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Insulating the Rim Board Between the I-Joists

Miles007 | Posted in General Questions on

I’ve read lots of Q&A posts here, regarding I-Joists.  And I’ve read this article, talking about leaving a space between the rigid foam and the framing.
https://www.greenbuildingadvisor.com/article/insulating-rim-joists

The concrete stem walls of this basement will get (2) layers of 2″ Polyiso rigid foam (which will get covered by 1/2″ OSB).

The Rim Board will also get (2) layers of 2″ Polyiso rigid foam.  The I-Joists are 11-7/8″ high.  Following advice from other GBA posts, I will fasten the foam at the rim board with a little bit of adhesive, and 4-1/2″ screws through 60mm Polypropylene washers.

Instead of cutting (4) notches around the I-joists in each piece, I am thinking about 1) cutting rectangles to fit between the inside faces of the I-joists (so, approximately 13-1/2” wide), and all the way from the mud sill to the subfloor (with the requisite gap), and then 2) filling the gaps on the sides (approximately 1” wide x 4” deep x 8-1/2” high) with canned spray foam.

Alternately, I could get some 1” thick rigid foam, and cut similar-sized pieces:  1” thick x 4” wide x 8-1/2” high.

Question 1:  Which would be better?  Since it’s a good idea to put spray foam around the edges of the rigid foam on the rim board anyway, it seems like the first option would be quicker, right?  Lots of cans, though, I imagine.

Originally, I was planning on running the rigid foam of the walls only to the top of the concrete stem wall.  But now, I wonder if it might make more sense to run it all the way up to the bottoms of the I-Joists. That would leave a gap between the wall foam and the mud sill that’s approximately ½” deep x 1-1/2” high.  Again, I could fill that gap with either spray foam, or rigid foam.

Question 2:  For this condition, which would be better?

Question 3:  Does it make sense to run the wall foam all the way up to the bottoms of the I-Joists, as described above?

 Please see attached photo.  This project is in the Columbia Gorge, WA.  Climate Zone 5B.

Thanks,
Miles

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Replies

  1. Malcolm_Taylor | | #1

    Miles,
    You want to completely seal the foam board layer both at the perimeter and at any joints. So with that in mind:
    1. I'd notch the larger board at the corners, which limits joints.
    2. For a gap that small canned foam is your best bet.
    3. Definitely run the board up the the underside of your I-joists.

    1. Miles007 | | #2

      Hi Malcolm. I was hoping you'd reply. Especially since you have already replied to other posts that I've done on this project. Thanks very much for the info!

      Miles

  2. Dancravn | | #3

    Hi Malcom and Miles,
    I have a project with a similar situation and I am planning to do closed cell spray foam in the interior at the rim joists/ sill of an unvented crawl space. It seems like a pretty straight forward way to insulate and help prevent any air leaks. I am also planning to have rigid insulation on the exterior side (2" GPS) from the top of the footing to just above the rim joist. This exterior insulation will then have a stucco finish. My question is, how would you inspect the sill plate for termite activity in the future if all of the wood members are encapsulated by either rigid foam or spray foam? I came across some articles that insurance companies are starting to reject homes that have spray foam covering wood due to the inability to inspect. The project is in zone 5A. Thank you.

    1. Miles007 | | #4

      Hiya, that's a good question. I will defer to the more knowledgeable people on this forum. I do know that closed cell spray foam is not a very popular choice, because of its environmental impact.

    2. Expert Member
      BILL WICHERS | | #5

      Common practice in termite prone areas is to leave an inspection strip below the rim joist on the masonry part of the foundary wall. Since the termites come up from the ground, you can see their mud tunnels along the masonry in that inspection strip area. You don't need to do anything special with the insulation itself in the rim joist area, just stop any insulation on the foundation wall an inch or two before you get to the rim joist area.

      For the OP: I recommend Loctite's (apparently now GE's) Tite Foam for foaming in rim joist insulating panels. I find it does a better job (denser and more durable foam) than the more common Great Stuff product. You will need a lot of cans for the job, I'd recommend buying a case or so and see if you can get a quantity discount. I find one can gets maybe 3-5 joist bays and then it's done.

      Bill

      1. Miles007 | | #8

        Hi Bill. Thanks for reminding me about that. The termite inspection strip is well illustrated here:
        https://www.finehomebuilding.com/project-guides/insulation/crawlspaces-that-work

        Good to know about the Loctite Tite Foam, too. And there is indeed a discount for buying (12) or more, at Home Depot. $4.48 per can. At least right at the moment, anyway.

  3. jadziedzic | | #6

    DAP's "Sharpshooter Extend Foam Applicator" for use with gun-grade canned foam works really well for hard to access areas like the rim joist: https://www.dap.com/products-projects/product-categories/spray-foam-insulation/spray-foam-tools-accessories/x-tend-wand/ . The hand-held applicator attaches to a gun-grade foam can via a 5-foot hose and is much easier to use in the rim joist area than the cans with the attached "straw" that require you to hold the can upside down which then bangs up against the subfloor.

    1. Expert Member
      BILL WICHERS | | #7

      That is awesome! Kinda spendy though (a bit over $60). I usually just using some tubing to extend the can or "pro" gun, but I'll have to think about that nice DAP one...

      Thanks for posting that.

      Bill

      1. jadziedzic | | #10

        Hi Bill - I was put off by the price at first, and then I found it available at Menard's for $35 after a bit of Google Fu. If you don't have a Menard's locally, Menard's will ship it for a reasonable price. Buy two, they're cheap ;-)

        One caveat: you can't leave the Sharpshooter attached to a can of foam for more than a day without a chance of it developing an internal restriction that reduces the foam flow significantly. It's best to clean the Sharpshooter immediately after use, and that keeps it reusable for at least a couple of sessions.

    2. Miles007 | | #9

      Thank you. That looks good. I bought some vinyl tubing at Tap Plastics, and I'm gonna give that a try.

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