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Community and Q&A

CDX over polyiso

remodeler2014 | Posted in General Questions on

Hi, I’m replacing some old LP osb lap siding with Hardie plank. The substrate under the existing siding is foil faced polyiso over the studs. I propose to install 1/2″ cdx over the existing Polyiso so eliminate the siding sinking into the polyiso when nailin, and providing a better nailing surface. Are there any thoughts of trapping moisture between the polyiso and the cdx? I’ve seen a diagram on your website using this system but I don’t see any use of a vapor barrier. Please advise. The home is in the Pacific Northwest.

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  1. Expert Member
    BILL WICHERS | | #1

    The biggest issue here is that if that CDX is expected to act as a shear panel in any way, the nails going through the foam can be an issue. It sounds like your sheathing isn’t going to be structural, so that shouldn’t be a problem. I do recommend checking to be sure that you don’t need any structural sheathing anywhere.

    Why not just use vertical furring strips instead of sheets of plywood though? You’ll accomplish the same thing in terms of fasteners squishing into the polyiso, but you’ll also add a rainscreen which is a free added bonus.


  2. Expert Member


    I agree with Bill: With or with out the CDX you need to mount the cladding on rain-screen battens in the PNW - and if you have the walls open, it's a great opportunity to add some seismic resilience.

  3. remodeler2014 | | #3

    Thanks for the replies. The house does have metal "l" shaped diagonal corner braces. the cdx is meant to create a uniform substrate for the hardie as we all know can show irregularities in wall framing. The original LP siding exhibits moisture trapped between the Polyiso and backside of the LP. I think I'll go with a woven rainscreen on top of the polyiso then cdx/hardie.

    1. Expert Member
      BILL WICHERS | | #4

      You need a vertical air gap to have a rainscreen. 1/2” CDX Isn’t going to do much to change the wall compared to vertical furring strips. If your wall is so off that stud screwiness will cause waviness in the hardiplank, then you’ll need to shim the furring or the CDX to correct it. When doing drywall, I rip 1.5” strips of luan and intentionally break them so that the ends delaminate a little to allow for more gradual transitions into the shimmed area. Luan lets you shim in approx 3/16” increments, which is usually enough to correct things enough to look visually flat between studs.


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