Roof insulation
I am trying to determine the the best way to insulate the roof.
This is in Canada. I have a flat roof with with a 2 ply membrane over rigid insulation slopes and sheathing.
My first thought is 5″ of closed cell polyurethane which gets me to R 31. This would leave nowhere for any water under the top membrane to go though.
I could also to a 2″ of closed cell and another 6″ (or more, if people recommend) of fiberglass. This would allow any water that gets under the membrane to evaporate downwards through the roof over time. I worry that it would also allow moisture to move the other way, up through the roof though, as per what I would expect from the default vapor drive.
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if this were commercial, then the best assembly would be sheathing; peel and stick vapor barrier; flat insulation; tapered insulation; coverboard; 2 ply membrane [likely SBS for a colder climate.]
the vapor barrier against the sheathing should be fully detailed as an air barrier so that it prevents infiltration from the conditioned space into the roof assembly.
generally you don't plan for leaking so there should be no water infiltrating the system from above.
not a commercial roof. Not at all the type of roof we are working with. I believe i saw on another post people suggesting a 2" polyurethane under a residential flat roof in order allow water vapor to escape. My question has to do with the viability of that proposal.
This insulation is not about moisture escaping but about moisture from the inside not making it to the roof deck. The right ratio provides condensation control which means the moisture from the house can't reach a surface cold enough to cause issues.
How much rigid do you currently have above the roof deck?
You generally need about half (or a bit more in colder climate) of your total R value as rigid + spray foam for condensation control. The rest can be fluffy insulation bellow.
The space under the roof membrane doesn't need to dry, if there is moisture there, your roof has failed.
ARKHOUSE,
And unfortunately if your roof fails, a drying path won't be of much use.