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Community and Q&A

Insulating rim joists: Is it OK to do so in winter?

GaryFD | Posted in Energy Efficiency and Durability on

I read Martin Holladay’s article on insulating rim joists in Fine Home Building, and also the comment thread I found on this site, and I am planning on using rigid foam to replace the fiberglass batts that are in our house. (House is a 1950s bungalow, we live in CAlgary, Alberta, and have issues with cold and condensation around the baseboards on the main floor).

My question is, is this a project I can/should undertake in the winter when the outside temperature is around -10 C, or do I need to wait until spring/summer when all surfaces should be dry?

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  1. GBA Editor
    Martin Holladay | | #1

    You don't want to do this work if there is any ice or frost on the interior of the rim joists. But as long as there isn't any ice, I think you'll be OK.

    I'm assuming that the rim joist can dry to the exterior. To make sure that assumption is correct, you should tell us what materials you have on the exterior side of your rim joist.

  2. GaryFD | | #2

    Thanks Martin.

    To answer your question, the exterior of the house is (from the outside in): concrete stucco, behind which I assume is probably tar paper, given the age of the house, and then the sheathing. Does that help?

    I've measured the depth of the rim joist cavities, and they are about 5". Should I use a double thickness of rigid foam, and if I do, should seal each one with spray foam, or just spray foam once both are in place?

    thanks in advance.

  3. GaryFD | | #3

    Oh, and the house sits on a solid concrete foundation, not blocks. I noticed that mentioned in one of the posts.

  4. GBA Editor
    Martin Holladay | | #4

    Older cement-based stucco is fairly vapor-permeable (as is asphalt felt), so I think you will be fine.

    In cold climates, most above-grade exterior walls are insulated to at least R-20 these days, and that's a good target to aim for when you insulate your rim joists. Two layers of 2-inch thick rigid foam would be a good idea. It's also a good idea to air-seal the perimeter of each layer of rigid foam.

  5. GaryFD | | #5

    Thanks Martin, this is really helpful. So just to confirm, here are the steps I should take:
    1- remove fiberglass
    2- cut rigid foam to slightly smaller than cavity
    3- install rigid foam in place and surround with spray foam
    4- repeat steps 2 and 3 for a second layer.

    any requirement to caulk the rim joists before step 2? or does the spray foam serve the purpose of blocking air?

    also - any time delay required between putting in the first and second "layers" (ie for the spray foam to cure) or can I just put them in one after the other?

    thanks again - you've been super helpful!

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Any requirement to caulk the rim joists before step 2?"

    A. It wouldn't hurt -- especially if you see a big crack or can feel air movement.

    Q. "Does the spray foam serve the purpose of blocking air?"

    A. Yes, but a belt-and-suspenders approach is a good idea.

    Q. "Any time delay required between putting in the first and second layers (ie for the spray foam to cure) or can I just put them in one after the other?"

    A. It's better to let the spray foam cure so you don't get it on your fingers. Use a light touch for the first layer -- too much spray foam will cause more problems than too little.

  7. GaryFD | | #7

    Thanks Martin. it turns out, on closer inspection, that there is some ice and frost below the rim joist cavity (its currently -18 C) so I guess I'm waiting for warmer weather to tackle this project.

    It also appears (or at least feels like - I can't actually see) that behind my finished basement walls there is a layer of poly sheeting against the concrete foundation wall, fibreglass insulation, then another layer of poly sheeting and then the drywall. Is this usual?

  8. Andrew207 | | #8

    I have some ice/frost on my rim joists right now. Should I wait to insulate until it disappears or until summer for a full dry of the sheathing? The exterior is vinyl siding.

  9. GBA Editor
    Martin Holladay | | #9

    You can either use a hair dryer to dry out each bay before you install the rigid foam and seal up the edges, or wait for a break in the weather (so you can perform the work when the foam is ice-free).

    If the rim joist is slightly damp, it will still be able to dry to the exterior. But you probably don't want to do this work when there is ice.

  10. GBA Editor
    Martin Holladay | | #10

    Gary Fredrich-Dunne,
    One of your questions (in Comment #7) was posted two years ago, and your question never got an answer. (Sorry about that.) My answer is probably too late to be useful, but I'll provide it anyway, for the benefit of any GBA readers who stumble on this thread: It's a bad idea to insulate a basement wall with polyethylene sheeting and fiberglass batts. What you want is rigid foam or closed-cell spray foam.

    For more information on this issue, se How to Insulate a Basement Wall.

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