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Vapor Barrier in summation

fall50 | Posted in Building Code Questions on

In talking to a friend of mine today who is in the tail end of GC’ing a complete remodel of a 1920’s 2 story Victorian in Minneapolis ZONE 6b he mentioned that he was going to use a layer of poly in the 2nd story ceiling, drywall and finally blow the attic floor with cellulose. His main rational for doing so is because the BI called for a poly vapor barrier. I suggested he instead consider VB paint and that would effectively meet code and also avert many of the much publicized concerns over using poly specifically raised in the blog post from the other week. Although it was interesting to read Doug M persistent defense of the poly as he is from MN as well…………… my friends concern is whether the building inspector will allow for the VB paint. Understanding all building codes are different and untimely he wil need to get the final ok from the BI on whether he will also for the paint ……..I did have a few questions.

1) Does the paint need to be a VB spec’d or does 2 coats of any latex based primer suffice?

2) He is using closed cell SPF in the walls, controlling interior humidity/vapor becomes extremely important as the house will be very tight. Thus the poly in the ceiling would seem to very much run the risk of condensation especially in lieu of them having AC during the summer. Right?

3 The downfall of using poly has been well documented, but does using VB paint have any disadvantages shortcomings etc.

Assuming the BI allows for the paint is their any good reason that poly would be preferable? His insulation contractor suggested in “remodels” paint is often ok’d, but in his case because the house is down to the studs, essentially its considered new construction and thus the BI will mostly likely want to see the poly? I see the rational on why the insulation contractor say it but again the logic seems to be lacking.

I guess if the BI plays hard ball then Membrain by Certainteed. Has anyone used it?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    M.S.,
    Q. "Does the paint need to be a VB spec'd or does 2 coats of any latex based primer suffice?"

    A. You'll have to check the local code book, but many codes call for a vapor retarder with a permeance of no more than 1 perm -- in other words, vapor-retarding primer. You need a paint that has been tested for permeance.

    Q. "The poly in the ceiling would seem to very much run the risk of condensation especially in lieu of them having AC during the summer."

    A. In an air-conditioned house during the summer, a ceiling without poly will stay dryer than a ceiling with poly. However, I haven't heard of serious problems with ceiling poly in Minnesota, so I don't think there would be enough condensation to cause problems.

    Q. "Does using VB paint have any disadvantages, shortcomings etc.?"

    A. No.

    Q. "Assuming the BI allows for the paint, is their any good reason that poly would be preferable?"

    A. No.

    Q. "The BI will mostly likely want to see the poly."

    A. I don't see any reason (other than ignorance) why this would be so. I don't think there is any residential building code in the country that requires poly in a ceiling.

    Q. "I guess if the BI plays hardball, then MemBrain by Certainteed."

    A. MemBrain would certainly work.

  2. Foamer | | #2

    Martin, as I understand it, poly was installed to prevent vapor from entering sealed areas where potentially damaging condensation could take place (i.e. wall cavities). Assuming that an attic is properly ventilated, can you think of circumstances when a vapor barrier would be beneficial (for example extreme humidity levels such as swimming pools)? We see moisture issues in unconditioned attics all the time but they are due to missing air barriers and inadequate ventilation - not missing vapor barriers. Where we find poly installed over the ceiling drywall, mold issues are not uncommon especially when insulation levels are inadequate (Zone 5).

  3. GBA Editor
    Martin Holladay | | #3

    Torsten,
    During the winter, the vapor drive through a cold-climate ceiling is from the interior to the exterior, so vapor-retarder paint makes sense. It will slow down diffusion of interior moisture that tends to migrate into the ceiling assembly.

    You claim that mold is associated with the use of ceiling vapor barriers. I'm doubtful of the association. I've often seen mold on bathroom ceilings, especially close to exterior walls. That happens because the insulation is very thin over the wall top plates, and the drywall ceiling is cold during the winter. That's why moisture and mold accumulate there. The solution is thicker insulation and raised-heel trusses.

  4. user-948834 | | #4

    If the attic is vented correctly, poly in the attic will work fine as long as the attic is also air sealed. We have used .Membrane for a while now and it works fine. Air sealing is as important as the insulation. Do not rely on any kind of paint for a vapor barrier.

  5. GBA Editor
    Martin Holladay | | #5

    Tim,
    I'm confused by your statement, "Do not rely on any kind of paint for a vapor barrier." You're right that paint is not a vapor barrier; it's a vapor retarder. But building codes don't require vapor barriers; they only require vapor retarders.

    Vapor retarder paints perform dependably, creating a 1-perm surface on gypsum drywall. And that's all that building codes require.

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