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
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
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?
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?
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.
Sorry I asked the second question before Martin's answer.
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
Dana,
Thanks for the link. Definitely one of my favourite threads!
Martin linked to it before I did (while I was off searching for it), but you're welcome!
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.