Not knowing how many house plants, fish tanks, people or pets who live in your house, (all generate water moisture), I would say you should start by putting down at least a .6 mil plastic sheet on the dirt floor and secure it around the edges as a barrier to prevent moisture from coming up underneath the dirt. It would also be a good idea to open up any basement windows you may have, (or instal vents that can be closed in winter). Then, you should be sure that you have adequate attic ventillation. There is a formula, which I seem to recall, (though I could easily be wrong, but hopefully others here will correct me), is one square foot of vent opening per 150 square feet of floor space. That is a minimum, and in the attic, should be open all year around. The best system for attic ventillation is soffit vents, gable vents and a roof ridge vent, but two out of the three will work too. I hope this helps, good luck.