Foam board under roof sheathing (but over rafters) for unvented cathedral?
I am in Zone 3 humid/coastal NC and want to build an unvented attic and cathedral ceiling without spray foam. Why do you need roof sheathing, then R-5 foam, then more sheathing? Can I not forgo the first layer of osb and place foam directly over the rafters, creating my air impermeable layer then attaching my roof sheathing with longer screws into the rafters? I have found panel systems with osb and foam attached that interlock, but they still require an initial layer of osb. You would have to do 1 foam board and 1 osb together at a time since you could not walk on the foam, but seems reasonable to achieve.
Another option is to use a thin layer of spray foam to create your air barrier, but wouldn’t foam board over the rafters, that are taped or spray foamed do the same thing? Just seems like a waste of money and labor for the two layers of osb. What am I missing?
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BDubbs,
You may find this related discussion useful: https://www.greenbuildingadvisor.com/question/zip-system-r-sheathing-on-a-roof
Sort of, but not really. He is still in essence using 2 layers of sheathing and venting it to avoid thermal bridging to radiant heating of the roof. I'm trying to build an unvented roof with foam board between the rafters and sheathing, instead of spray foam applied to the underside or having 2 layers of osb (osb, foam,osb).
BDubbs,
I linked it mainly for the discussion around Zip-R on roofs.
What you are suggesting is basically a site-made version. Zip-R provides some shear resistance on walls when using their fastener schedule, and I suspect they may eventually may get the same approved for roofs, but as huey_ce points out, currently neither it or your site build version could guarantee it would provide a structural diaphragm necessary to resist the forces it might encounter - especially in an area prone to hurricanes.
Disclaimer: I am not familiar with NC codes.
First issue is if you are building a prescriptive house to IRC. The roof sheathing section of the IRC is divided into two sections; lumber sheathing and wood structural panel sheathing. Foam sheathing is not part of the prescriptive code and would require an engineered design.
I'm not saying its impossible, but it will probably be more cost effective to just put down some OSB sheathing.
Second issue is that putting the foam down first means that your sheathing fasteners are no longer just in shear, the layer of foam results in some bending within the fasteners, which will freely bend through the foam. This needs to be accounted for with reduced capacities and stiffness. Are you relying on diaphragm action from the roof sheathing? Maybe you end up needing to add a lot of discrete bracing which eats up your savings on the OSB.
I went through some of the comments on the thread Malcolm linked and googled nailbase per one of the comments there. I found this:
https://www.insulfoam.com/insullam-nailbase-insulation/
"InsulLam is not a structural panel, and is suitable only for installation over fully supported structural decks."
Still need a structural deck.
TLDR;
You refer to the outer layer of OSB in your assembly as sheathing, but that is a bit of a misnomer. The outer layer of OSB in your assembly is just a nail base for your roof covering and isn't performing and of the structural functions of sheathing. That is why you need the inner layer of OSB. If you were to depend on a nailbase to act as sheathing you would need to consider the things I mentioned above. Not impossible, but I would expect to to be less economical than just installing the OSB sheathing.