Insulating a cathedral roof with rigid foam

Question

I am in the process of remodeling the 2nd floor of our cape and I am trying to figure our how to insulate it. Here is what the 2nd floor looks like, roughly.

Here is another drawing that shows the venting. You can see that it has a ridge vent, and some sort of small soffit vent. Not shown are the gable vents, the knee walls, and the collar ties, among other things.

I'm in zone 5 and I'd like to have more insulation than the R13 I could fit in the rafters. So because of that I am trying to decide between having spray foam insulation, or using rigid foam between the rafters. I'm hesitant to use spray foam since I know less about it, I don't know if converting to an unventted roof is a good idea, I feel like I might need to change some of my roof sheathing soon, and I am only able to redo half of the second floor at a time.

My thought for the rigid foam would be to do whats shown in the drawing below. Additionally I would use expanding foam to seal foam to the rafters and top plate.

So, besides costing quite a bit of money, is this a good idea? Are my reasons for avoiding spray insulation even remotely valid?

Answer 1

I for one like the new spray-in foam. It does such a good job because it makes the house so tight. Infiltration is where you loose your heat. It's the cracks that hurt you. Nothing seals the cracks like the spray-in expanded polystyrene. Regarding all that venting business; they are just trying to keep the roof surface cold so the snow won't melt and cause ice dams. Nothing out there can better keep the heat away for the roof surface.