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Rim joist exterior flashing for concrete porch and crawl space sealing / insulating?

BrunoF | Posted in General Questions on

Our zone 4A, Central NC home framing is complete and the last thing the framers did was to staple a thick, PVC? coated flashing material to the rim joist and sheathing where the concrete patio will abut the exterior of the home.

I am doing a sealed crawl space and with the recommendation here and from my insulation contractor we are going with closed cell foam on the inside of the rim joist and on the CMU walls.  I assume that with this method, any moisture in the rim / sheathing will have dry “out” since it can’t get through the foam insulation to dry “in”.

My concern now is that the flashing membrane and the concrete slab for the porch will prevent drying in that area and that over time the staples used to hold the membrane to the rim joist will rust and allow moisture to get behind the flashing with nowhere to dry to.

I would like to hear the opinions from the folks here who might have experience with this construction type as well as thoughts on what (if anything) I may have to modify.

as always, thanks!

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  1. Expert Member
    Akos | | #1

    This has come up a number of times before ie:

    Overall not a great idea, not code compliant yet it is done often. As long as it is a covered porch you'll be mostly ok as long as there is a drainage path so water does not pool near the rim joist.

    You can also add some drying capacity to the interior by only using 1" of closed cell foam which is around 2 perm at the rim joist to air seal and adding fluffy batts to add the extra R value.

    1. BrunoF | | #3

      What part of the building code is this not compliant with?

  2. BrunoF | | #2

    Thx. Yes, in my case both porches will be sloped and covered with a roof. The concept of 1” foam + batts is interesting…perhaps I should do that for the entire rim joist area and not only where it backs up to the porch?

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