Natural gas funaces produce humidity on their own. When you burn natural gas, the by product is mostly the composition of water. This will add humidity to the inside of the house and condense on your cold windows. If you had efficient windows, the water would not condense as much since the glass would not get cold enough on the inside. I would guess that your furnace is probably where most of your moisture comes from. If you have a crawlspace anywhere under your upper floor, that could produce moisture too if it is not vapor wrapped. (Those ventless gas heaters usually used in garages produce ALOT of moisture)!