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Community and Q&A

Can Polyiso be applied in a remodel on the exterior of a building without creating moisture problems?

atmosphere | Posted in Energy Efficiency and Durability on

This is a house located in the mixed climate of Virginia. It is currently sheathed with OSB over 2×6 walls. The walls are R19 batts with paper vapor barrier to the warm side. Can the siding be removed, PolyISO applied, furring installed and new siding be installed without creating a mold and moisture nightmare. Removing all the drywall and applying a vapor barrier directly to the inside of the studs is not an option.

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  1. GBA Editor
    Martin Holladay | | #1

    The answer is yes, as long as you know what you are doing. For more information, see:

    How to Install Rigid Foam Sheathing

    Calculating the Minimum Thickness of Rigid Foam Sheathing

  2. Expert Member
    Dana Dorsett | | #2

    The vapor-retardency of kraft facers is ~0.4-0.5 perms, which is sufficiently vapor-open to allow seasonal drying toward the interior, but it's not super-fast drying.

    Foil facers on iso are a true vapor barrier (< 0.01 perms), and would restrict all drying toward the exterior. The all-important thing to get right when putting vapor-retardent or vapor-barrier insulation over the exterior is to be DAMNED sure the flashing is all installed correctly to prevent bulk water intrusion. Whereas the "before" picture may have had a couple perms of drying capacity toward the exterior, almost any exterior foam (except unfaced EPS) would reduce that to a perm or less. With foil faced iso you're relying entirely on the ability of the assembly to dry toward the interior, making it less resilient to bulk-water events.

    It doesn't take a lot of exterior R over the sheathing to deal with wintertime vapor diffusion from the interior accumulating in the sheathing, even without the kraft facers. But WITH the kraft facers in place it's kind of "in the noise"- the kraft facer is protective. With R5+ on the exterior you could get rid of the kraft facers in your climate zone and do just fine with latex paint on the interior, but it's not worth ripping it out.

    If you have foil or vinyl wallpaper on the interior, don't use foil-faced goods on the exterior, use unfaced XPS at 2" (R10) or less, or EPS at 4" or less, since those interior finishes are also true vapor barriers that would otherwise create a moisture trap if you shut down exterior drying entirely.

  3. user-945061 | | #3

    When I look at real-world water management details, this assembly makes me uneasy. I've done this assembly a number of times, but really focus on water using all the great info from BSC. Recently I've taken to removing cavity insulation via an 8" rip in the sheathing and installing high density cellulose. Cellulose will allow for much better inward drying than the kraft facing.

    On the other hand, I've also taken apart this assembly several times and found surprisingly little water damage in instances where polyiso was installed by drunken rednecks with no concern for water.

  4. Expert Member
    Dana Dorsett | | #4

    I'd be happier with cellulose too, provided the kraft facers could be ripped completely out, but Rick doesn't seem to be headed that way.

    Cellulose fill with absent or misinstalled flashing on windows & doors could be courting disaster though. While cellulose can buffer a lot of air transported or vapor diffusion moisture, taking on bulk-water incursions isn't in the cards. Crummy low density R19 fiberglass dries much quicker, even though it's a real dog from a thermal performance & air retardency point of view.

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