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Discussion Forum

Sheathing and Rigid Foam Board Use

gjbles | Posted in General Discussion on November 16, 2015 01:38am

I have read a number of articles related to sheathing and the use of rigid foam panels for insulation and was wondering; is it possible to place a 1″ to 2″ rigid foam panel against the wall studs and then use OSB on top of that as the exterior of the building?  

The foam could be temporarily attached with glue and the OSB can be fastened to the studs with nails or screws thus securing everything. Correct me if I am wrong but the OSB would provide the sheer strength while the foam panel on the inside would allow for spray foam between the studs against the foam panels without having to worry about moisture issues or an additional vapor barrier. This also allows for easy attachment of exterior siding.

Everything I have seen places the OSB (or plywood, etc) against the studs if it is used at all then rigid foam goes on top of that.  Below are some articles that detail how moisture can be a problem when covering the sheathing material with rigid foam.

Any thoughts or experiences would be appreciated.

 

http://buildingscience.com/documents/insights/bsi-038-mind-the-gap-eh 

http://thehtrc.com/2010/building-green-homes-walls-air-sealing 

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  1. DanH | Nov 16, 2015 05:39pm | #1

    Conceptually, you have a problem in that the compressible foam combined with the flexible nails/screws doesn't firmly tie the OSB to the framing.  It's unclear, however, how thick the foam would have to be for this to begin to be a problem.  But unless some "agency" has studied this, either mathematically or with test structures, many inspectors would be reluctant to accept it.

    (There probably are ways to overcome any actual problems by, eg, using "splice" blocks (maybe a strip of OSB let into the foam) under the OSB joints, and using some sort of scheme to assure that the OSB is firmly tied to the framing in at least a few places.)

  2. renosteinke | Nov 17, 2015 08:29am | #2

    You'll be fine!

    It is common to place foamboard on the outside, over the sheathing, then siding atop that. Web sites for siding manufacturers often have specifications for doing so.

    It is common that it be specified to attach the siding to the studs, not just to the sheathing, through the foamboard. It's also usually assumed that there will be a gap, or 'rain screen,' between the foam and the siding.

    1. DanH | Nov 18, 2015 02:42am | #4

      But Opie want's to put the OSB on the outside, over the foam.  If the OSB is required for structural reasons, this would appear to create a problem.

      1. User avater
        deadnuts | Nov 18, 2015 06:58am | #5

        R-Zip OSB is on the outside

        Its called R-zip. Its essentially a single sided OSB SIP panel. Different thicknesses available. If it's required for structural reasons (wall brace panel), he can check with his engineer or, perhaps, manufacturer on fastening requirementsl. My guess is that it all comes down to the type (size& length) of fastener which will be transferring shear to the framing.

        http://www.huberwood.com/zipsystem/products/zip-system-rsheathing

      2. User avater
        deadnuts | Nov 18, 2015 07:09am | #6

        IRC allowance

        According to Huber, there are presciptive code allowances for R-zip installation in the IRC. Of course the acceptability of this allowance is dicatated by your local AHJ. When in doubt, you always have the option of presenting an engineered solution prescribed by a professional engineer.

        http://www.huberwood.com/assets/pdfs/ZIP-R-Sheathing-Installation-Guide.pdf

      3. renosteinke | Nov 18, 2015 07:54am | #7

        Oops- THAT Would Be Unusual

        I somehow missed that detail .... Yes, I agree that applying the foamboard directly to the studs would eliminate any shear strength the OSB might give the wall.

        What if ....

        Even if the wall has let-in bracing, I'd advise against placing the foam first. Let-in bracing is, by far, the weakest way to 'stiffen' a wall. This is evidenced by the codes recognizing such bracing only in the least demanding areas. There are specific nailing schedules for shear walls, and glue is often specified.

        The proper technique is stud - sheathing - foam board - spacers/lath over studs - siding. This is what siding makers expect, and recommend.

        Naturally, I have seen very few siding installers even try to sink the nails into the studs. Well, that doesn't make it right- no matter what sort of sheatthing is used. Siding fasteners should pass right through the lath, the foam, the sheathing, and into the studs.

        With the foamboard hung and joints taped, there's no benefit to having any house wrap. Such a wrap might even create a moisture problem.

        One thing I really like about exterior use of foam board is that this gives you an absolute thermal break; there's no point where the wall is 'solid wood' all the way through. My recent IR house scans had no problem spotting the places in the walls where there was only wood - places like the headers over windows, and clusters of studs where interior walls met outside walls.

        1. User avater
          deadnuts | Nov 20, 2015 08:11pm | #8

          proper?

          renosteinke wrote:

          The proper technique is stud - sheathing - foam board - spacers/lath over studs - siding. This is what siding makers expect, and recommend.

          There's nothing "proper" about this technique. It's simply one way to detail an exterior wall. IMO it's not always the best; particulary when the siding is, say, vinyl. That's a material that couldn't care less about moisture (from either side).

          In the case of zipR sheathing, you're getting insulation and a nailing surface in the same board. That results in good economy of material and labor. Since the insulation board is fully bonded to the sheathing, shear strength of OSB panel is transferred through the insulation board along with fasetners. Your'e also getting a full thermal break* just as installing foam board on the outside. In fact, installing insulation board over OSB in the field can easily result in bridging over uneven wall framing. Every bridge spot is a possibility for shortcircuiting the thermal break (particularly if not well air sealed from the outside) and rendering whole sections of insulation board compromised.

          BTW, since when did siding manufacturers drive the market toward good architectural detailing? Answer: Never.

          *minimum total wall R-values must be observed in order to avoid dew point issues. Same rule applies to insulation board being installed to exterior side of OSB.

  3. User avater
    deadnuts | Nov 17, 2015 11:21pm | #3

    Huber Industries

    already makes this for you. It's called ZIP panel and has a weater resistant barrier applied to the exterior face of OSB  to boot. All you have to do is buy it, install it, and tape the seams.

  4. LowCarbonKyle | Jan 12, 2023 03:47pm | #9

    I know this an older post, but I'm reviving it with a twist: Ideally there would be a ZIP-R type panel that was made with natural materials; however, this doesn't exist (to my knowledge). So the question persists, anyone have experience installing Sheathing over top of wood fiber? Right now (for cases which require nailing that doesnt lend itself to strapping such as cedar shakes), we are using sheathing, wood fiber, then another layer of CDX and it just seems silly. Its kind of like the strapping on a roof for over-venting, works great, but man its a lot of material and there has to be a better way. We are also dealing with contractors who are either used to ZIP-R or used to no exterior insulation at all and want that exterior side sheathing to nail to with cedar breather over top. All thoughts are welcome, but currently kicking around the idea of of wood fiber directly to studs and a layer of CDX on the outboard side of that with WRB. We would obviously have an engineer look at the shear strength before going that route, just wondering what experience or thoughts others have.

    1. Matt_Millham | Jan 12, 2023 05:33pm | #10

      I’d think you could sub in wood fiber for foam using the EP&B concept FHB and GBA covered a few years ago. https://www.finehomebuilding.com/project-guides/framing/better-wall-production-builders

  5. greg_robb | Nov 05, 2023 02:37am | #11

    I heard from my architect, she had a contractor client who made some shake table test walls. The wall that performed best had the rigid foam on the studs and the shear ply outside the foam. That was a test in Oregon shake table.

    The nails need to bend to work best as seen in the tests that used nails and glue where the wood itself tore apart at the glue line.

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