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Combining spray foam and interior rigid foam in roof

quantumgirl | Posted in General Questions on

I’m in climate zone 2, hot and humid. My home addition has a flat roof, 2×6 construction with plywood sheathing and EPDM. Unfortunately no exterior insulation and too late to add any since we already build a deck on top of the EPDM.

I’m now ready to insulate the rafter bays. I want to spray 2” closed cell foam under the roof deck, (to keep potential interior moisture from reaching the sheathing), followed by 3.5” open cell foam.
To reduce thermal bridging and increase overall R value, I want to add 4×8 sheets of 1 or 2” thick polyiso rigid foam underneath my rafters, followed by 1×3 strapping and then drywall.

Is it okay to combine polyiso with the spray foam or am I creating some sort of moisture trap doing this?

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