Spray foam for Outsulation on roof + compression strength
I have a flat roof, where I am thinking about what materials to insulate it with. After looking at different foams, and reading Martins posts on the matter, I have a few questions regarding my specific scenario.
For instance, does anyone have any experience with spray foaming a flat roof? Can the foam come out relatively flat? I plan to put a vegetative live roof and patio decking on top of the roof membrane, and wondering if the compression strength of the foam will be ok with the weight. Are there any other complications associated with doing the spray foam that I’m not thinking about.
Below is my logic:
My roof assembly is as follows:
-3/4″ Plywood (taped seams)
-9.5″ I joists with X” of rockwool batting insulation
So being in Zone 6, The exterior R value needs to be at least 51% to prevent rot issues. The max depth of outsulation I have to play with is 6″, going higher will cause further work and complications. Thus the Outsulation material will determine the thickness of the Insulation and the total R value of the roof.
I could go with 6″ of 20psi EPS, which is the cheaper option, but I would have to cut back on the interior batting to meet the recommended ratios and it would lessen the total R value of the roof.
So, In the interest of getting as much R performance at the roof, I was thinking 6″ of closed cell Sprayfoam at R-6 per inch (R36), and thus I could have 8.5″ of batting at R-4 per inch (R34) creating a total of R70.
What do you think of all this?