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Community and Q&A

Durability of Taped OSB Seams for Air Barrier

BrunoF | Posted in General Questions on

Is there any data or real-world experience on how durable plain OSB sheathing is over the long term?

Background: I have essentially decided to use regular OSB with Zip tape as my primary air barrier together with Benjamin Obdyke drainable housewrap for my WRB.  I will also caulk bottom plates and apply sealant behind the drywall top plate as a secondary air barrier.  My recent concern comes from observing the interior of my garden shed which has roof sheathing made of OSB.  Although I see no evidence of water penetration, I do see areas where the OSB is delaminating and flaking off but the board still seems solid when I tap on it.  If what I am witnessing would be a taped seam designed to seal air; it would be a total failure.

Is there any evidence of something like this happening with wall sheathing on a properly constructed house?  Is there any long term testing available on OSB to determine how likely or unlikely this failure mode is?


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  1. Expert Member
    Michael Maines | | #1

    I don't know of any specific studies but I'm sure they have been done. But it's commonly understood by anyone who has worked with OSB that it swells when it gets wet. It's also not always airtight. It usually is, but there are cases where it wasn't. That aside, if the panel swells, I don't see how tape would remain well-adhered.

    Even Zip sheathing and Adventech swell when wet, just not as much as commodity OSB.

    I would be ok with your system IF there was enough exterior insulation to keep the sheathing free of moisture accumulation. I would not trust your system if you are in a cold climate and have little to no exterior insulation, or if you're in an area with high humidity levels.

    1. BrunoF | | #3

      Michael, thank you! I am not in the "warm - humid" zone as defined by the ICC ( but am in the mixed-humid zone as defined by the BSC. The climate here is pretty mild so I am not planning on any exterior foam.

      If I fall into your classification of "high-humidity" what would you suggest as an alternative?


      1. Expert Member
        Michael Maines | | #5

        Bruno, it's hard to say because there are many factors, but generally, when possible I like board sheathing with a self-adhered WRB; my next choice would be CDX with primed and taped joints or a fully adhered WRB; next would be Zip sheathing, preferably with a secondary, mechanically fastened WRB; next would be a cross-strapped wall without sheathing; last and least would be commodity OSB.

        1. mikesmcp | | #19

          Michael, for my next project, I am planning a double stud wall with diagonal, site sawn board sheathing. Would you be willing to share your favorite "self-adhered" WRB and reasons why?

  2. Expert Member
    Akos | | #2

    This might help:

    Generally tapes will hold up much better if there is some continuous pressure, exterior rigid insulation does a decent job of that.

  3. jollygreenshortguy | | #4

    I seem to remember reading a blog or comment here on GBA in which the author identified that OSB makes a poor air barrier and lets a surprising amount of air through the field of the panel compared to the plywood. If I can track it down I'll post a link.

    - Found it! It's by Martin Holladay, "Is OSB Airtight?"*kbcec1*_up*MQ..*_ga*MTAyNjA3OTEyNi4xNjgxMDU1MzYz*_ga_SBNZMMC0G6*MTY4MTA1NTM2Mi4xLjAuMTY4MTA1NTQzMC4wLjAuMA..

    1. Expert Member
      Michael Maines | | #6

      Now you've opened a can of worms! The leaky OSB issue is rare but it can happen, at least with OSB produced in northern states that use aspen instead of southern pine. I was working for one of the builders quoted and who commented on that article, when we had issues with commodity OSB on one of our projects. Many people since then have informed me that what we saw was not true. Thus my vague statement, "OSB is not always airtight" in comment #1. It seems to be rare enough and the lack of airtightness small enough that most people won't consider it a problem.

    2. BrunoF | | #7

      I have read that article but believe that its validity has been called into question by several folks since it was sponsored by a company that makes and sells plain OSB alternatives. I find it hard to believe that sealed OSB will leak any significant amount under standard blower door test conditions.

      1. Expert Member
        Michael Maines | | #8

        JGSG, see what I mean? ^^^

        1. 5Stud | | #9


          My understanding was that if there was "harder" wood species (possibly birch) in the mix you get sharp channels along it's edges that aren't as tight.

          1. Expert Member
            Michael Maines | | #10

            Do you mean along the edges of the wood chip or along the edges of the panel?

            My experience is that a piece of poly sheeting taped to the center of a sheet of OSB would billow inward at ACH-50. I didn't do the work myself but was in the office while the boss and crew were trying to figure out what was going on. We/they couldn't get below 1.0 ACH50 when we routinely got to 0.4 ACH50 with our panelized system. After coating the interior of the panels with a vapor permeable, airtight fluid-applied membrane, we were able to get to our target. I don't recall now if that was 0.6 or 0.4 ACH50. I realize that 1.0 ACH50 is "pretty good" but air was definitely coming directly through the center of the panels.

          2. 5Stud | | #11

            Yes, edges of "harder" wood chip. Not panel edges.

            To me it is crazy that osb manufacturers don't provide the info. They know the number. I get that on the same production line, one panel is different from the next one but there shouldn't be a huge variance.

