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Double walls, dense packing, and bendy sheathing

ranson | Posted in Green Building Techniques on

Many articles on double stud walls suggest fiberboard or gypsum as the exterior sheathing to prevent winter moisture issues. Given that thick walls require higher cellulose density, what tricks are there to keep the sheathing from bulging?


Zone 5, Rochester NY

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  1. GBA Editor
    Martin Holladay | | #1

    The two tricks I know are:

    1. Let the sheathing bulge out, and then whack the bulges back into place with brute force (using short lengths of 2x4 or small plywood scraps to distribute the force of the blow). Not satisfying, but builders have done this successfully.

    2. Install two layers of strapping: first, horizontal strapping (often 2x4s), and then vertical strapping (to create a rainscreen gap that drains vertically).

    For more information, see Wall Sheathing Options.

  2. ranson | | #2

    Following up on my old question: Would a Slicker-style rainscreen material, in combination with horizontal wood siding (like shiplap), provide enough resistance to prevent dense-pack bulging in fiberboard and gypsum?

    I noted that Slicker is spec'ed to compress only 0.7mm at 150psf. That's well below the 2.9psi (417psf) that the cellulose is blown at, but I can't imagine the resting pressure of the cellulose on the rainscreen would approach that level, especially through the sheathing. However, I believe this is a short-term test, and does not account for creep.

    Here would be my concerns: Is the rainscreen material going to creep over time, and will it remain an effective rainscreen? And will the wood siding effectively resist the force of the cellulose through the fiberboard so that no bulging is visible in the siding?

  3. GBA Editor
    Martin Holladay | | #3

    I don't think that a plastic mesh product like Home Slicker will provide any resistance at all to bulging. Nor do I think that siding can be depended on to resist bulging.

    If you are choosing to install a type of wall sheathing (like fiberboard) that is known to suffer from the bulging problem when dense-packed cellulose is installed, I advise you to adopt one of the two approaches I mentioned in my first posted answer. Either whack back the bulges before the siding is installed, or install two layers of sturdy furring strips (one horizontal and one vertical). The horizontal furring strips should probably be 2x4s.

  4. ranson | | #4

    I'm also dubious about the ability of Slicker to keep its thickness under pressure from the cellulose.

    What's the reason for not depending on siding to resist bulging? Aren't most of Robert Riversong's houses dense packed behind shiplap without any sheathing at all? Bulging sheathing is not a complaint I've heard voiced about his designs. I would expect coupled fiberboard and wood siding to be significantly more resistant to bulging than siding alone.


  5. Expert Member

    The ability to resist bulging is very dependant on what siding you are contemplating. Perhaps a good test would be to imagine replacing the standard 1/2" plywood sheathing used as a roof underlay with that other material. While you would be quite comfortable walking the sheathed roof deck, would you do the same with corrugated steel, cedar t&g? (i would), Hardi panels or boards, cedar channel siding (no thanks).

    To address the larger issue: Installing any sheathing is a time consuming and costly part of framing. I don't think the advantage gained by moving to exterior gypsum or fibreboard in terms of its moisture tolerance offsets the losses in the qualities traditional sheathing supplies - that is shear and a fastening surface for the WRB, trim, furring and penetrations.

    It seems to be pretty well accepted that double walls that include a rain screen can safely be sheathed with plywood. If that still worries you I'd consider removing the exterior sheathing altogether and using a combination of more robust furring and sheet WRB.

  6. dankolbert | | #6

    Perhaps a row or two of horizontal blocking in the exterior wall would provide enough fastening to prevent the sheathing from bowing.

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