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Crawlspace Insulation; getting conflicting information

gjmw | Posted in Energy Efficiency and Durability on

I recently bought a 1974 single story ranch house in the northern front range of Colorado.  After insulating and air sealing in the attic I’m turning my attention to the crawlspace that is vented to the outside.  There is a a robust vapor barrier in place already.  My furnace, ductwork, and plumbing are all in the crawlspace.  

I spoke with a contractor yesterday who was adamant that the insulation I use on the inside of the stemwall should be vapor open (mineral wool or a fiberglass blancket).  He argued that if the concrete (no damp-proofing) can’t dry to the inside it could result in transferring moisture to the framing.  My understanding was that the insulation should be vapor closed (polyiso) so that warm, moist interior air doesn’t condense on the cold foundation wall.  

I’d appreciate any thoughts..thanks!

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  1. Expert Member
    Peter Yost | | #1

    Hi Micah -

    1. Is the portion of your crawlspace foundation above-grade without any moisture protection and absorbing bulk water? If so, you need to move that load off of the foundation and/or apply a moisture-resistant coating of some sort. You can paint the concrete.
    2. If your concrete crawlspace wall can dry to the exterior, then it does not matter whether or not your interior insulation is vapor permeable. More important that the unvented crawlspace has a continuous air control layer at the perimeter, particularly up in the rim joist assembly.

    1. gjmw | | #5

      That's helpful, thanks!

  2. user-2310254 | | #2


    Have you read Martin's article on crawl spaces (

    I think the biggest concern would be at the sill plate. Can you tell if there is a capillary break?

    I suspect the experts will suggest that your contractor is misinformed.

  3. Expert Member
    BILL WICHERS | | #3

    Where is the vapor barrier you mention? It sounds like it’s in the crawlspace, but does it go up the walls too? If it does, the concrete is already sealed from drying inwards. Concrete doesn’t need to dry, it’s happy to be wet forever. Your contractor is correct about the possibility of “rising damp” getting moisture into the framing in the rim joist area. Make sure you have a capillary break there (plastic or metal usually). If you do, you’re all set and can insulate with foamboard without problems. If you don’t have a capillary break, you might want to add one unless your wall is exposed on the outside.

    Personally, I think having the capillary break in place is the safest option.


  4. Expert Member
    Dana Dorsett | | #4

    Using 3" of reclaimed polyiso on the walls that stops a foot from the floor would allow ample drying toward the interior, even if there isn't much above grade exposure. Taping the cut bottom edge of the polyiso and installing 3.5" rock wool in that bottom foot would deliver the code-min R15.

    Used foam is dirt cheap (usually much cheaper than batts, WAY cheaper than rigid rock wool) and fairly avaiable in your area:

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