I'm redoing the insulation in my attic the floor/ceiling cavity and then the ceiling of the attic. I have 2 rolls of plastic that is 15 ft by 150 ft just sitting around -- I've had it for years. Is it a good idea to put this in between the ceiling and the insulation then the floor on top without a barrier so that the insulation is free to release any moisture through the floor into the attic? Or is the plastic not a good idea in this situation?
Air sealing is more important than vapor sealing. All assemblies (wall and ceiling) need to be able to dry out. If you prevent air passage AND provide sufficient insulation to prevent condensation, you don't need or want an impervious layer.
Cables,plumbing vents, ducts and lighting fixtures are notorious sources for air leaks. These can be stopped with many materials, including caulk, expanding foam and fire stop (special foam or mineral wool), as appropriate.