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Community and Q&A

Polyiso for fiberglass insulation retrofit

DKPotter | Posted in Energy Efficiency and Durability on

I’m new to this community, but have really appreciated the depth of information available as I’ve been reading though old posts the last couple of weeks.

My wife and I recently moved into a 1974 home in central Virginia (climate zone 4) which has a wall structure of: tongue and groove boards, 2×4 framing with foil faced fiberglass batts in the stud bays, OSB sheathing, felt paper and cedar shake siding.

We do not want to remove the cedar shake siding from the outside of the home right now, so are considering the best way to retrofit insulation in these walls. I have a reasonable amount of recycled 3.25″ fiber-faced polyiso insulation that we were thinking of cutting and spray-foaming (“cut and cobble”) in the stud bays from the inside. We have some places where we are removing internal tongue and groove as part of another project and this approach seems like a good first step based on what I’ve read here and elsewhere. My thinking is that it would provide immediate R-value and air-sealing improvement over the fiberglass, hopefully without creating any new moisture-related problems. I’m curious if others agree or if there would be any drawbacks to the approach. Any thoughts are appreciated! Thank you

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  1. GBA Editor
    Martin Holladay | | #1

    Your approach will work. If you can also install a continuous layer of rigid foam on the interior side of the studs -- not cut up into narrow strips -- before you install new drywall, that would be even better.

    Here are links to two relevant articles:

    Cut-and-Cobble Insulation

    Walls With Interior Rigid Foam

  2. Expert Member
    Dana Dorsett | | #2

    The difference in whole-wall-R thermal performance between 3.25" of R6/inch (R19.5) foam and 3.5" of R3.7/inch (R13) fiberglass is remarkably small, about R1.23 (for typical 16" o.c. framing), or about a 10% reduction in heat transfer.

    If you caulked the framing to the OSB sheathing inside every stud bay, installed R13 batts then put a continuous 1/2" of polyiso under the drywall it would yield a 25% reduction in heat transfer. That would only use about 20% of the total foam volume, but would take up a half-inch of interior space (1 square foot of room area for every 24 linear feet of exterior wall area), and may require some adjustments to the window trim (which are probably be needed anyway if you're gutting the interior walls).

    To bring it up to IRC 2012 & newer code min performance would take R13 batts plus an inch of continuous polyiso, or R15 batts plus 3/4" of polyiso.

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