Posted By Dana1 on 20 Nov 2013 02:18 PM
The lack of a vapor barrier on the cantilevered area isn't an issue in climate zones 5 or less, since there is no rain-wetting of the underside of the floor to worry about, and the drying capacity toward the exterior through plywood is high even in winter.
In zone 6 it would be better to use 5/8" exterior grade gypsum sheathing (eg Georgia Pacific DensGlas Gold) rather than plywood for sealing up the bottom. The gypsum is moisture tolerant and highly vapor permeable- it's fine if/when it takes on a bit of moisture, and dries extremely quickly (10x as fast as plywood.) If the joists extending to the cantilever are the same joists that are being insulated in the garage, there isn't any need to add blocking (foam otherwise).
In zone 7 or higher an interior side vapor retarder such as a flash-inch of closed cell on the underside of the sub-floor would be in order, as would the foam-board blocking between the joists.
Thanks for your info, I am in zone 5A. I think I will put an 1'' rigid foam board on top of the plywood just to get a better air seal, and to reduce thermal bridging a bit.