I’m finishing a “bonus room” over my garage.  It’s framed with 2X6 on 16″ centers, including the kneewalls.  This is a pretty big room, and I’d like to make it as efficient to heat as is reasonable.  I’ve seen a fair amount of discussion on the effect of framing on heat loss, but I don’t want to shrink the room by putting insulation on the inside of the wall.

I was thinking about putting standard 6″ batts in the kneewall, with rigid foam behind the framing (on the unfinished side of the wall).  Is there any reason I shouldn’t do this, assuming that the batts have a properly applied vapor barrier?  Is there a better way?

A section of the ceiling will be applied to the rafters.  This seems like another opportunity for improvement, since the batts are going to be squashed between the rock and the rafter vents.  Should I just suck it up, or is there a straightforward way to improve efficiency here?