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Insulating rim joists

Pcoyne123 | Posted in General Questions on

Hello. I’ve been reading q&a on this website for a long time and have always found the content very helpful. This is my first time posting a question and hope someone has some information for me.

I’m finishing my basement and planning the insulation portion. My basement is block walls and in the rim spot where a rim joist would normally be it is also a cement block. The house is well built and is of double brick construction ( brick wall on the exterior a small gap and an inner cinder block wall. ) the joists are resting on the basement block wall and they are about 2 feet above the outside grade. After a great deal of research specific to my location i have decided on the following approach for the basement walls: roxul board (r6 I believe) fastened up to the block walls.. A 2×4 wall built up against the roxul and the cavities insulated wth r14 roxul. Then a vapour barrier on the interior. Specifically a smart barrier which apparently allows drying to the interior while not allowing vapour to penetrate the walls from the interior. For the rims I plan to do 2″ rigid insulation board with “great stuff” foam sealing it in place around the edges. Here is my question: the normal rim joist bays are simple, however on 2 sides of the basement where the joists run parallel with the block wall I can’t insulate with the rigid in the same manner. The last joist is only about 1″ max away from the block wall and I have no idea what’s behind that joist. I considered simply installing the rigid on the inside of that final joist effectively treating it like a rim joist but I underdstnd that would be leaving the joist unheated and would become somewhat of an exterior joist and exposed to the cold. I know some would say spray foam up in the 1″ gap but I am avoiding spray foam at all costs.

I Know this was a lengthy post and I apoligize but I’ve been toying with this for a long time and wanted to properly explain it.

Thanks in advance. If necessary I will submit a photo.

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  1. Pcoyne123 | | #1

    In case a little clarity is needed here is a YouTube link to a video explaining my insulation plan.. The only difference is that the video doesn't discuss the rim joist area and appears to show roxul being used but I intendd to use rigid insulation board and tie the vapour barrier from the wall into the rigid.

  2. GBA Editor
    Martin Holladay | | #2

    P Coyne,
    Unless I missed it, you forgot to include your climate zone or geographical location. That information is necessary for us to provide you with good advice.

    I strongly suggest that you read this article: How to Insulate a Basement Wall.

    In that article, I explain why you don't want to install an air-permeable insulation like mineral wool against the interior of your basement wall. (Mineral wool is air-permeable, and it allows warm, humid interior air to contact the cold basement wall, allowing condensation to form.) What you want in this location is a continuous layer of rigid foam (polyiso, EPS, or XPS).

    In the video you linked to, the salesman was proud that the mineral wool allows the damp basement wall to dry inward. Why would you want that? No one should be inviting ground moisture into their home.

    When it comes to the 1 inch gap between your last parallel floor joist and the exterior wall, the best solution is to use spray foam. If you really don't want to use spray foam there, I guess the second-best approach would be to shove chunks of mineral wool up there with a drywall taping knife and a paint-stirring stick, and then seal the bottom of the crack with high-quality caulk. That's not as good a solution as using spray foam, but it sounds like you don't want to use spray foam.

  3. JC72 | | #3


    The OP basically described Roxul's method of application almost word for word as it pertains to finished basement walls. Perhaps Roxul assumes that the drywall serves as an adequate vapor retarder? *shrug*

  4. GBA Editor
    Martin Holladay | | #4

    Roxul is in the business of promoting the sale of their insulation products. I disagree with Roxul on their basement insulation recommendations.

    I'll repeat: You don't want to encourage a basement wall to dry to the interior.

  5. Pcoyne123 | | #5

    Thanks Martin for the quick reply. I was hoping you would have a look at my post as I have read many of your recommendations on the topic. I agree with you that I don't want to invite any additional moisture into the interior. I have a technical background and have been doing a ton of research on this topic so I have a fairly good grasp of the principles involved. I actually stumbled on my planned method by a suggestion of a friend who is normally up to date on the cutting edge research and he felt that this design was uniquely suited to my geographic area. I. guess the intent of the design is to deal with the moisture that ultimately will find a way in via migration. I have also looked extensively at the xps design as that was my first plan. In case your are interested I have an additional link which is a document that outlines the same info the in the video but with a little more detail. Apparently a college was testing the method but I haven't had any luck locating the results.
    I'm going to take another look at my plans based on your concerns and reconsider the xps design that I strayed away from. By the way I'm in Canada in a climate that is effectively zone 6.

    I will look to see if I can get the roxul up between that last joist as you suggested. I wouldn't be opposed to using "great stuff" up there Instead of hd caulk to seal it off , I just need to stay away from a two stage spray foam in any large quantity.

    Thanks again for your advice.. your suggestion brings up a follow up question. My understanding is that in my climate my vapour Barrier needs to be in the inner most side of the insulation. If I do the xps it becomes my vapour barrier.. In your opinion could I then add roxul to the interior of the xps, effectively insulating before the vapour barrier? Or do I need to do a thickness of rigid that will meet my goal of r20 to keep the vapour barrier in the correct position?

  6. GBA Editor
    Martin Holladay | | #6

    P. Coyne,
    I disagree with the approach used by Doug Tarry Homes.

    I disagreed with the approach when it was presented in a video.
    I still disagree with the approach when it is presented in a PowerPoint presentation.

    As I said, my advice is summarized in my article on the topic (How to Insulate a Basement Wall). In that article, I answer many of your questions.

    There is no need of an interior vapor barrier on this type of wall, as my article notes. No interior polyethylene. No interior "smart retarder." Just drywall, paper-free drywall, or cementitious backerboard on the interior side of the insulation, installed in a relatively airtight manner.

    If you don't want to take my advice about using 100% rigid foam to insulate the basement wall, you can first install 2 or 3 inches of rigid foam to the wall, followed by a 2x4 stud wall on the interior side of the rigid foam. If you do this, you can install mineral wool insulation in the stud bays if you want. But the rigid foam is an essential part of this wall assembly.

  7. Pcoyne123 | | #7

    Hello Martin.

    Alright I've changed up my plans and have made arrangements to do the walls in rigid only. I will be starting out up next week. I wanted to bounce one last idea off you.

    You had suggested that if I don't want to spray foam the area behind the last joist( the rim parallel to the floor joists) that I could at least squeeze some roxul up there and use a high quality caulk to seal the bottom of the joist. I had a look and may have another option. It looks like I would have enough room to slide up a 1/2" piece of rigid up there. What are your thoughts on me cutting a half inch piece the exact height of the joist, putting on some adhesive and sliding it up holding tight against the joist until it sets. Effectlvely insulating the outside face of this last joist and then I could send up as much "great stuff" (maybe window and door type to prevent any over expansion) as it will take and seal the bottom 1" gap up. The cavity behind this joist may be many inches deep to the block wall , I can't be certain, but at least I may get a good seal at the joist area. What do you think?

  8. GBA Editor
    Martin Holladay | | #8

    P. Coyne,
    Your plan sounds fine.

  9. Chaubenee | | #9

    Why not use a greener open cell foam? Something froM Aloha perhaps?

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