Generally, air exchanges utilize a damper to allow fresh air to circulate at controlled amounts. There is no way to completely keep moisture out of a house. When you get even small amounts of moisture, while circulating the same air without exchanging it for fresh air, the house becomes a peachtree dish for mold. There are times in the summer when I think that my house might be just a tad too tight. As far as figuring it, that's not exactly my wheelhouse. It has to do with amount of cfm, square footage, etc.