I'm insulating my shed and I have a cathedral ceiling. Initially I was going to fit foam board in between the rafters and give no kind of attic or ventilation space, but then I was made aware that the roof needs to "breathe", or else it would sweat and the rafters would rot out. I've also heard that if foam board is tight and sealed properly, roof sweating is no issue.
So first of all, I want to know if ventilation space is required for foam board? If so, I'm wondering how? At the top of the roof, I have both a copula and ridge vent. If I was to create some kind of ventilation space, what I was going to assume to do was run the insulation under the rafter ties, about 1' down and 1' wide, and leave that open space. Then I would run the rest flush with the ceiling.