INSULATING A 26’ SCISSOR TRUSS

I am putting on a 26’ x 12’ addition to my living and dining area.  The truss is a 5/12 over 3/12 scissor truss.  I want to vent from the eave to the ridge under the roof deck instead of using a “hot roof” where the insulation is tight up against the roof deck.

I am thinking about running 1” wide wood strips on each side of the truss up against the roof deck and then putting 2” rigid foam against the wood strips to from the eave to the ridge to create a positive vent.  I would then create an R-50 by spray foaming against the 2” foam.

I plan to put 5/8” drywall on the ceiling which would create a “dead air” space between the drywall and foam insulation.

Would this cause any issues since the insulation is not on the backside of the ceiling drywall?

If this would work, should I use poly above the ceiling drywall?  I am worried that this would create a double vapor barrier since the closed cell spray foam on the ceiling is a vapor barrier.

Thanks,

Rick