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Community and Q&A

Zone 6 dew point

David Dunn | Posted in General Questions on

Would a 2×6 wall with:
R-23 Roxul Batts in the cavity,

Exterior layers of:
OSB sheeting,
1″ R-6 R Max rigid insulation (taped),
R-6 Insultex House Wrap (taped)
And finished with stucco,
eliminate the dew point problem in a Zone-6?

I called Insultex and they recommended their product be installed over the rigid insulation

Thanks,
Dave

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    In zone 6 you'd need R11.25 on the exterior for dew point control even if it were R20 in the cavities, and with 1" polyiso you have less than R5 performance during the colder months that count in your stackup.

    With R23s in the cavities it would be better to go with something that delivers R12+ performance during the winter months, which you can get with 2.5" of Type-II (1.5lb density) EPS. Unlike polyiso, EPS increases in performance as the average temp through the foam drops. When the average temp through EPS is 40F (which it would be, most of winter) it delivers R4.5 /inch so with 2.5" you'd be at 11.25, meeting the IRC code prescriptives. But when the average temp through the foam is 25F (which it would be during the wintertime extremes) it would be in the R11.75 range.

    See Table R702.7.1: http://codes.iccsafe.org/app/book/content/2015-I-Codes/2015%20IRC%20HTML/Chapter%207.html

  2. GBA Editor
    Martin Holladay | | #2

    David,
    Insultex housewrap is not R-6. It's a scam. (The manufacturer of this product is trying to "insultex" our intelligence.)

    To read a previous Q&A thread on the scam, see Anyone familiar with Insultex housewrap?

  3. David Dunn | | #3

    With the R-6 Insultex along with the R-max derated to R-5 it gives you R-11 exterior, wouldn't that move the dew point to the Insultex?

  4. GBA Editor
    Martin Holladay | | #4

    David,
    Dana is right -- you need at least R-11.25 of real insulation (excluding any supposed R-value from housewrap manufactured by scammers). For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  5. David Dunn | | #5

    Sorry I asked the second question before Martin's answer.

  6. Expert Member
    Dana Dorsett | | #6

    Regarding the ridiculous performance claims of Insultex, we've walked this path before:

    https://www.greenbuildingadvisor.com/community/forum/green-products-and-materials/43346/anyone-familiar-insultex-house-wrap

  7. Expert Member
    Malcolm Taylor | | #7

    Dana,
    Thanks for the link. Definitely one of my favourite threads!

  8. Expert Member
    Dana Dorsett | | #8

    Martin linked to it before I did (while I was off searching for it), but you're welcome!

  9. Charlie Sullivan | | #9

    It's interesting that Insultex recommended that he use a layer of rigid foam under the Insultex. Presumably because they know their product doesn't actually work.

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