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.
Yes you can sheet rock with longer screws. However the screws will basically have no hold until they hit the wood. Your previous insulation will continue to shrivel. Your ceiling will sag (glue and screw your drywall to ceilings). And you will be like - "why didn't I prep this for a couple hours so I don't have to redo it." Nothing worse than completing a job and knowing that something you did could (probably will) go wrong.
Also I doubt your insulation is even/flat and I think this would make hanging it flat very hard.
I am all about saving time but if you are in the house for more than a year or two I think you are wasting time.
Also why are you insulating the roof of a detached garage?
Best Answer
Yes, just be sure not to squish or compact the batt or loose insulation. Part of it's insulating value is it's air space, or maybe 'fluff' would be a better word. If you compact it, you'll lose some of it's insulation value.
That is just generally speaking though. No need to be anal about it, just generally try to keep it loose.