We are going to be rehabbing a home in CT in which the third floor is part of the living space. We want to get the insulation level up by using either nailbase or 2 layers of 2" tuff-r with staggered seams on top of the deck. We are going to use close cell spray foam with polyisocyanurate insulation to increase insulation and stop bridging on the inside. Has anyone used nailbase over spaced wood slats that use to have a cedar shake roof over it? We believe that the closed cell spray foam underneath would act as a vapor barrier, the slats would provide more that enough support and the hot roof is o.k. but real-world experience would be much appreciated.
I would consider using cellulose insulation blown in between the rafters, rather than spray foam against the skip sheathing. Foam on both sides is risky, in my opinion, and will make any future repairs to the roof structure a real challenge. Since you are addressing thermal bridging above the sheathing I don't see the real advantage to more foam below. Proper air-sealing of the drywall lid will probably be adequate, and I'm assuming you are making plans to correctly control interior moisture load anyway.
An alternative would be to install all of the insulation below the roof deck. Install baffles made of rigid foam board between the rafters, with a 1"+ vent space, then install cellulose or possibly foam between the rafters, and follow up with foam board across the bottom. That way you preserve the vented roof. Obviously this approach uses up some of your headroom.
Building assesmblies need to be able to dry in at least 1 direction. Rigid insualtion on top will limit the drying to the outside so you should allow it to dry inward. I would agree with David and use cellulose. I would not use poly on the inside. Vapor diffuison through the drywall is not a big worry. What is a worry is air leaks carrying moisture in to the ceiling. You should consider and air tight drywall system along with a vapor retarding paint. You will also want to use more than 1 layer of rigid insulation with staggered and taped seams.
Do not use batt insualtion. Only use a dense pack material as that performs better than batts and cuts down on air movement close to the point of being an air barrier. I said close but it is not an air barrier.