Thanks Fred.
So with the false wall I could simply attach these to the back of the studs ie the part that is towards the wall. That would be smart so the actual wall is undamaged if we ever go to sell.

I did look at Owens corning 703 but can't find it up here in Alaska so the 3.b/cu ft is great advice! Guessing that I either need to specify rigid fiberglass as well / or all rigid is 3lbs / cu ft.

Will check those forums out too - thanks. I did see the links here about corner traps etc and they are very useful. One of the threads had some nice panels on the walls with the Owens corning, but didn't see how the wall panels were built up so need to look at that as well