Having vents in your crawl space are major problems for numerous reasons. When you think about vents in a crawl space, it's easiest to look at it this way: on cold days, we are venting our crawl spaces with cold air. On hot days, we are venting our crawl spaces with hot air. On wet days, we are venting our crawl spaces with wet air. All of the air you bring into your crawl space then flows upward into your house.