Good Morning everyone and thanks for adding me. I am insulating a flat roof with a membrane roofing material like a commercial building. I have a glue-lamb that runs the entire length of the roof and roof rafters are 2 x 8 on 16" centers. It would be almost impossible to vent each rafter bay as the glue lamb isolates each bay. This home is on the Oregon coast so it is prone to moisture. Would R-Matte Rmax Plus-3, 1/2 in. x 4 ft. x 8 ft. R-3.2 Polyisocyanurate Rigid Foam Insulation Board product be a good alternative to close cell spray foam application? Can you stack it to achieve R-20, I assume it would take 4" to achieve it? How would you hold it in place in the rafter? Any help is greatly appreciated. Thank you for the help