basement insulation question
I have a 3-level split home, with concrete block foundation. In the lower level, the foundation goes up about 4 feet to grade level and has a 3 foot stud wall (pony wall some call it) on top of the block. I'm sure many of you are familiar with this design and many times it offers itself to building a ledge around the room at grade level upon finishing.
However, I plan on building a straight wall in front of the foundation and continuing this straight up to the floor joists of the second level (no ledge). I will put batt insulation against a moisture barrier on the foundation part, and in between the original studs on the top part.
My question is, do I have to fill the resulting air gap between the original stud wall and the new wall with more insulation?...or can I leave an air gap between the top wall's insulation and the subsequent vapour barrier.
In other words, does it matter if I leave the top part of the new wall with no insulation (between the warm side of the insulation of the original wall and the vapour barrier), or should I double insulate the top part of the wall to fill this gap?
Hopefully I've explained myself well enough.
Thanks for any comments.