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So a 2,000-square-foot crawl space would need a 40 cfm exhaust fan. It's pulling just a little bit of air from the crawl space. The problem is that you don't really know where the makeup air for the crawl space is coming from. Yes, the code specifies a pathway for air to move into the crawl space from the house, but air doesn't always follow those arrows in the diagrams.
Think of a crawl space as a mini-basement. Its height can vary from 16 inches to four feet. The ground that it rests on can be prepared in many ways: by pouring several inches of gravel on top of a vapor barrier, by building a block foundation, or by pouring concrete.