We have a 75 year old cape, stone with slate roof. Replacing the roof with new slates. We want to insulate the roof. We are getting different opinons from contractors. Questions: What is the minimum foam depth that will suffice for thermal and vapor barrier? Can the masonry gables be foamed as well. How thick? Is it best to spray the entire roof deck or spray the back of the knee wall, attic deck and ventilate the soffit to a ridge vent? The roof currently has only gable vents. I assume the ridge vent option would create a good deal more cost. Any input is welcome.