We plan to act as our on general contractor during for our home build next year.  The construction will be in zone 5 (50 miles east of Boise, ID).  
I had been planning to make the air barrier wrap the entire exterior of the home at the sheathing (a sort of monopoly-style) from the slab, up the exterior of the walls and continuing to the peak of the roof. With 2 inches of rigid foam on walls and 4 inches on the roof (both over the airbarrier).  But with the cost of lumber being what it is I don’t know if I can still do the insulation on the roof as I’d need to add lumber over the top to secure the roofing (metal).  I would really like the house to be as air tight as possible and rely on an ERV to move/ventilate in a controlled manner.

Can I still use the roof as an air barrier if all of the ceiling/attic insulation is below the roof? I ask because trying to seal around all of the ceiling penetrations (lights, speakers, plumbing, electrical, network) seems near impossible.

Thoughts….
Thanks.