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Diagonal Board Sheathing and Air Barriers

lrs123 | Posted in General Questions on

HI all, 

I’m kicking around the idea of using pine boards for wall sheathing in a new construction home due to the high cost of OSB and CDX. I’ll be placing several layers of recycled polyiso over it which I could tape as an air barrier. I’m concerned that this might not be the most durable option over the long haul (insects mainly, no termites though).

Any thoughts? I like the boards because they come from a mill 3 miles away but don’t want to cut a corner now just to run in to headaches down the road. The alternative would be taped OSB sheathing followed by polyiso.

thanks!

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Replies

  1. charlie_sullivan | | #1

    We used site-milled horizontal board sheathing plus diagonal bracing. We foolishly tried to use a Mento heavy duty WRB for air sealing at the exterior and it was hard to detail that to make a real air barrier, but our primary air barrier is interior, so it's not a disaster. Taped polyiso should do better, if it's in good enough shape to tape well. A self adhesive WRB should do better than either, I would think.

    1. maine_tyler | | #6

      I know this is an old post... but Charlie, what made detailing the Mento difficult / ineffective? Might be looking at similar options...

      1. charlie_sullivan | | #7

        Here's my thinking. With a free floating membrane, if there's a little tear in a membrane anywhere, that becomes a leak that size. On the other hand, if there's a tear in the self adhesive membrane, and it's on top of a board, there's no air leakage. It's only when the holes line up with the cracks between boards that you get leakage. There's also the fact that sealing the membrane to the sill plate, for example, requires the tape to stick to the membrane, and the membrane to stick to the wood, so that's two points for possible failure. And if you have wind before the siding is up, the whole expanse of membrane is a sail pulling on that possibly questionable interface.

        If partly depends on your contractors skill level and learning ability. Mine had trouble understanding that tape doesn't stick to dirty surfaces. The success level with a free-floating membrane might be higher with more skilled contractors.

  2. Expert Member
    MALCOLM TAYLOR | | #2

    Luke,

    We often re-use the 1"x8" boards we used to form up the foundation walls as diagonal sheathing. That's easier here because we still use an interior air/vapour barrier, but you can do as you suggest and use the foam, or use an adhered WRB.

    Sheathing with boards is a task I hate. If they aren't long enough you have to taper the ends at the studs they join at, and trim the ends where they overhang the plates. There is a lot of waste and it is tedious, slow work.

    That said there is nothing wrong with the finished product. It's a good durable sheathing that isn't as susceptible to moisture problems as plywood or OSB.

  3. lrs123 | | #3

    Thanks for the responses.

    I agree that a self adhered WRB would be ideal, but that makes board the same price as taped osb with more work. I supposed i could do air tight drywall as a secondary barrier.

    Charlie - did you happen to have raised heel trusses on your build? I'm wondering how I'd address that with horizontal boards (or diagonal for that matter). Also did you use 1x or 2x for the let ins?

  4. Jon_R | | #4

    Consider not using wood sheathing at all - just diagonal steel strapping if required for racking.

  5. lrs123 | | #5

    Good suggestion, Jon. I hadn't considered it because it's not common around here but no reason not to I guess.

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