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Paper-Backed ISO for Exterior Wall Insulation

cmcgrath09 | Posted in General Questions on

Getting ready to frame my house in Northern NH. Zone 6. I have noticed some really good deals on 4×8 ISO on craigslist and other marketplaces. The down side is it is paper backed, designed for roofing the seller said. Would this be a bad product to use on my walls. (I feel like it is) Zip sheathing, then the ISO With a rain screen and wood lap siding. I originally was planning on 3in of xps, due to the downgrade in R of ISO in cold weather, but the price is right and available.

Thank you – Chris

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  1. Expert Member
    BILL WICHERS | | #1

    The paper faced "roofing" polyiso will work just fine. The only thing I'd do differently here compared to using foil faced polyiso is that I'd not rely on the polyiso as a WRB -- you'll need a seperate WRB layer (Tyvek, etc.) over the polyiso since the kraft facer can't act as an effective WRB even if taped.

    Even if you downgrade polyiso for cold weather, it will still perform around the same as XPS. Polyiso is usually downgraded to R5 per inch in cold weather, which is exactly the same as XPS is rated. XPS doesn't really offer much, if any, advantage here.


  2. Expert Member
    PETER Engle | | #2

    +1 on Bill's reply. Note that you can and should tape the face of the ZIP as your water and air barrier. Flash windows and penetrations to that layer for easiest air tightness control.

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