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Insulating a 1950’s partially embedded rim joist

user-7694953 | Posted in General Questions on

I’m zone 7A (Calgary AB) and am looking to develop my basement.  Foundation is well drained.  I’d like to insulate my currently un-insulated basement with 2×6 24″ O.C. and R22 Roxul (held in with typar) and due to codes here seal everything on the inside in with 6mm Poly.  The 2×6 wall would be 1″ away from the concrete so nothing would be touching it.

My issue is that I have a double rim joist that is partially embedded, anywhere from 1/3 to 2/3rds up the joist.  The top of foundation is anywhere between 8″-16″ away from the ground and the roof has a decent over hang (~3ft around all the sides except for the back which is 6″.  The back however is south facing so it gets lots of heat and is well sloped.) 

I’ve read all sorts of stuff about insulating partially embedded rim joists and moisture issues with them, so in order to help with that I was thinking about just sealing the air gaps in inner the rim joist with great stuff, install roxul on the top of the interior 2×6 wall, and poly it in on the inside.  I’m thinking this would allow the joists to “breathe” and still let some heat through to help dry things out.

Obviously there are more expensive/better approaches (spray foam everything, cut the joists and make interior load bearing walls etc.) but I’m trying to balance budget with longevity of the joists.  

Is this an ok approach?  Thanks!

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  1. user-2310254 | | #1

    Maybe these two articles would be helpful:

    Rim joists:


    Are you concerned that your plan for using air permeable insulation will lead to long-term issues? Have you thought about using R-15 reclaimed rigid foam attached directly to the wall? Can you install some type of capillary break between the joist and the foundation before air sealing that area?

  2. user-7694953 | | #2

    Thanks for you reply Steve,
    I'm the most concerned about trying to dry the joists embedded in the concrete out. As it's been uninsulated for 60 years, the major heat loss from the basement has done this no problem in winter, but if everything is sealed up an insulated, I'm trying to get dry(er) heat to the joist in a relatively controlled fashion.

    I did think about rigid foam attached to the wall, but we need R22 minimum up here in Canada so I'd still have to install roxul or something in the framed wall.

    I'm not sure there's a way to install a capillary break between the joist and the foundation except for possibly drilling holes into each inside rim joist and trying by best to spray foam the top of the concrete? Thanks!

  3. user-2310254 | | #3

    I'll let one of the professional chime in about whether you can use air permeable insulation in a basement.

    On the joists, if you search GBA, you find several threads that discuss this type of scenario. One solution is to build a new bearing wall inside the foundation and then cut off the ends of the joists that are in contact with the cement and then applying foam. Since you are considering framing a new wall anyway, this might be a workable solution (to me). But again, one of the building professionals reading this thread might have a better idea.

  4. GBA Editor

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