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Community and Q&A

Rim joist insulation

dasrks | Posted in General Questions on

Im building in zone 5. My Bond area consists of a mud sill, OSB rim joist and TGI floor joists covered with Henry Blueskin (33 perms) and 2”Halo Exterra (Sales rep says 9.4 perms but I think it’s .94 perms). I would like to use closed cell spray foam to insulate the bond area is this a Bad idea?  Is the wet spray cellulose ok as long as I give adequate time for drying? Thanks.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Dasrks.

    What's your air sealing strategy for this area? Does the Blueskin come down over the rim and lap onto the foundation?

    With continuous exterior insulation and all of the air sealing takes care of, closed cell spray foam is unnecessary. Any other insulation, installed properly, should be fine.

  2. dasrks | | #2

    Thank you Brian. The Blueskin stops on the mud sill. I was planning on using a liquidflash where the Mud sill meets the foundation to complete my air seal. Im a little concerned about the wet spray cellulose as well since it’s cold and will be difficult to dry. I had some moldy lumber that was delivered and installed, was wet for several weeks and propagated. I’ve had it remediated but adding that much moisture to the lumber is a little scary. Should I just use minerwool Or dare I say Fiberglass insulation instead?

    1. GBA Editor
      Brian Pontolilo | | #3

      I don't know enough about the installation process of wet-spray cellulose to say if your concerns are warranted. I would hope the installer could guide you. But if you don't feel confident in them, sure, use something else. Are you planning to finish the basement? Will there be a framed wall and a drywall ceiling?

      1. dasrks | | #7

        Yes we are planning on finishing the basement with framed walls with a drywall ceiling. With my wall detail is there an advantage to use mineral wool over fiberglass?

  3. dasrks | | #4

    Yes we are planning on finishing the basement with framed walls with a drywall ceiling. With my wall detail is there an advantage to use mineral wool over fiberglass?

    1. GBA Editor
      Brian Pontolilo | | #8

      Hi Dasrks.

      If you use rigid mineral wool it may be easier to cut and fit nicely. That's the first thing that comes to mind.

      1. dasrks | | #10

        Thank you

    2. Expert Member
      BILL WICHERS | | #9

      To add to what Brian said, mineral wool will usually support itself where fiberglass oftentimes requires some kind of retention (wire, staples, etc.). Mineral wool can usually stay put using nothing but its own structure and "springiness".


      1. dasrks | | #11

        Thank you

  4. walta100 | | #5

    Is there a deck attached to the rim joist?
    If so be careful as the contractors will try to remove the foam insulation over the rim joist giving you a cold spot.


    1. dasrks | | #6

      Good point Walt thank you. There is a deck but I’m the one putting it on so I’ll be sure to leave the insulation in place.

  5. walta100 | | #12

    Unless you build the deck so it is self supporting, the foam can become a structural issue.

    Before you finish the basement be sure you understand the newer deck support requirements required by today’s codes. Some would call them unbelievable.

    Going back later and trying to get the ledger flashing installed correctly almost never happens.


    1. dasrks | | #13

      Going to make it self supporting, thanks again for your help.

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