I have done extensive research on vapor barriers and different types of insulation. Whether you need a vapor barrier or not, in my mind, depends on the type of insulation.
Closed cell foam is probably your best option, in which case you wouldn't need a vapor barrier, but it's expensive. Dense packed cellulose (Nuwool, Applegate,or Zerodraft) is the next best option. Applegate insulation has a lot of helpful things on their website ("How to properly insulate a cathedral ceiling"). I recommend looking there. Lastly, if you have fiberglass insulation.......... you're doomed. Air molecules naturally travel from a warm to cool space. You can't stop it, and when that happens in a wall cavity, condensation occurs. If you can slow down or stop the air movement you drastically improve your insulating value and chance of condensation occuring (foam or cellulose). However, if you had a tight building like this you'd need an air to air heat exchanger, otherwise you'd be trapping moisture and stale air inside. My point is that air can easily pass through fiberglass insulation, and no matter how good of a job you do with plastic inside or tyvek outside, there will be pressure differentials in the house and air will pass through, normally around outlets, light fixtures or other places like these.
I'm building a new home now and I'm using dense packed cellulose in the walls and rafters, no vapor barriers, and an air to air heat exchanger. Hope this helps.
