text:  And the way you do that is they have crawl space fans that are basically 8 by 16 inches, which is the same size as a concrete block. So in lieu of one of the vents, you install this fan and you wire it to a humidistat, maybe mounted somewhere in the middle of the crawl space. And then when the moisture gets really high, the humidistat will kick on the fan and it will draw some drier air through the crawl space.