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Mitigating risk of low perm rating exterior insulation

dpilot83 | Posted in General Questions on

I’m trying to find a substitute for Rockwool Comfortboard or other similar products (due to poor availability) without compromising the durability of my wall assembly.

2” of Comfortboard 80 has a perm rating of 30 or so

2” of Halo Exterra has a perm rating of 1.2 but this is based on perforations in the facing which according to comments in this thread ( ) is really not going to increase the permeability of it. So maybe it’s not as permeable as advertised?

2” of XPS has a perm rating of 0.55

1” of unfaced polyiso is 2.5 to 4 (found in the same thread linked above). Not sure where that info comes from. Can’t find what 2” of polyiso would be.

I was really hoping for 3” or comfortboard 80 or similar externally to get to around R12 continuous insulation externally. So all of these are less than my target but I chose 2” for this thread because Halo Exterra is a max of 2” and you definitely can’t layer Halo Exterra and hope for it to have the ability to dry.

I’m thinking maybe I can get away with a vapor impermeable exterior insulation if:

1. There is a rain screen (firring strips) between the exterior insulation and the cladding

2. There is enough external insulation that it’s likely to keep the sheathing and everything interior to the sheathing above the dew point

3. The interior portions of the wall assembly are vapor permeable enough to allow drying to the interior of the house

Am I on the right path?

For perspective, this is the far SW corner of climate zone 5a (northwest Kansas).

Wall assembly from inside to outside was going to be (before availability concerns)

vapor permeable paint
2×6 framing with rockwool batts
zip sheathing taped carefully
3” of comfortboard 80
some sort of a windscreen material
firring strips
hardi board

Perhaps of note is that I’m very worried about rodent and insect damage on the external insulation, whichever way we go. I was under the impression Comfortboard was a little better that way than the various foam boards. Maybe that’s not right. Was thinking maybe something like this to help?–20-x-20-to-4-x-4?_gl=1*sitbz2*_ga*NTc4MTY4NTg3LjE3MDgwMzY2NjM.*_ga_JG6JT8G7NW*MTcwODAzNjY2NC4xLjEuMTcwODAzNjY3Ni40OC4wLjA.

Anyway, now with poor mineral wool availability I’m just totally lost on how to change the assembly without hurting durability.

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  1. matthew25 | | #1

    Atlas GCF is a fiberglass-faced polyiso and its spec sheet says it is 1.2 perms at 1”, so probably less at 2”. But this whole question may be unnecessary. Behind your polyiso will be your WRB, correct? There is nothing between the face of the polyiso and the face of your WRB that you need to be worried about trapping moisture in.

    You points #1-3 are spot on. Drywall with latex paint is still permeable enough to dry to the inside if necessary. And you should definitely have a rainscreen in general. And why wouldn’t you put enough exterior insulation? If you’re going through the effort of doing 2” you might as well do 4”. Look up ThermalBuck if you need help mounting flanged windows with exterior insulation.

  2. Expert Member


    Yes, you can use impermeable insulation, which if thick enough alleviates all your concerns. For climate zone 5, you would need R-7.5.

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