Hello GBA -

I live in a very wet climate in Alaska (Kodiak Island) where our mean winter temperature is around 32 or so degrees. It is almost always wet. I just purchased a new place and am going to take advantage of a state provided program to help out w/ subsidies to weatherize places.

I have a single slope cathedral ceiling with a brand new rubber type roof. My plan is to install polyiso beneath the ceiling to help insulate it. Here is a picture of my plan.

http://i.imgur.com/9wFvmaT.jpg
The big x's are the rafters and the polyiso would go below the rafters.

In order from TOP to Bottom
* Pink Board ( polystyrene) -
* Polyiso (4.5" R - 33) -
* Vapor Barrier -
* sheetrock -

Here is a picture of my roof:
http://i.imgur.com/xRAH6bi.jpg

I have a few questions.

1. What affect will the polystyrene (pink insulation) have? Is it worth the investment to put 4" up between the rafters (above the Polyiso?). There would be an air gap above the pink board allowing the roof to vent (I think I have 2 x 8 " up there..so roughly 4" of airspace)

2. Do I need a vapor retarder/barrier between the polyiso and the sheetrock? From the literature I am reading I do not (so long as I tape up the seems of the panels well.

3. If I do not use a vapor retarder/barrier how do I airseal where the polyiso/sheetrock meets the walls?

Please critique my design!
Thanks!