I've finished building my timberframe cottage in New Brunswick, Canada, and now I'm ready to insulate the roof. I currently have rolled ashphalt roofing over 1x6 T&G hemlock, supported by 4x6 hemlock rafters on 2' centers. I'm planning to add four inches of Styrofoam SM (the cost of winter panels is prohibitive in my area), held in place with 1x3 nailers, nailed through the SM into the rafters (I'm not going to use a vapour barrier, since the rolled ashphalt should do that job). On top of the nailers, I'm planning to put plywood, followed by cedar shingles. Comments? How would you suggest I ventilate the peak and edges of the roof?