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OSB as air barrier

Trevor Lambert | Posted in General Questions on

I read the relevant GBA article, and the entertaining comments section (which left me perhaps more confused than before. Since that is a pretty old article, I’d like to ask if there is any updated info. What is the latest opinion on it? Were the air leakage failures ever determined to be manufacturing defects or an inherent problem with OSB? My house was built with 7/16 OSB interior sheathing as the air barrier / vapour retarder. I’m finally getting close to putting up drywall, and wondering if I should do anything else before it’s closed up. Would a coat or two of latex primer on the be of any help? I know it’s not going to get every crack and seem, but if the OSB itself is potentially going to leak, I’m thinking that a coat of paint over 95% of it would be a significant help.

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Replies

  1. User avater GBA Editor
    Martin Holladay | | #1

    Trevor,
    I don't really have any updated information since I wrote "Is OSB Airtight?" -- except that I still encounter Passivhaus builders who are suspicious of the airtightness of ordinary OSB, and who therefore often choose other materials (for example, Zip sheathing) as an air barrier.

    As I noted in my article, some brands of OSB appear to be airtight, while others are leaky.

    In your case, the obvious path is to perform a blower-door test. If you hit your target number, you're done. If you fail to hit your target number, seal up any obvious leaks first -- and if that doesn't work, paint the OSB.

  2. Trevor Lambert | | #2

    The thing is, if priming the OSB is likely to help, I might just do that preemptively. It's fairly inexpensive and easy. A second blower door test would actually cost more than painting, and it could prevent yet another delay. So am I correctly interpreting your reply that you think if the OSB is a source of air leakage, painting it could reduce that leakage?

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

    Trevor,
    One of the suggested solutions that was mentioned in my article ("Is OSB Airtight?") was to paint the OSB with elastomeric paint. I have no idea whether that remedy would make a difference solving the air leakage problem with leaky OSB, but it sounds reasonable.

    Home Depot sells a brand of elastomeric paint for $126 for 5 gallons (about $25 per gallon).

  4. Trevor Lambert | | #4

    Yes, I read that but it also mentioned that the elastomeric paint was incompatible with some sealing tapes. I think the only benefit of the elastomeric paint vs latex is flexibility (for bridging cracks, and resisting cracking with expansion and contraction of the substrate). For an interior surface where the temperature variation will be minimal and a perfect seal is not a requirement, elastomeric paint would seem unnecessary. I see that it is latex based though, so it shouldn't interact negatively with the joint tape. As an aside, that exact paint is $195US at current exchange rates at Home Depot in Canada.

  5. Trevor Lambert | | #5

    We completed the blower door test without doing anything to the OSB. 0.22ACH, so I guess that's good enough. Prior to reading about this OSB stuff, my target was below 0.2, because that's what the house before us got. We had some design refinements that should have resulted in a more tight envelope. But they had something we didn't have, a completely airtight covering over top of the OSB (due to real or imagined allergies of one of the occupants). The guy at the builder told me they actually tested a bunch batches of the OSB they use, and based on their tests and the wall area and volume of my house, he expected it to contribute in the ballpark of 0.1ACH. Based on all the latest info, I revised my target to 0.3. Now I'm wondering if we could hit 0.1 if I do the painting and retest. Of course, my wife thinks I am out of my mind to even consider this. She is probably right.

  6. User avater GBA Editor
    Martin Holladay | | #6

    Probably out of your mind. I agree.

  7. Malcolm Taylor | | #7

    Trevor,
    Congrats on hitting 0.2.

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