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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