Calling all energy efficiency pros!

I have a cape cod style home. The 1.5 story homes with the knee walls and slopes ceilings upstairs. we are trying to make the upstairs more comfortable so we decide to gut upstairs and re-insulate and air seal.
we will insulate and air seal knee walls and the gable end walls.
I'm wondering if it is even worth insulataing the sloped portion of there ceilings. It is only 2x4 construction so the best I can do is 2" polyiso rigid foam which would give me an R-13. I would use 2" foam to leave a ventilation gap for the roof sheathing. The dimensions of the sloped ceiling are approximately 10ftx6ft each side.
what do you guys think?
would i gain comfort by insulating that slope portion of ceiling with r13? Is it worth it?
thanks

 

Heat loss is up, out and down so absolutely you need to insulate.

With only 3 1/2" might consider alternative, cellulose or spray foam to gain some extra R value.