Originally designed to control humidity in the crawlspace, these vents actually have been shown to increase humidity rather than to decrease it. Think about it: in the summer months, the outside air is usually hot and humid. When a crawlspace is wall-vented, the same hot-humid air outside is passing through…leaving condensation on the floor joists and air ducts. Excess condensation leads to mold, fungus, and foul odors in the crawlspace. Crawlspace vents need to be closed to seal out unwanted humid air. Dirt crawlspaces also need a proper vapor barrier installed on the floor and exterior walls. An encapsulated crawlspace will properly control humidity, leading to a healthier and more energy efficient home.
A study by Advanced Energy shows the outside relative humidity (green) vs the relative humidity inside a vented crawlspace (red) vs the relative humidity in a closed/encapsulated crawlspace (blue). Notice the striking similarity between the green and red lines: the vented crawlspace humidity follows closely with the outside air relative humidity…proof that the vented crawlspace simply does not work! A closed/encapsulated crawlspace maintains a steady, low relative humidity, and is not heavily influenced by outside air. See more at www.crawlspaces.org