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Smart Membrain installation details

CTSNicholas | Posted in Green Products and Materials on

I am installing CertainTeed’s Smart Membrain product over 5.5″ of Owens Mineral Wool insulation. The installation instructions are not very clear in the fine details unless I am installing the product as an ‘air barrier’ in addition to vapor retarder. Located in zone 5.

My insulation is obviously face-less and has no barrier. I have 7/16″ OSB and Greenguard 3D Rraindrop House Wrap over that, with Vinyl siding covering the house. The house wrap was taped, but I am certain every staple and siding nail has punctured it in many ways.

I am just stapling the edges of this membrain product per install instructions. It’s not clear about outlets and fixture openings. I’m considering cutting the product tight and fitting it over the outlet boxes and then taping that seam – but this seems pointless if I am not taping and sealing the product at windows, top and bottom plates, and where partition walls meet into exterior walls.

What should I be doing here? I’d like to install the product properly, but I am not sure taking time to seal around punctures in the product will do me any good? This also applies to every location that I staple to the stud. Without caulking, there will be leaks around each staple. Any suggestions on what installation method I should follow?

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  1. GBA Editor
    Martin Holladay | | #1

    Every wall needs an air barrier. The usual time to determine where your air barrier will be is at the design stage, before construction begins. For more information on this issue, see Questions and Answers About Air Barriers.

    I agree with you that taped housewrap does not amount to an air barrier. In your case, the easiest way to establish an air barrier would probably have been to use a high-quality tape to seal the seams of the OSB wall sheathing. This approach would also require careful air sealing at all wall penetrations (hose bibs, electrical penetrations, etc.) and at the gap between window rough openings and window frames.

    You don't mention whether you performed this work, so I'm guessing that you didn't.

    If you were installing MemBrain as a vapor retarder, then details at electrical boxes and seams wouldn't matter. A vapor retarder does not have to be installed in an airtight manner.

    As this point, your wall probably needs an air barrier. If you agree that you need an air barrier, you have two choices: (a) Install your MemBrain in an airtight manner, and call the MemBrain your air barrier, or (b) Install your drywall in an airtight manner, following the Airtight Drywall Approach.

  2. Dana1 | | #2

    A vapor barrier or interior wall that isn't at least somewhat air tight can pass a lot of air-transported moisture. From a moisture transport point of view a square inch of air leak can be worth a whole wall's worth of vapor diffusion through standard latex paint.

    In Canada they sell "vapour hats" to make air-sealing the vapor barrier around non-air-tight electrical boxes easier, but that's not the only way to air seal an electrical box. Some people do a better job that others, but there are many instructional videos out there online, eg:

    (I haven't fully reviewed these examples... searching youtube et al comes up with 100s of hits.)

    Anything you can do with air-sealing 4 or 6 mil polyethylene works with MemBrain too.

    Ideally any taped seams in flexible vapor retarders will happen where it is fully supported by the framing, but where that isn't practical, overlap the layers by a foot or so.

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