          3. Expert Member
            Michael Maines | | #12

            I've talked with reps of one popular brand and they do have testing for OSB made with SYP but at the time they supposedly had not air-tested their northern-made products. (Or they were having trouble getting consistent results, or they weren't getting numbers they like.) I am not aware of any requirement for structural sheathing to be airtight so it makes sense that nobody wants to be the first to start advertising values, as helpful as that would be.

  4. andy_ | | #13

    I think the biggest issue here, and it's been talked about already, is that the quality of OSB varies quite a bit depending on where you are and who makes it. Even locally I've seen OSB sheets that practically crumbled when carried and also 15 year old OSB in my garden shed floor that still hasn't swollen even though it regularly gets wet. Not sure how much I'd want to bet on which one I'm getting now.
    Even plywood is inconsistent these days. I've had batches delaminate with just condensation, others came moldy from the yard, while others have been indestructable.
    I will say that the Zip sheathing has been consistently pretty good and much better quality than any of the other OSB sheets that I've used. Might be worth the premium to go with Zip since you're taping anyway?

    1. BrunoF | | #14

      Thanks folks. The only reason I was hesitating on Zip is because I planned on using Benjamin Obdyke Hydrogap, drainable WRB. I wasn't sure that a WRB on top of a WRB (zip in this discussion now) would have the right vapor transfer. I was also concerned with what would happened if bulk water got between the zip and the hydrogap.

      1. andy_ | | #15

        I've used Hydrogap and really like it, but I'm curious why you're selecting it here. What's the siding? Any exterior insulation?

        1. BrunoF | | #18

          andy, I am using it as my WRB under fiber cement (no exterior insulation). I would love to do a true rainscreen but finding folks who can detail that around here is difficult so the next best thing is a drainable wrap.

  5. plumb_bob | | #16

    Michael Maines, you mentioned (post #5) that your preferred system is board sheathing, do you mean lumber? Why not plywood?


    1. Expert Member
      Michael Maines | | #17

      Hi plumb_bob, I like diagonal board sheathing for these reasons:
      May be cut from the site, low embodied carbon regardless of location, no toxic chemicals, scraps can be burned or allowed to rot, the most vapor-open solid sheathing, the most moisture-resistant sheathing (meaning it can get wet and dry out without swelling much or rotting), easy to handle individual pieces. They use a lot of it in Vermont, including projects by my friend Bob: who is a big fan of it. I have had little luck getting builders to do it here in Maine but I have used it on a couple of double-stud walls in recent years, with all lumber cut from the site.

  6. plumb_bob | | #20

    Interesting. So do you have a mill that travels from site to site? The lumber must be planed for it to achieve a wall flat enough for finishing? Is the lumber graded?

    1. Expert Member
      Michael Maines | | #21

      I don't have a portable mill but on one project, the owners hired one to come to the site. Unfortunately the sawyer was a drunk, the bandsaw blades were dull and the cuts were wavy, but I had designed a double-stud wall so we could put the best faces out and there was plenty of sheathing to choose from.

      On the other project, the owner was wealthy and into buying equipment so he bought a very nice bandsaw mill and had a steel-framed enclosure made from it. Those cuts were very nice.

      In both cases, here in Maine, visual grading is allowed. I did my own engineering and used conservative values. In both cases, and typically, bandsaw-milled lumber is flat enough to not require planing, though a little touch-up with a power plane is helpful before sheetrock goes up, but around here that's true of store-bought dimensional lumber as well.

      My house was built in 1830 and there are many houses of that era in New England so dealing with sawn board sheathing is nothing new for remodelers. Sheet stock is flatter and smoother, but if you're using a rain screen system you wouldn't know the difference.

      1. Expert Member
        MALCOLM TAYLOR | | #22


        We still form most concrete stem-walls with 1"x8"s here then re-use them as sheathing (although they are getting pickier about which walls we can use them on as seismic requirements increase). If you have enough, it's often worth just continuing on and using boards for everything - my own house is done that way.

        There are obvious benefits to having them as sheathing for moisture management, but they have downsides too. They are very time consuming to work with, especially at 45 degrees. All joints and ends need to be angle cut, and there is a lot of waste. The frequency of the gaps between boards also make attachment a bit hit or miss with a nail gun. Using them of course precludes the sheathing being the primary air barrier, and relying instead on a self-adhered WRB. Something that makes sense on renovations, but seems a bit of a compromise on new construction.

        1. Expert Member
          Michael Maines | | #24

          Malcolm, I agree that board sheathing takes longer to install, maybe 2-3 times as long as setting plywood or OSB. But on the scale of the entire project I don't think it's very noticeable. It wouldn't work for production framers but it seems like a good match for frame-to-finish carpenters who want to minimize how much glue they are inhaling and putting into the environment as sawdust and scraps.

          Self-adhered membranes are a natural fit for board sheathing but on the project I mentioned that I helped build, we used regular Typar, installed with cap nails and taped the seams, then installed vertical 1x3s as a rain screen so there was no problem with missed nails. We didn't test the project with a blower door as it was just an addition on an older home but I think it's pretty tight.

