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Polymer nails, water infiltration, and thermal bridging

burninate | Posted in Green Products and Materials on

Has anyone looked at these for their wall assemblies?

They make it sound like a miracle product (particularly in current discussion areas like sealing the leakage from nailing through WRBs), and they don’t even talk about the drastically lower thermal bridging you would expect out of a polymer compared to a nail.  With ASHRAE-calculated R-Value, small areas of highly thermally conductive material have dramatic impact on assembly R value, and steel is >400x as thermally conductive as wood.

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  1. Jon_R | | #1

    Note that very few people do the math right on this issue - the whole assembly impact isn't dramatic unless you are talking steel panel to steel bolts to steel panel.

  2. Expert Member
    BILL WICHERS | | #2

    I would NOT NOT NOT trust those nails for sheathing that is acting as a shear panel! 20,000 PSI tensile strength is a LOT less than the typical 80,000+ PSI for steels (and steel can go quite a bit higher). 150*F isn't much either, and I suspect they weaken as they approach that temperature. A typical fire sprinkler head will pop off at 155*F, for example, so in a commercial building those composite fasteners will fail before the fire sprinklers activate. I suspect these composite fasteners will fail in brittle fracture (snap toothpick style) in shear too, a nail will generally bend some first. This means a nail can deform a bit, which absorbs some energy, before it fails. Brittle fracture fails like a fuse -- you reach a certain point of stress and then SNAP! your fastener breaks. You don't want things like that in a structural application.

    The loss in R value due to thermal bridging of the nails is something of a myth. This is not to say it doesn't happen, it does, but the actual energy loss is minuscule. There is a Q and A on here where I actually ran the numbers for someone and posted the energy loss through a typical 4x8 sheet of plywood for just the steel nails. It was in the single digit BTU per hour range. You'd lose more through air leakage of a missed bead of caulk.

    The composite staples may be interesting for putting up something like housewrap though -- they'd never rust, so no risk of rust streaks to get exposed. I can think of lots of useful applications for these kinds of fasteners, I just wouldn't use them for framing, and I wouldn't use them for sheathing (due to the shear issue).

    I wouldn't worry about thermal bridging of fasteners regardless. There is sooo little energy lost through nails going through rigid foam that it isn't worth worrying about. There are other places where you can gain a lot more for less effort.


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