My historic 1.5 story farmhouse in Northern Vermont has 3 doghouse dormers with no insulation.  I’m replacing some rotted boards and reroofing.  I’d like to add rigid insulation on the exterior of the dormers.  I will have to build up the dormer roof to extend it over the rigid insulation on the dormer walls.  I’m wondering if anyone has any good details insulating and flashing a dormer from the outside?   

Please note  – I can add rigid insulation to the top of the dormer roof and walls.  However, I am not adding exterior insulation to the entire roof of the house at this time.  I couldn’t find a contractor to do it and the roof is leaking now.