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Using rigid insulation correctly

pnadler | Posted in Energy Efficiency and Durability on

For zone 6, when building a new construction wall assembly, why wouldn’t this method work well- starting from the exterior side- OSB sheathing, Roxul R-23 cavity insulation, 1-2″ foil-faced polyiso rigid board placed continuously over inside of wall, joints taped appropriately, furring strip, drywall. It seems that this would pose the least risk-as the continuous foil-faced polyiso will stop to a great extent vapor transmission. I realize that no assembly is completely vapor leak proof, but the OSB can dry to the outside if need be. I’ve taken apart many walls with only the traditional R-13 or R-19 cavity fiberglass, which allows moisture through, and rarely see signs of moisture damage, since the sheathing can dry to the outside. I don’t understand why the use of rigid foam on the interior side isn’t discussed more as a viable safer option.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Peter,
    You wrote, "I don't understand why the use of rigid foam on the interior side isn't discussed more as a viable safer option."

    There are several reasons.

    First of all, it is much harder to insulate rim joists and partition intersections if you use interior rigid foam than if you use exterior rigid foam. In most cases, builders who install interior rigid foam are unable to make the foam layer continuous. As a result, air leakage tends to be greater, and the whole-wall R-value is less.

    Second, if you install interior rigid foam, you still end up with cold OSB. Cold OSB is wetter and is more at risk than warm OSB. The risk can be mitigated if you install a ventilated rainscreen gap between the OSB and the siding, but building scientists who study moisture levels in OSB have discovered that the best way to keep OSB dry all winter long is to install an adequate thickness of rigid foam on the exterior side of the OSB.

  2. HarrisDwight | | #2

    I don't know much about it, but I just learned about "Extended Plate and Beam (EP&B) wall system" because it's on the schedule for an EEBA conference session. Seems like it accomplishes what you're looking for (Simpler exterior / window detailing) while still providing room for exterior rim board insulation. Window bucks and top/bottom plates are the only portions that are not covered by foam.

    I'd top it with Zip sheathing and it should be just as air tight. No benefit of a rain screen gap though.

  3. GBA Editor
    Martin Holladay | | #3

    Dwight,
    I'm not familiar with the "extended plate and beam" system, but I found this drawing online. This particular illustration doesn't appear to show any exterior rim joist insulation.

    .

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