Open-cell spray foam under a flat roof
I am finally at the insulation stage on my drawn out renovation of a 1905 all brick 2 family. The roof already has 4 inches of polyiso above the roof deck with a TPO membrane. My foam contractor is recommending the following:
1) 2 inches (all that will fit) of closed cell foam to the walls and parapets.
2) 7-8 inches of open cell foam below the roof deck between the roof rafters. He is concerned that if there is ever a roof leak, the roof will rot as there will be nowhere for water to go. Makes sense to me.
Is this a good assembly? My calculation is that 4 inches of foam above will keep my sheathing from being at condensing temp.
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It would be helpful to know your geographical location or climate zone.
Four inches of polyiso has an R-value of about R-25. Eight inches of open-cell spray foam has an R-value of about R-30, so the R-value of your entire roof assembly would be about R-55, with about 45% of the total R-value in the form of vapor-impermeable exterior rigid foam.
Conservative builders would say that your suggested approach will work in Climate Zone 5 or anywhere warmer, but might be a little risky in Zones 6, 7, 0r 8. (For more information on this issue, see "Combining Exterior Rigid Foam With Fluffy Insulation.")
But since open-cell spray foam isn't air-permeable -- it's just vapor-permeable -- the risk is lower than it would be if you were planning to install fiberglass batts on the interior side of the roof sheathing. So even in a cold climate, you'll probably be OK.
Thanks Martin. I am in zone 4a (st Louis). Sounds like I have a plan. I love the idea of a foam bubble on this old house to seal it up.
Already have a Panasonic whispercomfort erv installed to provide engineered ventilation and I’m using the new Fujitsu mid-static ducted system for hvac. I’ll report back on the performance as soon as I get some data.