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Should I seal my crawl space vents?

timgodshall | Posted in General Questions on

I recently finished a single-room addition on our house. It sits on top of a 4′ block wall crawl space, the floor of which is at grade on one end and about 18″ below grade at the other end. I put down 6 mil poly which runs up the wall about a foot and then is sealed to the concrete block. I insulated the underside of the addition floor with R-30 batts. I also put in some foundation vents around the perimeter, spaced about every 15 feet.

I’ve read a lot on GBA about how bad vented crawl spaces are. The correct way to do them, as I understand, is to seal them, insulate them, and condition them with just a bit of air from the house HVAC system.

However, I’m wondering, given my situation, what would happen if I just sealed off the crawl space and didn’t insulate it or condition it? I understand that’s not the ideal, but wouldn’t that be better than leaving the vents open? If no moisture is getting through from the ground (due to the plastic vapor barrier) and no hot moist air is coming through the walls, what sort of problems would I be likely to encounter by doing this? (the crawl space also tends to stay very dry even if there’s a lot of rain.)

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  1. GBA Editor
    Martin Holladay | | #1

    You didn't tell us where you live. Climate matters.

    Q. "I'm wondering, given my situation, what would happen if I just sealed off the crawl space and didn't insulate it or condition it?"

    A. The only way to tell for sure is to experiment and monitor the situation. There is a big risk to this approach, however. You might monitor the crawl space for one year and conclude that everything is just fine. If you don't inspect it regularly, however, you could end up with lots of mold in 5 years. Monitoring means monitoring -- with your eyes, with a strong source of light, and with a moisture meter. You can't guess, and you can't stop monitoring.

    If you sell the house, the new homeowner needs to get up to speed with your monitoring protocol.

    It would be safer to follow the usual recommendations for sealed crawl spaces. That means that you need to insulate the crawl space walls. It also means that you need to provide a little bit of air flow.

    I sympathize with homeowners who want to seal up their crawl space and see if it works without any conditioning. (This approach saves energy.) In some climates, and at some sites, a crawl space can be just fine without any fans or registers or air exchange. But you really have to monitor the situation regularly to be sure you know what is going on down there.

    More information here: Building an Unvented Crawl Space.

  2. timgodshall | | #2

    Thanks for the response, Martin. My location is Harrisonburg, VA. We definitely get some hot, humid weather here in the summer. You are exactly right about my reason for not conditioning the crawl space from the start. I figured it's cubic footage to be almost 30% of the total cubic footage of the addition, so I thought I'd try to save a little energy by isolating it from the conditioned space. I'll keep a close watch on it and see how things progress.

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