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Community and Q&A

Rim Joist Insulation Alternative

rshuman | Posted in Energy Efficiency and Durability on

I have read articles detailing the preference for using spray foam to insulate rim joists in unfinished basements. Facing difficulty in finding someone to do this work for me, an alternative was presented to me recently. That being using about 12″ of blown-in cellulose instead. Intello or a similar product would be used as a vapor retarder.

As near as I can tell, non-spray foam alternatives fall out of favor because they are not very effective in keeping moisture from the rim joist.  Although the cellulose could also come up short in this regard, would the inclusion of a smart retarder minimize or eliminate the potential for problems caused by moisture?

I suspect the spray foam will seal the rim joist area better than the cellulose. Would you agree?

Thanks in advance.

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  1. thegiz | | #1

    Cut 2 in rigid foam squares a little but loose. Push them against rim joist and used canned spray foam to seal the edges, from there you can stack more rigid foam or use fiberglass batts. This is basically a cheaper alternative to using spray foam. Spray foam by an installer would be best but this works ok.

    1. monkeyman9 | | #2

      The nice thing about this method is if you have to do any work later, the rigid foam boards are easier to get out. Just write wire or pipe on them if you're blocking one so you don't cut through it later.

  2. Expert Member
    BILL WICHERS | | #3

    You really need a vapor impermeable insulation here to be safe. Closed cell spray foam meets that requirement, as does rigid foam locked in place with canned foam. I used 2" EPS on my own rim joist, secured in place with canned foam. I cut the edges of the EPS with a bevel (using a hot wire cutter I built), so that the force of the expanding canned foam tends to press the EPS panel back against the rim joist instead of away from it. I used pieces of scrap wire to hold the EPS panels in place while the canned foam cured too (bend the wire into a bit of a "U" shape, then press it against the EPS panel so that the ends of the wire bite into joists or the sill plate and subfloor to act as a sort of clamp to hold the EPS panel in place temporarily).

    I used Loctite's TiteFoam as the canned foam, as I've found it to be a little more durable, and I wanted a secure installation here. I've had no problems with the final installation. The only downside to using pieces of rigid foam in this way is that it's a lot more labor to install than spray foam.


    1. Expert Member
      MALCOLM TAYLOR | | #4


      I've found that for basements and crawlspaces on new construction, the time to put in the foam board is during framing, before the sub-floor above goes on. I tack the foam with one framing nail, leaving them to be sealed at the perimeter later on.

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