Originally Posted by Calc
Is there a good insulation forum somewhere? A forum in which people read the question? A place where you don't have to wait many days for non-answers?
I read and saw quite clearly where you indicated fiberglass insulation.
Doesn't change whether or not it works effectively in this scenario.
Kneewalls are problematic for for fiberglass because it does not address the thermal bridging of the stud wall, the natural convection and resultant R-value loss, or the inadequate R-Value it provides in most cases.
Having fixed a bunch of cape cods, I can tell you that a bunch of clients have all the fiberglass they can shake a stick at and yet the summer temperatures are still quite unbearable.
I personally prefer 2x6 construction for the kneewalls and if you go that route, with an R-19 batt, and a 1" foil faced iso foam, it will make that insulation work very well.
Nothing about using the higher density or thicker batts does anything to address the thermal bridging of the wall studs and the big thermal conduit that they are.
If you are dead set on using fiberglass, run it inside the walls and another layer across the studs and cover it with and air barrier (i.e. Tyvek).
Fiberglass in the ceiling is relatively terrible because by the time you figure in the vent space, the amount of R-Value is minimum and the air wash over it derates it as well.