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Dupont says not to tape/seal plywood seams???

Daniel Weaver | Posted in Energy Efficiency and Durability on

I was doing a bit of research about sealing plywood seams on a retrofit project…I stumbled across this on Tyvek’s FAQ page:

15. Can I tape sheathing board seams?

Taping sheathing board seams creates a continuous vapor barrier, so moisture vapor would have no where to escape. Adhesion of tape to foam and wood would also be temporary, due to thermal expansion and contraction of the wall assembly. A more effective and efficient means to provide air infiltration resistance and bulk water holdout is to properly install DuPontâ„¢ Tyvek¯ using our best practice guidelines. Since DuPontâ„¢ Tyvek¯ flexible, it will continue to provide protection to the sheathing when undergoing thermal expansion and contraction.

Is this true? Seems like it would be wise to tape the seams – does this make it a vapor impermeable as Dupont suggests? Then read Martin’s blog and saw that Marc Rosenbaum recommends using Vycor to tape seams. Hmmm. I’m in Climate Zone 3.

Thanks for guidance,

Daniel

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Replies

  1. User avatar
    Armando Cobo | | #1

    Daniel,
    I specify taped Tyvek over plwd/osb sheathing and taped rigid foam on top to act a second moisture/air barrier, and depending on the cladding, I specify Home Sliker, Delta-Dry or 1x4 batten rain screens. The moisture vapor would get trapped if you install a non-permeable insulation in the wall cavity; the big issue is to make sure the wall assembly dries to the inside.

  2. Daniel Weaver | | #2

    Armando,

    This is a remodel situation wherein we need to align the planes of existing exterior finish that are in the same plane. We won't be using exterior rigid insulation - putting up with a bit of thermal bridging. We're using blown in cellulose - should be able to dry to the inside. Did you tape the seams of your plywood or osb? Trying to prevent air leakage here. Thx

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Daniel,
    The simple answer is that DuPont is wrong. Housewrap makes a lousy air barrier; taped sheathing is a much more effective air barrier. Of course, you have to use the right tape. Don't use Tyvek tape.

    Plywood is a smart vapor retarder; the higher its moisture content, the higher its permeance.

    OSB is not as permeable as plywood, and there are many reasons to prefer plywood to OSB (whether the seams are taped or not).

    Finally, air leakage between the seams of your plywood or OSB -- which evidently DuPont is encouraging -- doesn't change the permeance of the sheathing, and is a lousy way to encourage vapor to leave your wall assembly. No reputable building scientist would advocate deliberately encouraging air leakage at panel seams.

  4. Daniel Weaver | | #4

    Thanks Martin...that's what I thought. I presume that water vapor can diffuse to the outside through plywood, at least to a degree. Dense pack cellulose and drywall will allow the assembly to dry to the interior provided that there is not an overabundance of moist interior air which I don't anticipate.

  5. User avatar
    John Semmelhack | | #5

    This kind of makes me wonder what kind of crazy stuff is in the water over at DuPont...

    Whoops...just found the answer:
    http://www.opednews.com/articles/DuPont-s-Contamination-Of-by-Jack-Swint-120801-918.html

  6. Aj Builder, Upstate NY Zone 6a | | #6

    I would have post exactly what Martin did... but alas... I was trying to not be the one that spoke that poorly of a billion dollar company. Anyway... same as Martin from this guy.

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