Acoustical sealant that never hardens goes in every joint and along every stud. Special tape then covers the joints.
At the top plate, the air barrier switches from the exterior sheathing to the plastered ceiling. Vycor wraps from the sheathing over the plate.
After the trusses are installed, strips of OSB go in around the perimeter of the ceiling with the Vycor adhered to the top side.
At the dormer in the middle of the house, the ceiling is framed flat with the OSB strips around the perimeter.
After the drywall is bedded in sealant and attached to the ceiling, a skimcoat of plaster completes the air barrier.
To create vapor open sheathing on the outside of the trusses, locally available and inexpensive rough-sawn pine is the perfect solution.
Vertical furring strips provide attachment for the siding above the wall trusses. The rigid foam is the drainage plane for that part of the wall.
Bronze screen on the bottom of the trusses keeps out the critters, and spaces between the decking boards allow moisture to drain out.