I live in western North Carolina in a home I built in 2000 with a crawl space. The home is on a slope so the crawl space is about 5 feet high at on end tapering to about 2 feet at the other. The home is about 2000 sq. ft. The only appliance in the crawl space is a high efficiency furnace. My question is this: should I close tightly all the crawl space vents during the winter months to keep the floors and house warmer or could that create a moisture problem? The furnace has an air intake vented directly for air and the entire "floor" surface is covered with heavy black poly. My mind tells me to seal the vents for the winter, but I am from norhtern Minnesota and have a mindset that is used to dealing with extreme cold. PLEASE HELP