To meet code-min in south eastern NY & L.I. with a "compact roof" system you'd need to bring it up to R30. That would take 5" of iso, or 6" of XPS. With nailbase iso that would be a 3" nailbase panel on top of another 3" course of fiber-faced iso. If you used vented nailbase it adds about an inch to the structure, but it preserves the nailbase by allowing it to dry. From a thermal point of view the venting does very little for the energy use of the building, but it lowers the summertime temp of the shingles, and allows the nailbase to dry quickly after leaks or soaking under a foot of snow for months, etc.

Going with an unvented nailbase and making a DIY version with 1x furring and 7/16" OSB is always possible, and is usually cheaper if you don't have a lot of cut up dormers & valleys, etc.

A vapor barrier would not be necessary with an all-foam all-exterior insulation setup. AIR-sealing it by foaming/taping the seams of the panels is always a good idea though.

Now am evenly split between prefab vented panels and DYI - aside from price, it's the quality of local labor that I can afford that will tell the tale.  I also came to two 3" layers; am considering the advantages in foil facing.

Foil facing won't buy you anything without air gaps at the foil facers. A foil facer is a true vapor barrier as well, so some consideration of where it lies in the stackup relative to susceptible materials and the anticipated vapor drives. A foil facer next to the ventilation gap will buy you a modest amount of cooling load reduction, but not as much a simply using a lighter colored finish roof would. (A CRRC rated cool roof material is even better.) Sealing foil facers with purpose-made FSK tape is sometimes easier than foaming/caulking fiber facers.

Thought I could attach a sketch of the solution - anyway, am going to nonfoil faces insulation and no vapor barrier due to costs, but focusing on air tightness. Thanks

Most of Maine is US climate zone 6, but the very northern tip is zone 7. Code min in zone 6 for "compact roof" is R38. In zone 6 you want to split the R between exterior rigid foam and air-permeable (fiber) under the roof deck you need a minimum of R25 above the roof deck, per section R806.4 of the IRC 2009.

See:

http://www.instruction.greenriver.edu/mcabe/pdfs/supporting-documents/IRC-chapter-8-sect-803-807.pdf (scroll to the last page)

and:

http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling

See also: http://www.energycodes.gov/publications/techassist/residential/Residential_Maine.pdf

http://www.sprayfoamofmaine.com/files/Download/11_AtticVentilation.pdf