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Vapor barrier height and polyiso question — unvented crawlspace

agurkas | Posted in GBA Pro Help on

I am looking to convert my poorly vented crawlspace to unvented one. Because I am in Northeast, I also have to manage radon levels, so I am planning on installing 20mil vapor barrier and already have active radon system installed (but not running yet, until I have the vapor barrier)

Question 1: Considering I will be insulating the walls with polyiso, how far up the wall to I attach the vapor barrier? Foundation is cinderblock and is from 3′ to 5′ high.
Question 2: Do I attach vapor barrier behind the polyiso or on top of it?

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  1. GBA Editor
    Martin Holladay | | #1

    You should consult with your radon mitigation contractor, if you have one. But radon can enter a crawl space through cracks in a CMU wall as well as through the soil.

    Because you are trying to limit radon entry, you should extend the polyethylene up to the top of the walls and should secure the polyethylene to the blocks near the top of the wall in an airtight manner (using a horizontal batten attached with screws, and plenty of Tremco acoustical sealant). The rigid foam insulation can then be installed on the interior side of the polyethylene.

    For more information, see these two articles:

    Building an Unvented Crawl Space

    All About Radon

  2. agurkas | | #2

    I actually built my radon system. Had 12 in full size basement and now it is down to 1.4 month later.

    Reason I asked about how high I can install vapor barrier was because I have seen numerous mentions in articles here warning against any kind of poly behind insulation in crawlspaces.

  3. GBA Editor
    Martin Holladay | | #3

    Polyethylene can cause problems in walls insulated with fiberglass. But in your case, the polyethylene will help keep out the radon.

  4. agurkas | | #4

    Thank you!. That was the confusion. I am taking out all fiberglass. Looks like mice loved that stuff with previous owners of the house.
    P.S. This site has been great help. I have done a lot of projects myself with great results, because of advice I got here.

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