Just for conversation sake (and asking someone who knows more about the stuff than I do) have you looked into the spray-on foam that is also a vapor barrier? I don't know how it compares to the fiberglass for R value (and price), and realize that it'd be a greater PITA to do any work in the wall after the fact. I'm thinking that it might hold up better over the long run and not settle down like other insulations do? Do the glass bats hold their place for an extended length of time? Will there be any condensation on the opposite side of the plastic vapor barrier even tho the outside of the house is wrapped?
