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Community and Q&A

Polyiso inside 2×6 wall?

WLSchebaum | Posted in General Questions on

Hello, I’ve started in on some conceptual designs for a renovation project that was built in North central Vermont in 2006 and have discovered some interesting wall assembly details. From what I’ve learned so far the wall assembly from the outside is this:
– Tyvek WRB
– 5/8 CDX
– 2×6 framing 
-2″ Polyiso foam
– 3-1/2″ fiberglass
– 6 mil poly
– drywall

I’m somewhat concerned about the assembly in terms of its drying potential.  Has anyone encountered this particular assembly before?  The project renovation scope is of enough substance that this could all be torn out and corrected with sufficient continuous exterior insulation and new cavity insulation.  


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  1. Expert Member
    Michael Maines | | #1

    The relatively high ratio of R-value on the exterior is keeping the interior condensing surface--the interior side of the polyiso--above the dewpoint temperature for enough of the year that the moisture that gets into the wall is not likely to condense, and therefor not a major concern. If you are opening up the walls for other reasons, that would be a good time to swap out the poly for a variable permeance membrane, or just go with painted drywall, if blower door testing shows that the house is relatively airtight.

  2. WLSchebaum | | #2

    That was my initial thought as its an odd version of the flash and batt. I think it's just my usual suspicions of two very vapor impermeable surfaces with fiberglass between. There will be some immediate openings of select walls in the near future that will help drive the larger renovation goals in terms of how much is torn out and replaced. I have a hard time imagining the installation of all the polyiso inside the stud cavities being done in a fastidious manner compared to wide open exterior wall installation that is typically done and how that would affect performance.

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