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Sheathing between foil-faced polyiso and closed-cell spray polyurethane foam

user-4435615 | Posted in Energy Efficiency and Durability on

I’m working on the details of an R-40 Wall in Southern Indiana, Northern Climate Zone 4. Please see the attached file.

Among many options is to use 2″ of properly layered, sealed and taped FFPIC [foil-faced polyisocyanurate] exterior to the sheathing and an Advanced Framed stud cavity filled with CCSPF [closed-cell spray polyurethane foam] on the interior. The house will have 3′ Overhangs, Brick Cladding, and a 1″ Rain Screen.

Will sandwiching sheathing (weather Plywood or OSB) between FFPIC & the CCSPF create a condition which traps moisture and causes damage to the sheathing ? It would seem that construction and materials would keep moisture out, but I’m concerned that any moisture that did enter the sheathing would have a difficult time drying.

Is there any advantage to the Impermeable Liquid Applied Air/Water Barrier in this installation, or is it just superfluous ?

Would that assessment change if the LAA/WB was permeable vs. impermeable?

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  1. GBA Editor
    Martin Holladay | | #1

    Whenever I hear that someone is interested in installing foam insulation as part of their wall assembly, I generally urge them to put the foam on just one side of the wall sheathing -- either the exterior or the interior -- but not both.

    Spray foam between your studs doesn't do much good, because of thermal bridging through the studs. The best place to put the foam is on the exterior, so I vote for the exterior polyiso -- the thicker, the better. (You could also use EPS in this location.)

    Foil-faced polyiso is vapor-impermeable. You want your OSB or plywood sheathing to be able to dry to the interior, so closed-cell spray foam is a bad choice. I suggest that you install dense-packed cellulose between your studs instead. If you insist on installing spray foam between your studs, open-cell foam would be better than closed-cell, because the open-cell spray foam allows drying to the interior.

  2. Expert Member
    Dana Dorsett | | #2

    5.5" of ccSPF has a vapor permeance of about 0.2 perms which is pretty vapor tight.

    Even at a 15% framing fraction (do-able with advanced framing, but you have to really work at it), and a full cavity fill (which isn't possible with ccSPF, since it isn't trimmable) you're at only R21 for the the studs & foam layer after thermal bridging. With open cell foam or cellulose you'd be at R15.5.

    From a performance point of view you could make up for that with an other inch of exterior foam, at much lower risk & cost, but for the difficulty of finding sufficiently deep masonry ties for the brick.

    Mind you, allowing R1 for the combined sheathing & gypsum, and R12 for the average performance of the 2" polyiso, and R0.5 for the brick, your proposed wall comes in at only ~R35, not ~R40 (unless you are unrealistically generous about the thermal performance of the air films, which I have left out.)

    Also note, you still have a HUGE thermal bridge to the exterior at the stud plate to the concrete foundation, a common but serious performance problem with brick-clad structures. If you want a true high-R structure for a zone-4 location, it's better to lose the brick, since that thermal bridge at the sill plate is a double-digit percentage of the total wall losses.

    The vapor permeance of the liquid WRB is irrelevant, since you have multiple foil facers at less than 0.05 perms each between the sheathing & exterior. The sheathing & framing MUST dry toward the interior, since there is ZERO drying into the 1" cavity behind the brick.

  3. iLikeDirt | | #3

    You don't need to abandon the brick to avoid that thermal bridge. Just put a layer of foamglas on top of the brick ledge on the concrete foundation and under the first course of bricks. It's insulating (about R-3/inch) and strong enough to hold up those bricks. Then you can insulate the concrete foundation however you like.

    As for the walls themselves, I agree with others. Replace the interior foam with cellulose and spend the money you've saved on more exterior foam.

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