The type of spray foam we are using in the joist ends and cantilever areas is called a 2 pound foam. This is a closed cell type of foam that is impermeable, so can insulate a space without a poly barrier.
The foam does such a good job of tightly sealing hard to reach areas and the underside of plywood where you want to avoid a cold floor. There does not seem to be much other choice than to use a spray foam if you are trying to achieve a comfortable interior. In the example below we are even sealing in the warm air duct in the bedroom above the front entry.
