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Community and Q&A

Wall Assembly With Exterior Continuous Insulation

adschmidt2 | Posted in General Questions on

Hello and thank you for the marvelous resource and community

Im in St Paul, Minnesota and and building a wall system as follows – my question is will I be trapping any dangerous amount of moisture

from inside to outside

1. Latex painted drywall
2. 2×6 walls with blown in cellulose
3. 1/2” plywood sheathing
4. prosoco cat 5 r guard fluid applied WRB/ARB to min 12 wet mils
5. 2” unfaced EPS
6. untreated 1×2” nom furring Strips
7. painted lp smart side

is it worth transitioning to rockwool or am I not that worried about the perm rating of 1 that the unfaced eps has?

thanks in advance for the advice and or poking holes in this assembly

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  1. Jon_R | | #1

    Here (table 2B) is what you need when is comes to perm ratio or exterior foam methods of keeping a wall dry. Keep in mind that air sealing (with testing) is always important and that smart retarders further lower risk.

  2. adschmidt2 | | #2

    Thank you - that’s a valid and valuable reply.

    What I take from this is that my R-21 2x6 wall filled with high pressure filled cellulose (Assume 3.8 per inch r value per mfg.) is the denominator and the exterior insulation R value is R8 given the mfg rating of R4 per inch.

    The math tells me I need to have a ratio greater than 0.2


    8 (exterior continuous) / 21 (interior) = 0.38

    Looking at Table 2B that tells me that I’m greater than the 0.20 ration that is required for class II vapor retarder of the EPS which has a perm of a little under 1 per the mfg. rating in climate zone 6. All of Which tells me that I’m ok to avoid condensation moisture build up in the proposed wall assembly.

    Does that square with your perspective?

  3. Jon_R | | #3

    You were probably proposing a class III on the interior - but you need to use special vapor barrier paint to get under 1 perm *on the interior*. 1 perm EPS provides a little outward drying but only meets the specs if you use it with a class I on the interior (not recommended and would need to switch to Table 2A).

  4. Expert Member
    Akos | | #4

    Your assembly is a bit shy of the required amount of exterior rigid insulation in Zone 6 to be able to use class III interior vapor barrier.

    Your assembly is perfectly fine if you put up poly, use craft faced bats or install one of the smart vapor retarders.

    P.S. You want to install at least 1x3 strapping. 1x2 is way too easy to split. If you can get it from your local lumber yard, strips of plywood work even better as they are flatter and hold screws without any chance of splitting.

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