We have a Florida room attached to our home that we will enclose fully to make it part of our living space. The Florida room has a flat roof that is a 4 over 12 pitch. Currently there are no soffit vents in the soffits. We have 2×8 rafters on 16 in centers. How do I insulate this area with best R value? I read on here probably keeping it unventilated is best. I found some R tech boards and other brands and I guess I could stack 3 layers of 2in RTech board would give me a R value of 23.1 and another inch of John Owens foam board that has a R value of 5 would net a total of 28.1. However this is quiet costly for a 20×12 room. Any other ideas? I’d like to insulate ceiling and walls as best as possible as there will be no AC vent in the ceiling space of this room?

For this ceiling space – I do not want to use spray foam insulation, should I do rigid foam boards or mineral wool such as Rolux, a combination of both or maybe include a radiant barrier? Thank you