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Polyiso insulation outside — what goes inside?

ddubx6 | Posted in General Questions on

2×4 walls covered with sheathing, housewrap (taped), 4 inches of foil faced isofoam board, strapped with wood siding. All gaps in foam board are spray foamed and the window/door openings are sealed and flashed properly.

Plan to fill inside cavity with 3″ of closed cell spray foam with drywall covering, no vapor barrier. House is in zone 3, cold northeast valley.

After reading many ideas and/or opinions on the subject of moisture in walls I’m not sure if this is a good idea. Any better solutions or will this work?

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  1. GBA Editor
    Martin Holladay | | #1

    I've said it before and I'll say it again: If you like low-permeance foam insulation, choose to put it on just one side of your wall sheathing. You can install polyiso on the outside of your sheathing if you want, or closed-cell spray foam on the inside of your sheathing if you want -- just don't do both.

    The wall needs to be able to dry out in at least one direction.

    I advise you to insulate your stud cavities with cellulose. If you really prefer spray foam, make sure that it is open-cell foam, not closed-cell foam.

  2. Expert Member
    Dana Dorsett | | #2

    The difference in whole-wall R between 3.0" of R7/inch closed cell foam and 3.5" of trimmed R3.5/inch o.c. foam with a 25% framing fraction (typical 16" o.c. framing) is only about R1. (Really!) The performance hit from the thermal bridging of the framing is that bad! If your framing fraction is really low, say 20% the difference in whole-wall performance is still less than R1.5.

    Closed cell foam is WASTED as cavity fill at any thickness beyond what it takes to air-seal, and 3.5" of open cell does about as well in that regard, with a truly negligible difference in thermal performance in 2x4 framing. The 3" cc foam cavity fill runs on the order of $3/square foot, whereas 3.5" of oc cavity fill runs LESS THAN HALF the cost, and damp-sprayed (or even dense-packed) cellulose would run a bit lower still (depending on your local market for foam & cellulose.)

    How much are you willing to spend for another R1 - R1.5?

    Bumping up to 4-1/4" ( a mere quarter inch thicker) on the iso adds R1.5 for a lot less money!

    As Martin points out, rather than 0.3 perms of interior drying capacity with closed cell, with either open cell foam or cellulose the sheathing would now have 2-5 perms of drying capacity, dependent primarily on the permeance of the interior paint.

    Using 3" or 3-1/4" roofing iso for the layer closest to the sheathing and adding a 1" layer of foil-faced for the exterior (seams taped and staggered at least 1' between layers) works great, and gives you the same half-radiant-barrier performance of the foil facer + air gap for another effective ~R2 - R3 on the sun-drenched sides during the cooling season.

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