You just insulate
around the ventilation openings, same as you go around rather than over your windows -- no big deal
Insulation is not absolutely necessary in NW PA (unless you have a metal roof, then you pretty much do need to insulate at least *that*) -- however it will make your life and your chickens' life more pleasant, so if you can, I would suggest it.
Insulation allows the coop to hold more of the heat it acquires from daytime temperatures, sun thru windows, andthe chickens' own body heat. Thus you can have more ventilation open without things getting overly cold.
The ceiling is
at least as worthwhile to insulate as the walls - moreso if it is thin and will get condensate forming.
I would not personally use a vapor barrier, though, unless I was just using it to block wind from coming thru gaps in old siding or something. IME livestock outbuildings are really just
different from houses in a few important ways, and this to me is one of them. I'd rather have the building structure be able to absorb some humidity when there is a temporary peak. It will, I promise, dry back out soon thereafter when the air is drier. And meanwhile it will have reduced your humidity peak in the coop.
If you have PLENTY of ventilation, there is probably nothing much *wrong* with using a vapor barrier, but it is really not at all *necessary* IMO.
JM$0.02worth,
Pat