Why not install roof exterior insulation screws in decking?
DamionL
| Posted in General Questions on
I’ve found some interesting discussions regarding locating rafters when installing exterior insulation, but nailbase panels are just lightly glued on to osb or plywood and they all recommend installation into the decking. I understand that there is a thermal bridging issue with thousands of screws penetrating the structural sheathing, but I’ve never seen anyone employ this strategy. They always do what they can to screw into the rafters. Would love to hear some opinions on why not just go into the structural sheathing, assuming the wind and snow loads are met.
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Replies
DamionL,
I'll be following this discussion with interest. I understand the logic on walls where there are large shear forces, and maybe with steeper pitches it makes sense to need more backing, but I'd like to see some engineering around how much pull out strength is necessary for most applications.
Except possibly in very high-wind areas, in theory screwing into the sheathing alone should be adequate for both shear and uplift forces. At least when new. Over time, if moisture accumulates at the sheathing, it can weaken, reducing the pullout strength. Having enough R-value in the nailbase compared to the interior to prevent condensation should eliminate this potential issue.
There is another condensation issue, though. Because the screws are thermal bridges, and penetrate the sheathing, they will be cold enough to condense water on the interior, weaking the sheathing over time. Depending on your location and situation it may or may not be an issue, and it's not a problem that would show up for years or decades, but over the long term it could be enough for the nailbase attachment to fail.
Here’s a link to the DrJ report on number of fasteners to satisfy wind and snow loading in plywood and osb decking.
https://www.drjcertification.org/report/download/1872
If you’re bored you can always go around caulking the penetrations from the inside to at least alleviate the potential condensation issue.
I checked with Hunter and they claimed that this is just how they test their panels and although there is a thermal hit, it should be minor. Sure is much easier to find installers though when the installation requirement is as easy as just driving screws per diagram regardless of rafters location.