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Vapor retarder on interior if exterior has vapor barrier?

user-1044776 | Posted in GBA Pro Help on

This question stems from a different conversation on here and we need some more help.

We’re in Zone 5A. Our exterior has plywood, tar paper, 1″ of foil-faced polyiso foam, and red cedar shake. The framing is 2×6 wood construction.

The interior will be heated with wood stoves only. We know to never have 2 vapor barriers. What we’re wondering is if we need an additional vapor retarder on the interior? If so, what sort of barrier do we need? (Spray foam is prohibitively expensive).

We were planning on having R-21 cellulose insulation and then 1/2″ gypsum on the interior. Would too much moisture get in in the winter?

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  1. GBA Editor
    Martin Holladay | | #1

    We need more information.

    First of all, is this a new house or an existing house?

    Second: if it is a new house, has construction begun yet?

    Your exterior rigid foam has an R-value of about R-6 or R-6.5, which is less than ideal for a 2x6 wall in climate zone 5. The minimum recommended R-value for a 2x6 wall in your climate zone is R-7.5.

    For more information on the minimum R-values of exterior wall foam, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

  2. user-1044776 | | #2

    Hi Martin,

    It's a new building, the construction has begun. All exterior work has been done (sheathing, tar paper, foam, shingles). We decided to use the exterior foam as additional to the interior insulation, not to meet code. We're coordinating rough electric and plumbing right now. We want to make sure we make the right decisions on the interior regarding vapor barriers.


  3. GBA Editor
    Martin Holladay | | #3

    Well, you already a mistake by choosing exterior foam that isn't quite thick enough to keep your plywood sheathing above the dew point in winter. In some areas of the country, that could be a code violation. Even if it's not a code violation, it's unfortunate.

    All of this is explained in my article, Calculating the Minimum Thickness of Rigid Foam Sheathing.

    You don't want to have an interior polyethylene on your wall. To find out why, read these two articles:

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

  4. Expert Member
    Dana Dorsett | | #4

    Again, the best solutions at this stage are to either :

    A 1" flash foam of closed cell foam on the interior of the sheathing, which will bring the condensing plane I(the interior surface of the flash-foam) to a warmer location, with a semi-permeable foam layer (0.8-1.2 perms) that won't wick moisture toward the sheathing, but would still allow the sheathing to dry at a reasonable rate toward the interior.


    Install Certainteed MemBrain between the cellulose & gypsum. MemBrain is a class-II vapor retarder (<1-perm) when the proximate air is less than ~35%RH, which can be controlled by ventilation rates of the house during the susceptible winter season, but rises quickly to 3+ perms when the interior RH reaches 50% RH in the spring, or should the cellulose take on moisure from a bulk-water penetration. The result is that it limits adsorption rates into the sheathing during the months when the sheathing is below the dew point of the interior air, but allows it to dry quickly during warmer weather, when the conditioned space air is more humid. The stuff really works!


    The MemBrain solution is pretty cheap, whereas the flash-foam solution is somewhat pricey, but either would keep the sheathing from getting punky from years of high wintertime moisture accumulations, and would be able to dry at reasonable rates if it ever got wet from wind-blown rain getting by the flashing.

  5. user-1044776 | | #5

    Thank you both. Dana, my husband was just afraid that the MemBrain was going to act as a second vapor barrier which we kept reading we should never have. I'll show him the specs and your further details. Thanks again!

    Martin, what is interior polyethylene? Is that what the MemBrain is?

  6. user-1044776 | | #6

    The MemBrain specs say it was designed to use over non-faced fiberglass batts. Should we use those instead of cellulose?

  7. GBA Editor
    Martin Holladay | | #7

    Q. "What is interior polyethylene?"

    A. Polyethylene is clear plastic that comes in a roll. In the 1980s, it was commonly installed on the interior side of an insulated stud wall to act as a vapor barrier. Since you asked a question about interior vapor barriers, I thought you might have been considering the use of polyethylene.

    While polyethylene is a vapor retarder with a fixed permeance, MemBrain is considered to be a "smart" retarder with variable permeance. When it gets damp, MemBrain becomes more permeable.

  8. user-1044776 | | #8

    Thanks, Martin! I will not use that!

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