I have been reading on crawl space vents. There are many differing opinions in there use. I think the best answer to the crawl space vent issue for use of them in the Greenville, SC area came from a pest control company. The reason for crawl spaces vents is just what they say- to ventilate the crawl space to keep the moisture levels down to avoid mold, mildew, and structural issues. The problem with well-ventilated crawl spaces is that it can cause drafts in the home in the winter, and loss of energy raising your energy bill. So, my suggestion is to shut your crawl space vents in the winter but do not forget to open them in the Spring. It is all part of a well maintained home. As we head into colder weather, save your self some money and have a warm cozy home in the winter! But, remember to open them in the Spring! If you are going to be forgetful, please leave your crawl space vents open all year. The pest control company said that is the biggest cause of unwanted excess moisture in the crawl spaces!