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Community and Q&A

Vapor Retarder or not?

Dublin1997 | Posted in Energy Efficiency and Durability on

Hiya Folks,
First time poster but long time reader. I’m currently insulating a detached workshop here in the arid SW (Zone 5B, zip is 87048). This building is 2×6 stud construction with a flat roof. At this point in time, I don’t plan to heat or cool the space unless I am present. I’ve installed a mini-split system (21 SEER) for the times I’m present. To date, I’ve sealed all the exterior cavities between stud and sheathing, filled all my holes in the studs/top plates, sealed the bottom plate to the stem wall, and am now working on insulation. The exterior has 2 layers of tar paper, Tyvek, then stucco. Insulation in the walls is Roxul (R23) and the ceiling is getting the same (R60) but installed in two layers perpendicular to each other and covering the bottom chords of the trusses. For roofing, I have a TPO membrane over the standard rigid insulation they use (about an inch worth IIRC). The underside of the roof is vented with parapet vents on all sides. Insulation will leave approx. 9″ between the insulation and the deck at a minimum increasing to approx. 12″ at it’s “peak”. When insulation is complete, I plan to drywall and paint both walls and ceiling to finish it all up.

Over the years it seems that the philosophy of using vapor barriers has morphed quite a bit. Moving more into vapor retarders determined by geography/climate and now I’m starting to hear of more folks recommending not installing a separate retarder at all, at least in my climate. I did an addition on my home a few years ago with very similar construction and used Membrain under the drywall. It’s only been a few years but I haven’t seen any signs of issues. I’m just wondering what the experts think would be appropriate in my situation. Would drywall/latex be enough especially given this building will only be used on occasion and not fully habited? I certainly don’t mind putting up Membrain again but it would be nice not to incur the cost of that if not necessary.

Thank you very much for your time and inputs!

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  1. GBA Editor
    Martin Holladay | | #1

    Building codes require, and building scientists recommend, the use of an interior vapor retarder in your climate zone.

    Either vapor-retarder paint on your drywall or MemBrain will work fine.

    For more information, see these two articles:

    Vapor Retarders and Vapor Barriers

    Do I Need a Vapor Retarder?

    -- Martin Holladay

  2. Dublin1997 | | #2

    Thank you Martin. After speaking with my building inspector, he indicated that it was not required to have a retarder (aside from latex on the paint) and that code was allowing this (at least in our locality). This has prompted me to ask the questions to begin with. Are you recommending one or the other (ie. paint or membrain) but not both? Or would both be preferred? And finally, I've been given so many recommendations and advice locally and I've read myself silly online, should membrain be put on the ceiling? I believe, given I have a flat roof that is vented, that yes I should but thought I'd try to confirm. Thank you for the help!

  3. GBA Editor
    Martin Holladay | | #3

    As the articles I linked to indicate, attention to airtightness is far more important than addressing vapor diffusion. You'll be fine with ordinary latex paint.

    That said, I'm not sure that your roof has adequate ventilation. For recommendations on flat ("low-slope") roofs, see Insulating Low-Slope Residential Roofs.

    -- Martin Holladay

  4. Expert Member
    Dana Dorsett | | #4

    If there is a vented air gap between the Tyvek and stucco it would meet the "Vented cladding over wood structural panels" exception in TABLE R702.7.1 of the IRC, which would then allow standard latex paint to be the interior side vapor retarder:

    If the metal lath for the stucco is tight to the Tyvek and there are no weep screeds or top vents to the cavity, it would need a Class-II or "smart" vapor retarder.

    If it fails to meet the vented cladding defininition, MemBrain would be preferable to half-perm "vapor barrier latex" paint as the vapor retarder, since half perm paint limits the drying rate toward the interior no matter how damp the air in the cavity becomes, whereas MemBrain becomes vapor open at humidity levels high enough to support mold.

  5. Dublin1997 | | #5

    Thank you folks. Appreciate the education! My exterior walls have the lathe tight to the Tyvek...there are weep screeds at the bottom but I have yet to ever see them work...I know the installation of them needs to be meticulous. I have vents at the very top of the walls but those are blocked off from the cavities below by the top plate so I wouldn't think that counts as satisfying the "vented cladding over wood..." definition. So at this point, I think I'll go ahead and poney up the $$ for membrain and go ahead and do it. As for the venting of my roof, there are parapet vents on all 4 walls and when I've been working up there, I can feel the breeze going through my trusses. Far from scientific and far from definitive I know but I'm going to run with it and hope for the best. When we did the addition on the home, we had to tear into a lot of the existing house which was built with the same style parapet vents and not a sign of moisture damage anywhere.....I guess living in a place with 15% RH has at least this advantages (and another being nothing ever rusts)! Thanks again to everyone for their time and comments!

  6. Anon3 | | #6

    OSB is a smart vapor retarder

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