          It all depends on priorities. If the priority is to minimize labor, then don't use board sheathing. If the priority is to use a locally-sawn material with reduced risks to health and environment, and for better long-term durability, I think it's worth considering.

  7. plumb_bob | | #23

    I am used to working around it on older houses, including my own, often with concrete residue as they were first used as forms as Malcolm mentioned. But I have not seen it on new construction around here, ever. But would be worth looking at if the owner has a mill or access to cheap logs.

  8. BrunoF | | #25

    Coming back to my original question, would a premium OSB like Advantech sheathing work for better butt joint integrity and still allowing drying in and out of the wall?

    1. Expert Member
      Michael Maines | | #26

      Bruno, yes I believe that Advantech sheathing would hold up better than commodity OSB. Advantech uses more expensive glue and their sheathing seems to be denser than commodity OSB. I don't believe they add as much paraffin as they do in their floor sheathing so their roof and wall sheathing does not seem as impervious as their floor sheathing, but don't quote me on that. I have talked with their technical reps about it but the only OSB wall sheathing I spec is the Zip system and I can't recall all of the details about their other products.

      1. BrunoF | | #28

        Thanks! I can't find the advantech sheathing vapor transmission spec online so I will give the mfg a call Monday. I also need to price the material and hope that it is cheaper than zip.

  9. Patrick_OSullivan | | #27

    Here's my backyard test on a bunch of sheet goods over about 3 years:

    - 5/8" CDX stacked above ground but completely uncovered has curled but is completely usable for utility grade stuff.
    - 5/8" Zip stacked above ground but completely uncovered is swelled about 1/16", but cut the edges and it looks like a brand new board.
    - 3/4" Advantech floor sheathing laying on the ground is greyed but otherwise looks and behaves completely new.
    - 3/4" commodity OSB bumming around the yard is swelled to about 1 1/4" and unusable for any real purposes.

    This is all relatively recent info as I was cleaning up my yard of construction remnants and loading a dumpster. Take it for what you will. In my view, for Zip to rot, it would take a constant barrage of water with limited drying, but eventually that will kill every material.

    1. Expert Member
      Michael Maines | | #29

      My backyard test for 3/4" Advantech floor sheathing is that when we bought our house 10 years ago, the previous owners had made a few A-frames from it to protect plants. We put them into our outdoor chicken run instead, fully exposed to weather, where they provided shelter and a spot to roost. For 9 years they have been outdoors, sitting in rotting/composting straw with lots of chicken poop. They had no rot for the first 5-6 years. Since then the bottom ±6" are rotting away and the rest of the sheets have swelled and are somewhat soft but not rotten. Amazing stuff.

  10. kooshball | | #30

    My buddy may have one of the best backyard test for Zip...he built a hyper insulated shop 15-years ago sheathed and taped with Zip but he never got around to putting siding on it. The taped Zip has been in direct contact with all the elements for 15-years and it is still watertight, all the tape is still adhered and it actually still looks good.

    If I go with Zip + a drainable WRB I need a solution for any water that might get between the Zip and the WRB.

  11. BrunoF | | #31

    My buddy may have one of the best backyard test for Zip...he built a hyper insulated shop 15-years ago sheathed and taped with Zip but he never got around to putting siding on it. The taped Zip has been in direct contact with all the elements for 15-years and it is still watertight, all the tape is still adhered and it actually still looks good.

    If I go with Zip + a drainable WRB I need a solution for any water that might get between the Zip and the WRB although I just learned that the Zip surface is textured to allow maybe I don't need a drainable WRB at all?

    1. Expert Member
      MALCOLM TAYLOR | | #32


      "If I go with Zip + a drainable WRB I need a solution for any water that might get between the Zip and the WRB"

      At some point you have to have faith that the assembly you are using can keep bulk water out to the extent that it will diffuse and not need to be drained. That is the siding, rain-screen gap, and the WRB behind. If enough water is getting through all that and wetting your sheathing enough that it needs to drain, something has gone badly wrong.

      1. BrunoF | | #33

        That is a good point. Should I trust just the Zip, or it is worth it to add a drainable WRB on top?

      2. BrunoF | | #34

        BTW, creating and / or trapping condensation between the ZIP and the WRB is my main bulk water concern.

        1. Expert Member
          MALCOLM TAYLOR | | #35


          As long as you are using a permeable WRB, any interior moisture that moves through the sheathing will diffuse through the WRB. How easily it moves on from there depends on how permeable your siding is, and whether you include an air-gap.

  12. briancornwell | | #36

    I would not depend on it. OSB is leaky, unless you go with the Advantech OSB style. Go with Advantech 5/8" sheathing and make it beefy.

  13. user-1072251 | | #37

    I've had excellent luck with fir cdx, Henry VP100 Blueskin as the WRB. We go over that with 1x3 strapping, although 1/4-3/8' wood lath works just as well. (and much cheaper) Using a wood rain screen as we do is not hard and is not complicated. The lath adds next to nothing to the depth, so you can leave existing window trim in place.

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