Suggestions needed on how to best insulate attic
I recently bought a home in NY that was built in the 1920's. When I purchased it, the home inspector said that the attic insulation was done incorrectly, and needed to be addressed. The previous owners installed two layers of R-19 inside the joists with the vapor barrier up, as well, they did not remove the vapor barrier from the top layer.
As I have been looking into the attic to determine my course of action, I am a bit confused. I am thinking the attic space is unvented. There is no AC ducting in the attic (yet). My questions are: Should I be insulating the joists or the rafters? Do I need to worry about air flow? Is there air flow without vents? Considering the ceiling is plaster and lathe, how much will new and proper insulation help in retaining heat/cool air?
Thank you in advance, and I appreciate any light you all can shed on my questions!
Happy New Year!