I am putting on a 26′ x 12′ addition to my living and dining room.  The truss is a 5/12 over 3/12 scissor truss.  I am thinking about venting from the eave to the ridge under the roof deck instead of insulating against the roof deck creating a “hot roof” with no vent.

I am thinking about running 1″ wood strips on each side of the truss from the eave to the ridge to create a positive vent between each truss.  I would then put 2″ foam against the wood strips.  The next step would be to spray closed cell insulation against the rigid foam to create a R-50.  (Live in zone 7)

I plan to put 5/8″ drywall on the 3/12 slope.  This would create a “dead air” space between the drywall and insulation.

Would this work or is it “stinkin’ thinkin’ ?

Does closed cell spray foam against 2” high density foam cause any shrinking of the high density foam because of the heat that it creates?

If I had 2” foam installed first for an R-10, I would need 8” of closed cell foam against it.  That sounds expensive.  Would open cell spray foam be more cost effective?  What else could I do to make it more cost friendly?

Would I have moisture problems with the dead air space above the drywall?

Should I put a poly vapor barrier on the back side of the drywall, or would this be a double vapor barrier because of the rigid foam and closed cell insulation above the dead air space?

Is there a less expensive method of getting a R-50 value in the ceiling than using spray foam?  I don’t want to put an attic access into a living/dining room ceiling.