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2 inches of polyiso and 8 inches of dense-packed cellulose in Climate Zone 4

imoldfella | Posted in Green Building Techniques on

Location is Philadelphia, PA. Brightbuilt double stud wall is 2 inches of polyiso and 8 inches of dense pack cellulose. Isn’t that too low a ratio for the external foam? Depending on how you derate the polyiso, you only have 25% or so of insulation outside the sheathing.

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  1. Expert Member
    Dana Dorsett | | #1

    In zone 4A the thickness of the exterior foam doesn't really matter. The IRC doesn't a prescribe any amount of exterior insulation for that climate zone to be able to use a class-III vapor retarder on the interior. (The IRC does pick a number for 4C.) The assembly dries just fine toward the interior, and won't accumulate enough moisture in the sheathing & insulation over a winter to matter.

    Marine zone 4C is not as cold as 4A , but has a much longer heating season. There the IRC prescribes a minimum of R2.5 for a 2x4/R13 type wall. At 10" you're a bit less than 3x the amount of fiber insulation. So if you simply tripled the prescribed insulating sheathing for a 2x4 wall it would make it even in clammy foggy-dew 4C.

    See R702.7.1 Class III Vapor Retarders:

    1. imoldfella | | #2

      What protects the sheathing during the winter when its dry outside? Doesn't the inside water vapor drive out to the cold sheathing and condense there, trapped by the foam? A few places Martin writes about how a little polyiso can be risky, but maybe he means for colder climates.

  2. GBA Editor
    Martin Holladay | | #3

    Dana's advice aligns with mine. See my article, "Calculating the Minimum Thickness of Rigid Foam Sheathing."

    In that article, I wrote, "If you are building a house in one of the warmer climate zones — zone 1, 2, 3, or 4 (except for 4 Marine) — you don’t have to worry about the thickness of your foam. Any foam thickness will work, because your sheathing will never get cold enough for 'condensation' (moisture accumulation) to be a problem."

    Q. "What protects the sheathing during the winter?"

    A. It never gets very cold, because outdoor temperatures are mild in Zone 4. Since the sheathing never gets very cold, it stays safe.

    1. imoldfella | | #4

      Thanks for weighing in! That article confused me a little, btw, because I can't find image #3 with the ratios. I wouldn't say it _never_ gets cold here. We can get to zero F for a week., but it doesn't stay that way for a month. Nothing like where I grew up in northern NY. I'm glad to hear that the brightbuilt design is considered a resilient as well as high performance option. We have a small number of high performance build options around here.

      1. GBA Editor
        Martin Holladay | | #5

        You're right -- finding the images in older articles has become confusing since GBA made the transition to our new web platform. We used to include most of the images at the bottom of the page; now the images are at the top of the page, so older articles that say "see image below" are misleading.

        I have edited the article in question to clarify where the images can be found.

  3. Expert Member
    Dana Dorsett | | #6

    Plywood or OSB sheathing isn't very vapor permeable (less than 1 perm when dry) and in winter with no foam the sheathing temperature stays mostly below the dew point of the indoor air. The notion that it simply drys-through in winter isn't very accurate- it accumulates moisture whether there is a low-permeance layer on the exterior or not, and doesn't pass very much through, even if there is a rainscreen gap on the exterior.

    Exterior foam (any thickness) raises the average temperature of the sheathing in winter, lowering the amount of moisture accumulated.

    With a rainscreen and no exterior foam the drying rate is faster, since as the average temp of the sheathing rises & begins to release it's accumulated moisture it can dry in both directions. This works even in climate zones 4C & 5, but not zone 6 & higher.

    With low-permeance foam on the exterior it really only dries toward the interior, but since there is less moisture accumulation due to the enhanced sheathing temperature of the exterior R it doesn't need the faster drying rate. The IRC prescriptives are there as a guide to the minimum amount of exterior R needed to keep the sheathing sufficiently warm, but it's not absolute, just a guide. With a competent WUFI simulation it's possible to go a bit lower than the IRC prescriptive exterior-R and still have margin, but not in all cases.

    In zone 4A the winters are colder than 4C, but the heating season is much shorter, with a much longer drying season. In 4A the cold periods aren't long enough to develop damaging levels of moisture in the sheathing, with or without exterior foam, with or without a rainscreen to enhance the drying rate. But in zone 4A adding exterior foam of any thickness lowers the moisture burden in the sheathing, both winter & summer, blocking humid-summer air moisture drives, and keeping the sheathing warmer & drier in winter.

    "We can get to zero F for a week., but it doesn't stay that way for a month."

    That's simply not the case. For a daily LOW temperature it might hit that for a week (extremely rarely), but it doesn't stay 0F or below for a week the way it can in the Adirondacks.

  4. walta100 | | #7

    To me your wall does not make sense it seems like you are trying to combine 2 good ideas. This looks like an expensive wall to build.

    If you are putting the money into labor and materials to build a double studded wall it costs almost nothing to make the wall thick enough to hold all the insulation you want.

    If you want exterior foam add more and skip the expense of double studded walls labor and material.


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