How to insulate garage ceiling under finished bonus room
I am finishing a garage and a bonus room above the garage in upstate NY. The 2 by 4 garage walls will be insulated with fiberglass bats, but due to the two 10 foot uninsulated steel doors, will not likely be warmer than outside temps in the winter. I do not plan on heating the garage. The previous owner insulated one side of the garage ceiling with unfaced 10 inch fiberglass bats and covered it with untaped sheetrock. There is no vapor barrier.
I received a bid to apply 3 inches of closed-cell foam in the garage ceiling joists. The R-value is advertised as 7 per inch. The salesman said the foam would provide a much better air and vapor barrier and would provide warmer floors in the bonus room than I could get with fiberglass bats.
My contractor suggests keeping the existing insulation and adding 10 inch, R-29, fiberglass bats to the other side of the ceiling. The ceiling would be sheetrocked, taped and painted. This option would be approximately $2,500 less expensive than the 3 inches of foam.
Should I choose the foam or the fiberglass insulation? Is 3 inches of foam enough? If I decide to go with the fiberglass, how do I get an effective vapor barrier on the warm side of the floor? Should I rip out the unfaced insulation and replace it with faced insulation?