.org/index.html