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Foundation-to-floor connections and sheathing/foam placement

TODD EGELAND | Posted in Green Building Techniques on

I am building an 8″ wide poured concrete walkout basement with a typical 2×6 plate, floor joists, decking, and 2×6 walls above.  The walls above the basement will have exterior 5/8″ sheathing, WRB, 2″ rigid foam, furring, and cedar siding.  (I plan to install innie windows).   I have gone through GBA’s Detail Library, but none of the section drawings that I found included sheathing.  My questions are:

1.  Assuming the exterior concrete basement walls will be left raw (no stucco or stone applied), should the 2×6 plate be attached to the concrete basement wall 5/8″ inward to enable the sheathing to be flush with the concrete and allow for the WRB plane to also be flush, or must the sheathing stand proud of the concrete?  What is the best practice on placing the sheathing?

2. This question may be moot give the answer to #1, but: Does the exterior rigid insulation always have to be fully proud of the concrete basement wall below it?  Let me explain:  If I move the 2×6 plate inward 2 5/8″, I can now have the sheathing, WRB, and rigid insulation directly above (flush) with the concrete basement wall below.  (My thought is to keep the foam board 1/2″ above the concrete to allow for drainage and install a metal flashing from the sheathing over the concrete).  With furring strips, the siding will now stand proud of the concrete and still have a ventilated rain screen.  Can this be done?

Thanks in advance for your expertise!

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Replies

  1. GBA Editor
    Peter Yost | | #1

    Hi Todd -

    Structurally, there is no difference between the two configurations you have posed. From a moisture management perspective, I strongly prefer running the exterior sheathing proud of and past the concrete foundation, along with a flashing/sealant detail to shed water off the bottom edge of the sheathing.

    But boy, there are a lot of opinions/details out there on this one:

    https://foundationhandbook.ornl.gov/handbook/section3-2-concrete-exterior.shtml

    https://www.doityourself.com/forum/exterior-paneling-all-exterior-sidings/581712-exterior-wall-angles-rim-joist-how-compensate-before-cladding.html

    https://vermontpassive.com/2011/03/thermal-break-foundation

    I think whatever you decide, just make sure that the bottom edge of the structural sheathing is airtight connected to the below-grade wall, that there is no capillary bridge between the sheathing and the concrete, and water is directed away from the bottom of the wall, rather than draining down and turning the corner to wet the bottom edge of the sheathing.

    Peter

  2. TODD EGELAND | | #2

    Thanks Peter, your thoughts are exactly what I needed.

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