Closed cell on skip sheathing?
Please excuse the blurry photo. What it shows is 1×6 skip sheathing over 2×4 rafters. There is asphalt felt over the skip, and a snap-lock metal roof over that.
The owner (local government entity) wants to convert this attic into conditioned space, and is requesting quotes for a project that includes spraying closed cell foam under this roof deck.
I’ve only had ccspf sprayed under solid plywood sheathing. Seems like in this case, if they do it, the foam will be partly adhered to the felt (to the extend that it adheres) and the rest to solid wood that will shrink and swell daily as the sun hits the roof and then later goes down.
What say you?
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David, I think your concerns are bang on. In addition to the problems you've described, neither of the two metal roofing suppliers I use allow asphalt paper as an underlayment because it heats and adheres to the roofing, then tears. So you will have discontinuous foam as an underlayment with pieces of building paper attached to the surface. Not a good idea.
Might be a good idea to use rigid foam to create your conditioned attic instead. Make sure to tape the seams. If they're insistent on ccspf no matter what, you could provide a continuous spraying surface for the foam by adhering plywood or sheets of foam to the boards, and then apply spray foam over that.
Skip sheathing does not present more stress to the foam bond due to different coefficients of expansion any more than continuous OSB/plywood does. If anything, it allows slightly more mechanical compliance/flex to occur. The fact that the ccSPF is also bonded to the 3/4" deep sides of the sheathing gives it a great deal of flex. Starting at ~3-4" of depth the foam itself begins to become as structural as the sheathing. This would only be a potential problem if the foam was thinner than 2" and even then the potential that the foam would crack or break is low IMHO.
The strength of the bonding to the felt is of no consequence, since the felt isn't structural. But if there is no slip-surface material between the felt & metal roofing, you still have the compatibility issue Malcolm Taylor describes, where the felt gets degraded by bonding to the metal. The foam won't aggravate that problem, but it's probably not going to mitigate it either.
Malcolm you taught me something. Still, thousands of Metal roofs are laid over felt here. I have done one years ago and all is fine I think. No calls, the roof is still there unchanged as I drive by it. So far I have witnessed only one shingle roof redo over the decades of mine, the Roofing material twenty years ago was horrendous in my area. Fiberglass shingles cracked five years old, asphalt shingles just plane disappeared, after ten to fifteen. The felt saved the fiberglass, no help for asphalt. Back then a square cost $30, today all up we call it $150 for the complete "system". Lifetime warranty now.... Latinos do all the roof now.
The specs for metal roofing systems almost always call for a slip-surface type underlayment, since temperature coefficients of expansion of metals are so different from wood. i don't know how long it takes for the felt to fail, or how long it would take for the failure to be detected, but it's probably worth doing it to spec using the manufacturer's specified underlayment. I don't get to watch enough metal roofing installation to say just how often they do something else in my area.
Local roofers install Kynar-painted metal roofing over 30# felt here, no bond breaker of any kind, and it doesn't stick to the felt. Metal roofing is always slightly oil-canned and I bet it's not quite in contact with the felt for the most part. I have unsnapped and peeled back metal roofing many times to do repairs, build additions, add skylights, etc., and none of it has ever been glued to the felt.
David, I'd bet it's completely a matter of climate. You and I working in the PNW don't get our roof surfaces hot enough to degrade the asphalt in the paper. My roofing supplier, serving the whole of Western Canada, doesn't change their specs for us lucky few.