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Rim joist insulation confusion

PJClem | Posted in Green Building Techniques on

I am building 3-level house, including basement of poured concrete foundation wall, so there will be 2 rings of rim joists, both above grade.  Air barrier for enclosure will be at exterior sheathing, which will be in assembly able to dry to interior as well as exterior.  I would like to avoid spray foam entirely and minimize use of foam board.  Climate zone is 3A (Atlanta).

https://www.greenbuildingadvisor.com/sites/default/files/The%20Best%20Way%20to%20Insulate%20a%20Rim%20Joist_FHB189.pdf and https://www.finehomebuilding.com/2013/09/12/insulating-rim-joists suggests insulating rim joists with impermeable insulation (i.e., foam) so that moisture does not condense on inside of rim joists.  I generally understand the articles, but I am confused as to how interior rim joist condensation in my build would be any different from condensation at interior of exterior sheathing, which will be insulated only with permeable insulation (i.e., fiberglass or maybe cellulose).

Can someone please help me clear up my confusion?

If I were to use permeable insulation at rim joists, could I not expect the same drying of moisture that I expect at the sheathing to keep rim joists free of rot and mold?

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    I’ll leave the condensation part to others more knowledable about that aspect than I.

    A big part of the recommendations to use spray foam on rim joists is to ensure good air sealing of a complex area with many connections and gaps. Fiberglass and other air-permeable insulating materials will do nothing to air seal the rim joist area. Rigid foam, when sealed in place with canned spray foam, also provides an air seal, but it’s more labor intensive to install and is more likely to miss some gaps that spray foam would have sealed.

    Bill

  2. Expert Member
    Rick Evans | | #2

    PJ,

    The insulation in your walls (between your studs) is covered by drywall on the interior. Although drywall is vapor open (50 perms) it blocks air and therefore blocks a lot of vapor- especially when that air is warm. (Warm air holds more moisture than cool air.) This keeps the sheathing pretty dry- although it still gets condensation in the winter months to some extent.

    Take away the drywall, the air-permeable insulation will let through A LOT of warm air and allow the moisture to condense on the cool rim joist. Spray foam and rigid foam work because it acts as a kind of air barrier.

    Floor joists over a concrete wall have additional issues as the concrete can add additional humidity especially in the presence of capillary action.

  3. PJClem | | #3

    Thank you for the responses, but I still must be missing something.

    Primary air barrier will be at exterior sheathing that will run continuously over ceiling joists. Secondary air barrier will be at drywall, which will be continuous at all interior planes of rim joists in my build, same as it will be at exterior walls. Sill plates on foundation wall will be separated from concrete by air sealing gasket that will also serve as capillary break.

    The only differences I can think up between condition at exterior wall stud cavities and condition in flooring structure bound by rim joists are:
    1) There are some HVAC ducts in floor structure, but they will be very well sealed and kept at reasonable distance from rims.
    2) The rim joists are in addition to sheathing so at rim joints, the wood assembly is considerable thicker, but it is still not as "thick" as at wall studs.

    Are these differences significant enough to justify the different insulation treatment? Or, am I missing something more substantial that would lead to problems from permeable insulation at the rims?

  4. GBA Editor
    Martin Holladay | | #4

    P.J. Clem,
    Rick is correct -- when you insulate rim joists on the interior with fiberglass batts, the fiberglass batts don't prevent movement of interior air through the batts to the cold rim joist.

    If you are planning to cover the fiberglass batts on the interior with drywall, the risk is reduced. The fact that your house is in Zone 3 (a relatively mild location) also reduces the risk compared to Zones 6, 7, or 8. So you can probably get away with it.

    1. PJClem | | #5

      OK. I think I can live with reduced risk that is probably okay. I am still, however, confused and interested in why the rim joists are receiving so much more attention than sheathing that seems to be exposed to near-identical conditions.

      Is it because the rim joists bear vertical loads whereas the sheathing only shear loads?

      Forgive me for asking asking about an article you wrote 5 years ago on rim joists, but can you help me understand the significance of "Two-story homes usually have another ring of rim joists above the first-floor ceiling. If you need to insulate these rim joists, it’s best to hire a cellulose-insulation contractor."

  5. GBA Editor
    Martin Holladay | | #6

    P.J.,
    In the 1980s and 1990s, many New England basements had casually installed fiberglass batts shoved up against the rim joist. This often led to rim-joist rot, (a) because rim joists in New England get quite cold in winter, and (b) because the fiberglass batts don't stop air movement, so warm interior air easily had contact with the cold rim joist.

    You're in Atlanta, where rim joists don't get as cold as they do in New England. And you told us that you plan to install drywall on the interior side of the fiberglass insulation. The drywall is an air barrier. So, as I already said, the risk for you is low.

    Rim joists get more attention than walls (when it comes to rot risk) because walls have interior drywall, and rim joists usually don't.

    If you have uninsulated rim joists between the first and second floor in an older house, the easiest way to insulate the rim joist is often from the exterior, using cellulose and the so-called grain-bag technique. (That way, you don't have to disturb the interior ceiling). For more information on the grain-bag technique, see "How to Install Cellulose Insulation."

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