Flash-and-batt roof assembly in Climate Zone 4a
Dana was so generous in laying out an insulation solution for my completely uninsulated duplex in Mixed-humid 4a climate here
But two small follow up questions–
would comfort board work as well against a pine plank party wall as polyviso foam? If not, could I buy used polyviso foam, or am I not buying enough quantity? (Washington, DC area)
The latest contractor said that closed cell foam against the roof decking (there is plywood above the original skip sheathing) could cause a problem when it’s time to re-roof, so people usually re-do the roof first. Is that right?
Wouldn’t it still be a problem in 25 years, since closed cell foam is forever?
The previous owner replaced the roof in 1993, but it is in okay shape, no leaks. We had expected we’d replace it sometime in the next five years, though I’m not sure if we’d need to convince the owners of the other half the duplex to do it at the same time (and convince them to add polyviso foam to the construction as we would like to do)
The roof geometry is pretty basic, but there is a dormer window on each side in front, pictures available here
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>"would comfort board work as well against a pine plank party wall as polyviso foam? If not, could I buy used polyviso foam, or am I not buying enough quantity?"
Sure comfort board would be fine, and wouldn't need any sort of ignition barrier to meet code. Reclaimed polyiso or EPS/XPS would also be fine, but needs to be covered with a timed thermal barrier (half-inch wallboard would work..) This reclaimer in the Baltimore area may have the size and quantity you need at a reasonable price:
Most foam reclaimers will sell in small quantities if you're providing the transport, but often have minimums (and a delivery fee based on distance) if it's going to be delivered.
If you run this search every week or so you might something VERY local and even cheaper:
>"The latest contractor said that closed cell foam against the roof decking (there is plywood above the original skip sheathing) could cause a problem when it’s time to re-roof, so people usually re-do the roof first. Is that right?"
The only problem it could cause is if a sheet of plywood was damaged by a roof leak and needed to be replaced it would be "glued" in place by the closed cell foam. With the skip sheathing under the plywood with less than 2" gaps between planks it's not a huge amount of bonding area, and it's unlikely that it would require major surgery to separate out a bad piece of plywood for replacement. I wouldn't sweat this one. YMMV.