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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.
Venting a crawl space would seem to create a situation. more like that of a house on posts. By the 1950s, crawl. space ventilation became mandated by building codes. The. basis for the code requirements was the combined idea that. (1) the primary source of crawl space moisture was. evaporation from the crawl space floor and (2) crawl space. ventilation would allow for dissipation of this evaporated. moisture.