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Minimum Plywood Thickness As An Air Barrier

Jaccen | Posted in Energy Efficiency and Durability on

Hello,

A number of articles and blogs have been written on using OSB ( https://www.greenbuildingadvisor.com/article/flatrock-passive-insulation-and-an-air-barrier ), plywood/zip ( https://www.greenbuildingadvisor.com/article/study-finds-osb-sheathing-not-a-reliable-air-barrier ), and smart vapor barriers ( https://www.greenbuildingadvisor.com/article/urban-rustic-air-sealing-the-attic-floor ).  They all illustrate various methods of air sealing a building using those materials along with their pro’s and con’s.

My question is: what is the minimum thickness of plywood one could use for air sealing (ie. not structural, but for air sealing purposes only).

It seems to be *generally* accepted that 1/2″ or greater is required for OSB.  Zip is…….Zip and any of the smart vapor barriers are whatever they are.  But I do not think I ever recall an article detailing the minimum thickness of plywood and that they all used at least 1/2″. Has anyone tried 3/8″ or even 1/4″?

Thanks.

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Replies

  1. Kyle Bentley | | #1

    I think the recommendation for 1/2" or 7/16" osb have been resoundingly in terms of structural first, and air barrier second. This site is primarily residential leaning, though commercial topics come up time to time. While code minimum is 3/8", I'm not sure many builders, even production builder spec it. Having said that, I don't see any reason that 3/8" plywood wouldn't make a good air barrier, as the odds of voids aligning to create such a pathway would be pretty low. The 1/4" plywood is probably more trouble than it's worth. It's flimsy, prone to warping, and the cost per inch of thickness is high there.

    If you didn't need structural sheathing, I'm certain you'd be better off using fiberglass faced gpysum panels, like you might have seen on commercial construction as you drive down the road. It's the yellow 'Dense glas" panels if you recognize it that way, or sometimes the white "glass rock" panels. They're cheaper, flatter, probably stronger than 1/4" plywood, are more rot resistant, and have better sound absorption. They also have a higher vapor permeability than plywood, and much more than osb, if that is what your climate needs.

    I know it's sometimes a chore to explain all the details or the background of a question, but if you've got a specific use case that you're wondering about, put it out there for discussion.

  2. Expert Member
    Malcolm Taylor | | #2

    Jaccan,

    If y0u are blocking on all edges I don't think there is a minimum thickness. If not I'd be worried that 3/8" and thinner wouldn't effectively stay in plane, and the tape would fail.

  3. Walter Ahlgrim | | #3

    If the air barrier serves no structural purpose, why chouse a rigid product at all and use house wrap or polyethylene.

    Walta

  4. Expert Member
    Zephyr7 | | #4

    As an air barrier, just about any solid material should work. You could even use 1/8" hardboard here (masonite), but it's not particularly durable in large sizes. The thinnest I've used myself is 1/4" waferboard, which is a sort of non-structural version of OSB. It works fine, but it's not very strong so it's prone to breakage (especially corners) if you try to install full sheets of it. As an air barrier though, it works just fine. I've built smaller things with luan, which is a sort of sub-1/4" plywood, and that works too -- and its a little more durable than 1/4" waferboard.

    The big issue is that you really don't save much money going down below 3/8" or so. I mostly use the thinner products for things like baffles where I want them to be lightweight so that I can more easily install them in awkward locations. Malcolm makes a good point about keeping a flat surface too -- really thin panel products have a tendancy to bow in or out when hung on a surface, which can complicate other things in the assembly.

    Assuming you are trying to seal up a wall, I'd stick with the usual 7/16" CDX sheathing here (or equivalent OSB), or something even thicker. This gets you a nice solid wall, and you don't really save much trying to go thinner.

    Bill

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