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Thermal separation of sealed crawlspace from room above. Do I need air sealing?

agurkas | Posted in GBA Pro Help on

I have posted this before, but I think it got lost during one of the site maintenance periods, since I can’t find it.

2/3 of my house is over full basement and 1/3 is over crawlspace. For radon mitigation I am sealing up the crawlspace and insulating the walls with polyiso.

Challenge I am encountering is because cralwspace and basement are connected/vented via two large former basement windows (they did crawlspace for addition), temperature difference between basement/crawlspace and floor above is at least 10 degrees.

I got a really good deal on 3″ Roxul AFB and I am planning on putting it between the joists in the ceiling of the crawlspace. My logic is to stop the wood floors from getting too cold, because of temp difference with crawlspace. I also don’t want to spend my energy $ on conditioning large crawlspace.

Question: do I need air barrier between Roxul and the floor boards or crawlspace and Roxul? Or I don’t need any at all?

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  1. Expert Member
    Dana Dorsett | | #1

    I assume you're putting down a full ground vapor barrier, and sealing that to the crawlspace walls?

    Rather than putting the 3" AFB between the joists, put it on the underside of the joists, which thermally breaks the joists, and and is a heluva lot less cutting & trimming. Since it's all inside the pressure & thermal boundary of the house, you don't absolutely need for air-barriers or vapor barriers on the AFB. The density of the AFB make it fairly air retardent, and you don't lose much thermal performance to convection except at the seams. If you want it to be perfectly even and block convection through the seams completely you could staple sheet goods (anything from vapor permeable housewrap to polyethylene sheeting- it doesn't matter) on the bottom side of the joists before installing the AFB. Leave the old window ports open to the basement.

    Be sure to air-seal & insulate the rim-joist & foundation sill with the polyiso too, carefully sealing the polyiso to keep air-transported moisture from getting to the band joist. You can put pieces of 3" AFB on the inside of the polyiso in the band joist/foundation sill to fatten out the R-value there. You may want to do cover the polyiso on the walls with the AFB too, which adds R-value, and functions as an ignition barrier for the foam.

    From an energy savings point of view you get more out of the AFB added to the wall polyiso than to the floor, but from a comfort point of view there will be some benefits. By putting it on the underside of the joists, the joists are fully inside the warmer part of the house, and you don't get temperature striping on the floor from joist conducted heat loss the way it otherwise would be with insulation between the joists, even if the crawlspace drops to the freezing level (which it won't, if the basement is insulated too.)

  2. agurkas | | #2


    Yes, everything from ground all the way up to band joist is getting covered in 12mil vapor barrier I got from radon mitigation supplies company. It will help with moisture too. Lastly, those CMU walls aren't exactly nice and air-tight either.

    Band joist will need to be sprayed. Just too many irregularities and weirdnesses there. Going to wait for summer to do that, so I don't risk curing problems.
    Walls are getting 3" of polysio

    Totally see attaching AFB as you recommend. Couple of questions that come:
    1. Electrical - sound like junction boxes would need to be relocated to surface (not a big deal, I can do that)?
    2. How would you fasten the AFB to the joists?

  3. GBA Editor
    Martin Holladay | | #3

    1. Yes, electrical boxes have to be accessible -- not buried in insulation.

    2. Mineral wool insulation can be attached with cap nails, which are manufactured in lengths up to 8 inches:

  4. agurkas | | #4


    Any idea where I can find those?

  5. mackstann | | #5

    The big blue and orange stores have them. Search "cap nails" on their websites.

  6. GBA Editor
    Martin Holladay | | #6

    I can buy cap nails for installing rigid insulation at my local lumberyard.

    If your lumberyard doesn't carry the length you need, call up Simplex Nails (800-622-3354; Americus, Georgia) and ask them to give you the name of a distributor or retailer near you.

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