Polyurethane foam is an excellent insulator, and it needs no added vapor barrier. Depending on your climate and local building authority requirements, you will need anywhere from 4 to 7 inches of foam. The remaining airspace does not need to be filled. It will help prevent your new ceiling from getting wet from inevitable condensation. Alternately, you could install baffles against which to spray the foam, creating an air gap below the roof sheathing and creating a sort of cold roof, if that has any benefit in your climate.
The foam is such a good vapor barrier, you will need a plan to remove excess moisture from your house. Frequent use of bath and kitchen fans may be adequate, depending on climate and remaining construction. You might consider whole house ventilation through an air to air heat exchanger or similar.
If you are removing the ceiling, you can pretty much use any type of insulation that makes economic sense in your area, but in very cold climates, 9.5 in of fiberglass is not really adequate. In warmer climates, you could avoid stripping the ceiling off by blowing in fiberglass through holes in the ceiling, then patching or covering the holes. You can also apply foam through holes, but you may have to fill the whole cavity, which would be expensive overkill.
I don't think it's possible to accurately place foam through holes such that the cavity is not totally filled, but it might be worth questioning an experienced applicator. If you blow fiberglass, then you will need to add a vapor barrier somehow. Some heavy solids paints meet the vapor barrier spec of 1 perm, so could be considered to be a leaky but adequate barrier. Or you could add a thin layer of foil backed sheetrock. Or just sandwich polyethylene sheeting between layers.
Without knowing your climate or local prices, I couldn't say what the best option might be, but I hope I've given you enough ideas so you can decide on your own. Cheers.
Spray foam insulation is a petrochemical product that is mixed on-site and blown with a blowing agent. The actual process is not great for anyone's health, which is why applicators wear protective suits. It is also exothermic, and if done incorrectly can cause a fire (rare). Additionally, if mixed incorrectly, or even in a small percentage of cases, the foam will offgas nasty stuff for a long time, rendering the house uninhabitable. I must stress that these cases are rare, but they do happen. Furthermore, fully-cured spray foam is flammable at elevated temperatures and will char and emit smoke at typical household fire temperatures.
Are there better insulations out there than spray foam? Absolutely. Spray foam is about the most expensive insulation you can get, and it is terribly mis-used most of the time, applied where its benefits are wasted. For insulating walls between stud cavities, wet-sprayed dense-packed cellulose, sprayed fiberglass (under the brand name "Spider") or mineral wool batts are better choices. For turning an attic into conditioned space, installing rigid foam boards above the roof sheathing is the preferred approach if already re-roofing. If that's not an option, then you can fill the spaces with the aforementioned choices of cellulose, fiberglass, or mineral wool batts, as long as you 1) create a ventilation channel of 2-4 inches in the rafter bays below the roof deck, 2) install a ridge vent and soffit vents that allow every rafter bay to be ventilated, 3) use an interior-side smart vapor retarder membrane such as Intello MemBrain (note: NOT a vapor barrier like polyethylene sheeting), and 4) you finish the assembly off with drywall, being careful to make the drywall layer as airtight as possible. Alternatively, you can detail the vapor retarder membrane as an air barrier and skip the drywall or use non-airtight finish materials like tongue-and-groove pine boards or something.
Best Answer
Spray foam is fine here. Hire it out, don't try yourself. You won't save much if any money and this stuff is better installed by pros. Make sure the roof decking is bone-dry on the day they install it. Determine what kind of R-value you want, too. The stuff is so expensive that installers will often try to convince you that 2 to 4 inches is sufficient for a variety of phony-baloney reasons; ignore that. If you want an R-50 roof (which is a good idea, and only slightly higher than code-minimum for your area in the latest version of the energy code), that'll require 7" of foam, which can very expensive. Typical price is $2+ per inch thick per square foot. for an 800 square foot attic, 7" of foam would cost more than $11,000!
Spray foam may be unnecessary. If you don't install the AC unit and its ductwork in the attic, then you can skip the spray foam and pile cellulose or fiberglass on the attic floor to get better performance for 1/3 to 1/10 the price. And it's easily DIY-able.