Insulation, Radiant and Vapor Barriers - old house attic insulation

I want to insulate the ceiling of the attic of my old row house in Philadelphia.
The attic is not vented.
There is a floor in the attic with insulation under the floor.

How do I insulate the attic ceiling between the rafters with fiberglass batts?
Is a poly film necessary?
What about foam vents?

My primary concern is not trapping moisture between the insulation and the ceiling/roof.

My eternal gratidude to anyone who can answer. I can't find the answer anywhere on the net.

In northern climates, your goal should be to insulate the living space of the house. Leave roof uninsulated so it can remain the same temperature as the outside. This involves having a vapor retarder over attic floor to prevent flow of warm, humid air into attic where it can condense and cause moisture problems. You should have a free flow of outside air from the eaves of your roof through soffit vents all the way to the gable vents, louver vents, or other type of roof vent. Use baffles to prevent the obstruction of airflow where insulation butts up against rafters or roof sheathing.

Northern climates, an R-value of 49 is recommended for adequate attic insulation. Fiberglass insulation has an R-value of roughly 3.0 per inch of thickness. Cellulose is has an R-value of almost 4.0 per inch. Attic floor will have to be removed to add additional insulation.