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Community and Q&A

Continuous insulation in Climate Zone 5

lhbasle | Posted in Green Building Techniques on

Trying to understand if I’m ok using 2” polyiso CI on Huber zip sheathing in zone 5 (Chicago) or if I need to go more  vapor open with mineral wool?  I will have 3/4” fir strips outside of the insulation before cladding.

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  1. Expert Member
    AKOS TOTH | | #1

    You can check here for details:

    If you have less than the ratio of exterior rigid rigid shown in the table, you would need an interior vapor retarder.

    1. lhbasle | | #2

      So as long as the exterior insulation is greater than 30% of the total I shouldn’t have an issue. My concern was trapping moisture behind the CI at the WRB (zip sheathing) and it not being able to dry outward through a foam board.

      1. Expert Member
        AKOS TOTH | | #4

        Usually with exterior rigid you are worried about moisture from the interior of the house. With sufficient ratio, the sheathing is warm enough that the moisture won't condense, so the only source of water is from bulk leaks from windows and siding.

        For water to get there, it has to first get through your siding than the rigid foam. So not much can make it there.

        Having a textured WRB, or just anything that will keep a very small gap (1/8" is more than enough) between the foam and the WRB will allow for a fair bit of diffusion drying.

        People install foam over ZIP all the time, it works fine. You can also always go with one of the permeable polyiso products (IKO enerair) which does allow for a bit of drying towards the outside as well.

      2. Expert Member
        MALCOLM TAYLOR | | #5


        This helps explain Akos' point about water infiltration from the outside:

  2. Jon_R | | #3

    Research shows that building right at the code minimum % exterior R without an interior side Class II vapor retarder and without the ability to dry outwards will have some condensation/sorption and some elevated risk - I wouldn't do it.

    1. lhbasle | | #7

      Jon R, are you recommending the more open mineral wool then?

      1. Jon_R | | #8

        Various solutions will decrease risk, but I suspect that Class II (Table 3) paint on the interior would be the most cost effective and least disruptive to your plans.

        Consider using this calculator, but increasing the climate zone by 1 to be conservative.

  3. lhbasle | | #6

    Since I already have the wrb I would hate to spend the money for the wrinkled wrap to put over it. Even if I do, most of the 2” boards give me about R8. That limits the total to R26.7 and the interior to R18.7. So that means I can only go R19 on my 2x6 wall while up to R23 is available. I had wanted to get to R30 but that doesn’t seem practical at this point.

    1. brianvarick | | #9

      Polyiso should get you way more than R8 on the exterior.

    2. Expert Member
      BILL WICHERS | | #10

      A sheet of typical 2” polyiso is going to be about R13. 2” EPS would be a bit over R8.


    3. Jon_R | | #11

      See here for why you want to significantly de-rate polyiso for this application.

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