Crawl-Space Vents

What purpose do they serve, and can you do without them?

Ask This Old House Crew
Photo by Matt Kalinowski

I recently bought a 1950s home; half the house is over a basement, while the other half was built over a crawl space. The crawl-space half has gratings built into the foundation wall, which I'm told were to ventilate the hardwood flooring in this portion of the house. The gratings are badly rusted, and the mortar is loose around them, so I was thinking about having the gratings removed and the holes filled with cinder block. Does this sound okay?
— Mike, Newark, DE


Norm Abram replies: It depends. Foundation vents are typically required by building codes to allow the escape of moisture that accumulates in crawl-space areas. So if the crawl space is separate from the basement, you probably should install new vents to prevent moisture from building up in that area. (Get the kind made of rustproof plastic or aluminum.) But if your basement is open to the crawl space, you'd be better off without vents.
In either case, if the crawl-space floor is dirt, cover it with 6-mil plastic sheeting to reduce the amount of moisture vapor entering the area.
Before you do anything, however, talk to your building inspector about the crawl-space ventilation requirements in your area. Codes often differ from town to town, so local experts are your best guide. Take some photos of the vents so the inspector can get a better idea of what's going on.


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