Insulation, Radiant and Vapor Barriers - plaster and lath tearout,new insulation

I am gutting a plaster and lath room. Some of the walls slant,and are actually the roof. Do I treat these slanted area like a wall or like a roof? These slanted areas are where you would normally put the soffit vents. There never was soffit vents on this house,but when the house was re-roofed,a cobra vent was installed. I also installed cable vents on the east and west side. Will I have problems with the shingles curling,by insulating these roof cavities? If I insulate them,should,should I still install soffit vents? If the rooms are cool in the summer,and warm in the winter,shouldn't that keep the shingles in good shape? here is a nice picture that is like a thousand words http://babysbathroom.blogspot.com/

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Do I treat these slanted area like a wall or like a roof? These slanted areas are where you would normally put the soffit vents.
You want to use the foam ventboards . Next to the roof all the way up.start at the knee wall. Then the insulation with paper to the room side. This is so air can get from your overhang vents up to that vent you have up by the ridge. Insulation in that knee wall to paper to the room

Styrofoam proper vent baffles are really designed for use with trusses or wider rafters, and IMO you wouldn't want to use them with 2x4 rafters. The problem you will have if you use the styrofoam vents with your 2x4 rafters is that it will only leave about 2" for insulation. Then when you try to put fiberglass in there too, it would compress the fiberglass, making it less effective. R-13 fiberglass that is squished down will not really be R-13. Squeeze the insulation too tight and you might even crush the baffles.

So what I'd suggest is that you forget the styrofoam baffles and make your own. Put a 3/4" cleat down each side of the rafters, then lay a piece of foil faced 1" thick XPS foam (like Thermax) across the cleat, foil facing the roof. This will leave you with a 3/4" air space if you'd like the ventilation. It will also give you an extra r-7 which your styrofoam baffles would not provide.

Then if it fits in with your remodelling plans, I'd suggest that you furr the bottom of those 2x4 rafters down, if at all possible, by stripping them with 2x2's to give your insulation greater depth. Then install your 3 1/2" fiberglass.

If adding 2x2's to the rafters is not an option, then you might as well forget using fiberglass and fill the remaining space up with additional layers of Thermax. It will give you much better R-value in this small space... up to R-21.

It also wouldn't be a bad idea to install a solid layer of 1/2" foam over the studs in this area to create a thermal break between the studs and the GWB (gypsum wall board).

I'd hate to see you have a cold, under-insulated ceiling that sweats in the winter when you take a shower or draw a bath.