Hi, I am building a garage with a apartment above. I am a bit confused by the air-barrier on the inside of the ceiling and walls. In addition I am being challenged by my more knowledgeable albeit dated carpenter on my choices. ( “why do you need to build the most insulated building in the county, its causing problems”). 38″ x 26′.
Walls- T & G pine cladding, rainscreen, 2″ of Rockwool, Zip sheeting (taped of course), 2×6, Dense pack blown cellulose, then ?.  
Questions:
1.Should I put in a air-barrier like Intello, or just utilize the drywall and paint to create an air-barrier?.  Cost is becoming a major issue.
2. I will frame in a plumbing wall inside of the 2×6, but I plan to wire in within the 2×6 and seal the outlets. Good Idea?
3. Ceiling-  Its a cathedral ceiling with scissors trusses. I plan to not have any can lights, but will have a few outlets in the ceiling for track lighting.
Insulation will be blown in cellulose.  Carpenter wants to hang drywall . Should i put up an air barrier between the truss and drywall?
I have a lot of Stegowrap left over. will it work?
Roof is asphalt shingles, and I will vent. 
Climate- Zone 6, Vermont, so code is not used. But I am building to code of course.
Thanks for all of your kind help.