Hello – I live in Zone 4 (Long Island), but not the far from Zone 5.  My HVAC equipment is located in the attic (I know, terrible place for it), so I’m going to create a conditioned attic.  My questions are as follows:
1) From what I read, open cell would be acceptable in zone 4, however, would closed cell be preferable?
2) If I use closed cell, how many inches would be necessary for dew point control? Contractor is recommending 4.25 inches of closed cell.
3) To address thermal bridging, can I spray open cell over the closed cell and cover the rafters?  I have 2×8 rafters.  The bottom slope of some of the rafters would be difficult to reach if I wanted to put batts.
4)  My existing attic floor insulation is pitiful. I can see the drywall ceiling in some spots.  Should I remove all of the floor insulation?  The contractor said it wouldn’t be necessary to remove all of it, but from what I’ve read that doesn’t seem to be true.  I have pull down access to attic so that would be one area for air exchange.

Any advice would be greatly appreciated.