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CDX and OSB as Vapor / Air Barrier

sb1616ne | Posted in General Questions on

I have read most of the articles about CDX and OSB as an air and vapor barrier. It seems that CDX is the best option for air tightness due to a lower leakage rate but a coat of vapor preamble paint(most basic primers) should do the trick?

Intello seems like great stuff, but it seems like for some scenarios a rigid durable surface behind the drywall such as CDX or OSB could make sense for alot of spaces?

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Replies

  1. Expert Member
    BILL WICHERS | | #1

    OSB and CDX are NOT vapor barriers, but they CAN be air barriers. The usual way to use either product as an air barrier is to put a bead of sealant around the perimeter as the panels go up on the exterior. You still need a water resistant barrier (WRB) over the exterior face of either CDX or OSB though.

    On the interior, the drywall alone is your air barrier, and ideally will be detailed in a similar way to the OSB/CDX using a bead of sealant around the perimeter as the drywall panels go up. Drywall is NOT a vapor barrier though, and doesn't really do much as a vapor retarder, either (unless you use special paint). Intello would act as a vapor retarder behind the drywall if you need a vapor retarder, and it can be detailed as an air barrier too.

    I don't see where you would need a layer of plywood or OSB behind the drywall, unless maybe you were planning to put up a lot of shelving on a particular wall. You certainly don't need plywood or OSB behind drywall to act as an air barrier, because the drywall alone already services that purpose. If you just want a more solid feeling drywall wall, I would just use 5/8" drywall instead of commercial standard 1/2" drywall. You'd be surprised how much more durable that extra 1/8" makes the drywall panels.

    Bill

  2. mr_reference_Hugh | | #2

    I don't have Bill's level of construction experience. I only have the research that I have done in the last 2 years.

    We used Huber Zip OSB 7/16" as a vapour barrier on our 12" wall. We have an engineer stamp and acceptance by our extremely detailed in rigorous (tough cookie) bulding permit reviewer/approvers. The OSB is located in the middle'ish of the wall assembly: 4" from the face of the interior wall, and about 8" from the face of the exterior wall.

    I wanted to know for sure what the story was so I contacted the Canadian Wood Council. https://cwc.ca/en/home/

    I also contacted some of the product suppliers. I did find a few "official" reference documents that brought me piece of mind.

    Here are a few references you can review:

    https://www.aiaaustin.org/sites/default/files/file/BEC/BSC%20List%20of%20Permeabilities.pdf

    https://www.greenbuildingadvisor.com/question/zone-7-wall-help-2-foam-exterior-3-4-plywood-to-interior-need-vapor-retarder

    https://www.performancepanels.com/permeability

    I have attached two documents.

    The first is a detailed document on plywood that examined plywood permeability. There is a table on perm rating on page 8 of 10 page numbers (at bottom of the pages). You might comparet this older data to what is found in the other links I provide above.

    The second is a relatively recent document on the perm rating of OSB specifically.

  3. Andrew_C | | #3

    Interesting question. In my mind, the permeability ratings are used for evaluating VAPOR retarders. My understanding is that the permeability of OSB is more variable than for plywood, varying by manufacturer and perhaps batch.
    In terms of being an AIR barrier, regular OSB is not as good and more variable than plywood. They make a point of this in the new PGH book (p. 104). Regular OSB, by itself, is not a great air barrier IMO, though wrt high leakage houses built by typical builders in the past, there are probably bigger leaks to address first. If you're making building new to PGH air leakage goals, the OSB needs some help.
    ZIP is another thing altogether.

    That's my understanding, feel free to correct.

  4. mr_reference_Hugh | | #4

    In my mind, the permeability ratings are used for evaluating VAPOR retarders.
    > agreed. I said vapour barrier but I guess I should say Class I vapor retarder.

    My understanding is that the permeability of OSB is more variable than for plywood, varying by manufacturer and perhaps batch.
    > That is in the research I shared. But I would never use simple OSB as a vapor barrier. Then again, I would not use regular OSB other than to build a shed.

    In terms of being an AIR barrier, regular OSB is not as good and more variable than plywood. They make a point of this in the new PGH book (p. 104).
    > I just prefer plywood but it is still a manufactured product with its weaknesses.

    Regular OSB, by itself, is not a great air barrier IMO, though wrt high leakage houses built by typical builders in the past, there are probably bigger leaks to address first.
    > Yes, lots of areas to improve on track housing. Lots of low hanging fruit.

    If you're making building new to PGH air leakage goals, the OSB needs some help. ZIP is another thing altogether.
    > Agreed. My builder wanted to switch from ZIP to regular OSB because inflation was off the charts during the pandemic. There were no other price increase (at the time) from our original contract so I paid more for the ZIP to be put back into the assembly.

  5. Expert Member
    AKOS TOTH | | #5

    I ran into an issue when trying to use OSB as a vapor barrier. Technically anything more than 5/8" has less than 1 perm, so class I, but it is not labeled. Since there is no label or datasheet providing that is has been tested as a vapor barrier, my inspector would not allow it.

    If you are in cold climate, there is nothing wrong with 6mil poly, cheap, easy to install and detail as a secondary air barrier. It also makes the inspector happy.

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