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Community and Q&A

Rim Joist Insulation – Exterior Foam, No Capillary Break

user-7653783 | Posted in Green Building Techniques on

I’m in climate zone 5a (New Jersey)and have 1″ of exterior XPS insulation.  My sill plate sits directly on the concrete foundation wall with no capillary break.  I don’t have any signs of water intrusion and my foundation is at least 12″ exposed above the soil.

I’m considering how to insulate my rim joist using either closed or open cell spray foam.  I’m thinking open cell so it will be air tight but still allow my rim joist and sill plate to dry inward.  I hear closed cell is typical for this situation but I am concerned about a “foam sandwich” that doesn’t allow drying.

Would open cell, closed cell or some other method be the ideal way to handle this situation?

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  1. Expert Member
    BILL WICHERS | | #1

    Closed cell will help block moisture from getting in too. My own preference that I used in my own home with a similar situation was to put blocks of EPS rigid foam against the rim joist and "foam them in" with canned foam around the edges (I used Loctite's TiteFoam here, which is a bit more durable than the more common Great Stuff foam). EPS allows for a little bit of drying, which I think is a plus, but doesn't allow for much moisture migration and doesn't tend to hold on to moisture the way open cell spray foam can.

    I'd also look into installing a capillary break on top of the foundation wall. It's often possible to jack up the wood framing a little (~1/8" or so) in sections and slide in thin sheets of HDPE to add a capillary break. This sounds very difficult but is usually not that bad to do.


    1. user-7653783 | | #2

      Thanks for the reply. EPS sounds like a good idea to seal the air but still allow some drying inward

      I think that would generally align with this building science corp detail except I don’t have the capillary break

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