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Can Liquid Tyvek be used as indoor vapor barrier on T&G ceiling

Regulajo | Posted in Green Building Techniques on

I have an unvented low slope roof and am covering the ceiling with T&G wood. I understand with a T&G ceiling I need a vapor barrier since usually the latex paint would serve that purpose on a drywall ceiling. Can I use Liquid Tyvek (since I have a bunch extra) as the vapor barrier. The ceiling is covered with 7/16 OSB that I propose to tape seams and then paint with the liquid Tyvek before applying the T&G to. The OSB is already hung.
Need to know ASAP as I am painting the OSB this weekend.

Thank you,

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  1. brendanalbano | | #1

    You'll get better responses if you list the full stackup of materials and insulation in your roof from top to bottom.

    I'm pretty sure liquid tyvek is typically an air barrier, not a vapor retarder, so if you do indeed need a vapor retarder, that probably won't be what you need.

  2. Regulajo | | #2

    Brendan, thanks for your suggestion. The stack up of material from exterior to interior is as follows: membrane roof material, 1/4 inch dens deck, 5 inches Polyiso taped seams, 5/8" OSB, 12 inches cellulose, OSB sheeting on interior ceiling with seams taped, vapor retarder either liquid Tyvek or latex paint, t & g wood.
    Climate zone 6. Utah at 6,000 ft. Elevation.

    Thanks again!

  3. Expert Member
    Dana Dorsett | | #3

    You have about R25 of polyiso (derated for climate) and R44 of cellulose, which is nowhere NEAR an acceptable ratio for an unvented roof in zone 6(!) with class-III vapor retarders on the interior. The IRC prescriptive R25 for exterior insulation is predicated on an R49 total R, and that ratio needs to be preserved to have the average temp at the roof deck to be in the same range. The average temp at the roof deck determines what it's moisture burden will be.

    If you backed off to 6-7" of cellulose you'd be fine, but you'd still need an air barrier on the interior side. Taping the seams of the OSB and caulking it to the rafters might be enough, but taped seams unsupported by framing may come undone over time. The OSB itself is a "smart" vapor retarder, and would behave a class-II vapor retarder when the roof deck was cold, as long as you kept the interior humidity under 40% RH @ 68F in winter. In the spring when the roof deck is releasing it's moisture burden the moisture content of your interior-side OSB rises to the mid class-III level (about the same as latex paint on gypsum board), allowing the moisture to dry toward the interior.

  4. Regulajo | | #4

    Dana, thanks for your detailed explanation. Since the roof structure is pretty well finished, do I or don't I need to put anything on the interior OSB as a vapor retarder? The house has an HRV system, as well as a dehumidifier to control humidity levels.
    The house is also designed to be cooled in the summer by opening windows at night.

  5. GBA Editor
    Martin Holladay | | #5

    I think that you are confusing air barriers with vapor barriers.

    If your roof assembly has thick exterior rigid foam, you don't need an interior vapor barrier or vapor retarder. However, with your plan to use tongue-and-groove boards as your finish ceiling, you definitely need an air barrier between the tongue-and-groove boards and the cellulose.

    -- Martin Holladay

  6. Expert Member
    Dana Dorsett | | #6

    There are no magic bullets. If you have R44 cellulose inside of R25 polyiso you'd be fine if you moved the house to climate zone 4. As it is you have to be ultra-serious about air sealing the OSB. You might gain something by painting it with vapor barrier latex and crossing your fingers, but that's a Hail Mary move.

    If you can find somebody competent to run a WUFI simulation of your stackup in your location both with & without vapor barrier latex on the OSB you can see which is least likely to fail, and the indoor humidity limits that would give it a reasonable shot.

    The right thing to have done is to follow the IRC prescriptive bearing in mind the importance of the R-ratio, but if it's already all installed, STOP, and do a WUFI simulation. If you have to fix it by pulling the OSB and cellulose and putting a couple inches of closed cell foam on the under side of the roof deck (R37 of polyiso + closed cell polyureathane outside of R37 cellulose is about the right ratio) to have any margin it'll be cheaper to do it now than after the roof deck is rotting. A couple inches of ccSPF would be a about a half-perm of vapor retardency between the fluff and the roof deck, which is more than enough to protect it, but you'd still want the air-tight OSB as an interior side smart vapor retarder.

    BTW: Are the rafters true-dimension 12" TJIs, or are they 2x12s (and thus 11.5", which would be roughly R42.4 for the cellulose, not that it's sufficient to save the day here with only R25-ish foam up top.)

  7. Regulajo | | #7

    Thanks everyone for the input. I will check into getting a WUFI simulation, and proceed from there.

  8. GBA Editor
    Martin Holladay | | #8

    As Dana has explained, your biggest problem is that you have a dangerous ratio of exterior rigid foam to cellulose. To learn why your existing insulation stack-up is dangerous, read this article: Combining Exterior Rigid Foam With Fluffy Insulation.

    In your climate zone, at least 51% of your roof's R-value has to come from the rigid foam layer. At this point, your assembly only has 36% or 38% of the roof's R-value in the form of exterior rigid foam. (The percentage is not definite, because I'm not sure how much you want to de-rate the R-value of the polyiso to account for its poor performance in cold weather.)

    Unlike Dana, I'm not a believer in the value of WUFI simulations. Almost all consultants who use WUFI don't know what they are doing, and the results are mostly worthless. To learn more on this topic, see WUFI Is Driving Me Crazy.

    I strongly urge you to reduce the thickness of the cellulose layer to ensure that your roof doesn't develop moisture problems. Once you've solved that problem, you can choose what type of air barrier you want to install between your tongue-and-groove boards and the cellulose. (I think you should install taped drywall.)

    -- Martin Holladay

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