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Insulate rim joist with rigid foam: exposed concrete?

runner9 | Posted in Energy Efficiency and Durability on

I’m trying to understand the different in recommendation in this FineHomebuilding article:

If using spray foam it states “For good thermal performance, the foam should cover all of the exposed concrete at the top of the wall.” and the picture shows the same.

On the section above for using foam board, it does not show the top of the concrete being insulated.

I’m using 2 layers of R13 2 inch foam board in the rim joists. I could cut a small piece of foam board and foam seal it in place over the top of the concrete.

Is there a reason this isn’t recommended in the article?

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  1. GBA Editor
    Martin Holladay | | #1

    In Steve Baczek's illustration (reproduced below) of a rim joist insulated on the interior with spray foam, the spray foam covers the top of the wall. That makes sense. However, the illustration raises another question: Why is the basement wall uninsulated?

    Here's the real scoop: all of the following areas need to be insulated:

    1. The rim joist.

    2. The exposed top of the concrete wall (the horizontal "shelf").

    3. The concrete basement wall.

    The article you linked to is about insulating the rim joists. it shows a few steps in the construction process. Let's interpret Steve's illustrations this way: Steve shows a few steps, but the house isn't yet complete. After the builder finishes insulating the rim joist -- the topic of the article -- he or she still has to insulate the basement wall. When that work is being performed, the builder has to verify that the exposed top of the concrete wall is also insulated.

    You wrote, "I could cut a small piece of foam board and foam seal it in place over the top of the concrete." Yes, you could. And you should. And once you've done that, you should keep on going -- and insulate the entire basement wall, down to the slab. (See the second illustration, below.)


  2. JC72 | | #2


    Q: Is there an assumption here that there's an adequate capillary break at the mudsill or does it even matter?

  3. GBA Editor
    Martin Holladay | | #3

    Q. "Is there an assumption here that there's an adequate capillary break at the mudsill? Or does it even matter?"

    A. If this is a new house, there had better be a capillary break (usually, sill seal made of closed-cell foam) between the top of the foundation wall and the mudsill. It's a code requirement.

    If this is an older house, most builders go ahead and insulate the rim joist on the interior, even when a capillary break is lacking. How risky is this? Somewhat -- because the interior insulation reduces the ability of the sill or rim joist to dry inward, and these wooden components may be damp due to capillarity.

    In most cases, these wooden components can dry safely to the exterior. Risk factors include: a very cold climate; bushes near the foundation on the exterior; high exterior grade; a damp basement; and a lack of roof gutters. The more of these risk factors exist, the more you have to consider retrofitting a capillary break.

  4. Dana1 | | #4

    As long as the foundation wall has reasonable drying capacity and minimal bulk wetting the capillary break would not be absolutely necessary (though always a good idea.) Deep roof overhangs, surface grading directing water away from the foundation, a foot or more of exterior exposure and proper drainage at the footing, and drying capacity toward the interior all help lower the moisture content of the concrete where it meets the sill.

  5. charlie_sullivan | | #5

    If you are concerned about drying of the top of the foundation wall, I suggest mineral wool insulation on the exterior of the foundation wall, perhaps in addition to interior insulation. That will help keep it warmer (to enhance drying) without impeding the drying.

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