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Community and Q&A

Seeking Advice: Sealing Crawlspace Vents/Other Penetrations

user-7006241 | Posted in Green Building Techniques on

I haven’t been on the site in awhile and I’m not sure if the thinking around sealed crawlspaces has changed, so I was looking for feedback from the community. I have a property that has some high moisture levels in the  crawlspace. I have a good vapor barrier and I can do some additional grading to encourage groundwater to flow away from the building, but ultimately I need to improve the crawl a bit.

This is a rental property, and a nice one, so I’m going to invest in it but I can’t afford to do a full blown sealed/closed crawlspace.

As I see it my options are to seal the foundation vents and any foundation penetrations and…

1) install a dehumidifer along with a foundation vent fan to blow air out of the crawl and encourage air to cycle from the house to the crawl and out
2) don’t install the dehu and instead purposefully allow some air from the ductwork to get piped into the crawl (to put conditioned air into it)

Thanks for any help! (especially if there are other options I’m not aware of)

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  1. GBA Editor
    Kiley Jacques | | #1

    I’m giving your question a bump. While you wait for feedback on your proposed solutions, here are two articles that might be of interest: Details for a Closed Crawlspace and Sealing a Crawlspace.

  2. Expert Member
    BILL WICHERS | | #2

    If you can't do a "full blown sealed/closed" crawl space, then what are you doing? Just sealing the vents? If you leave the walls uninsulated, you could have condensation issues, even with a dehumidifier running. I'm assuming you have a full liner too -- you don't want to have the crawl space connected to the interior home air space without that liner.

    If you're not able to put in a liner and insulation at this time, then a small exhaust fan is probably your best bet, with the fan blowing to the outdoors. I'm not sure how much benefit a dehumidifer would be in this case, since the small fan may well be enough to overwhelm the dehumidifier. You don't want the dehumidifier running continously just to dry out air you're going to blow outdoors anyway.

    I would try to at least put in a liner and some (even if below code minimum) continuous insulation on the interior side of the crawlspace foundation walls. Any R value you add will help reduce condensation issues.


  3. user-7006241 | | #3

    thanks for the suggestions. for context, my climate zone is 4 (NC). so it's pretty gnarly humid in the summer.

    i do not currently have a liner on the foundation walls, nor do i have insulation on the interior of the foundation walls obviously. i do have insulation between the floor joists. yes i am planning to seal all of the foundation vents.

    after reading through some old posts on this topic it seems like the best bet is to install a foundation exhaust fan (blowing air out of the crawlspace). i can always monitor the conditions in the summer and install a dehumidifier to run a little bit if needed.

    so i guess the question at this point are

    1) whether i need to at least run a liner on the interior of the foundation wall if i'm not going to insulate the wall and

    2) should i mechanically pump in a little air to the crawlspace through a supply register in the crawl. in previous articles it seems like this is what the IRC code wants. although if you are exhausting air out of the crawl won't it pull enough air from the house through all kinds of little cracks and crevices?

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