Probably the best way to build for a cold climate is to eliminate the attic entirely and have a vented cold roof. IE, this can be done with 4-6in of polyiso or DOW board on top of a T&G board ceiling. Then nail sleepers with pole barn nails to the ceiling beams/purlins. This space only is vented from the soffit to the attic. Classic Post & Beam in NY used this method and it had held up well
A slightly more expensive, but less labor intensive method to achieve this would be to simply use stress-skin panels and install the sleepers on top of them and deck and shingle/roof.
