Use insulation in the ceiling below with an integrated vapor barrier.The OSB is already attached to the joists. I had asked about putting vapor barrier under it, but was told that the OSB should be glued directly to the joists.
I'm not sure how I could put it underneath now.
You have an option of using a closed cell spray foam.Thanks!
After reading some of the information about fiberglass, I am wondering if it even makes sense to used batts in this location. I am not sure what other options I would have.
Thanks for the suggestion, but spray foam is too expensive for my application. The best quote I have to spray 4 inches thick is a little over $5000.You have an option of using a closed cell spray foam.
http://www.sprayfoam.com/spps/ahpg.cfm?spgid=6
I believe I need to put 5/8" drywall there in any case to meet the code here, no matter what kind of insulation I use.
For the XPS I meant two layers against the bottom of the joists and then the drywall below it - it just seemed a lot easier than cutting all those strips and foaming. There's probably good reasons not to do it.