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Is it Safe to Sandwich Rim Joist Between Rigid Foam and Vapor-Impermeable Flashing?

gfranke | Posted in Green Building Techniques on

The contractor on my renovation project has put a thick, flexible, black plastic or rubbery flashing material (York Vascoseal) around the bottom of the wall where it meets the CMU crawlspace. It is roughly the same width as the rim joist and covers it completely. He said the it pleases the inspectors, is required behind deck ledger boards, and helps protect the bottom of the house from splashes and whatnot, so they just run it around the entire house for good measure. I assume it is vapor impermeable just by inspecting it, due to its thickness.

I would like to insulate the inside of the rim joist with foam board cut to fit as described in various places. I’m concerned that by doing so I will be sandwiching the rim joist between two vapor impermeable materials.

Martin responded to a similar question in a previous Q&A:

Researchers have looked into the question about this type of rim-joist sandwich. When it comes to rim joists, the researchers haven’t noticed any down side to making a sandwich like this. Rim joists will stay dry when they have rigid foam on both sides.

Obviously, you need to use common sense. Don’t install the interior rigid foam on a day when the rim joists has frost on it or appears damp. Wait until the rim joist is warm and dry.

Would the above advice apply to a rim joist with rigid foam only on one side and a non-insulating vapor barrier on the other? If not, any advice on vapor permeable flashing material that I could suggest as an alternative would be appreciated.

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  1. gfranke | | #1

    A few more details. The rim-joists are OSB-style, not solid wood, and the floor joists are engineered wood I-beams.

  2. GBA Editor
    Martin Holladay | | #2

    Q. "Would the above advice apply to a rim joist with rigid foam only on one side and a non-insulating vapor barrier on the other?"

    A. Yes. I wouldn't worry. That said, for builders who have trouble sleeping soundly at night with this type of assembly, you can always specify a vapor-permeable peel-and-stick product like Blueskin VP 100.

    Siga and Pro Clima are both developing vapor-permeable peel-and-stick membranes to complete with Blueskin VP 100.

  3. gfranke | | #3

    Thank you for the answer. I'm always glad when the contractor can do things his normal way.

    Out of curiosity, I wonder why such assemblies are safe? Is it because they rarely get wet? Or because there is adequate potential for the joist to dry upwards, downwards, and through the floor joist intersections?

  4. GBA Editor
    Martin Holladay | | #4

    In general, building materials that are dry stay dry as long as the rate of drying exceeds the rate of wetting. This rim joist is protected from rain by the peel-and-stick. It's protected from capillary wetting from the foundation by the sill seal. It's protected from vapor diffusion from the interior by the rigid foam. Therefore it's not getting wet.

    It can dry, albeit very slowly, from the top and (perhaps) to the interior through the rigid foam (especially if the rigid foam has no foil facings).

  5. gfranke | | #5

    Perhaps then I will seal the crawlspace and let the dehumidifier run for awhile before starting on the rigid foam project.

    Thanks again.

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