One of the questions about insulating a double stud wall with Roxul is how to insulate the space between studs?  My new answer: Use roxul for steel studs, which comes 16 1/4" or 24 1/4" wide  to fill the inter wall cavity then add Comfort bats for wood studs between the studs.  The steel stud version of comfort bats are available for 16" and 24" framing in thickness of  2 1/2", 3 1/2  & 6".  Apparently the price per r sq ft is about the same for the steel stud but the big box stores (Lowes) don't carry it.   I found it priced here:  http://pricecatalog.steels.com/index.php?catid=&calculator=219  If you insist on a wall spacing that can't be filled by the 3 choices of comfort bat you could use the higher density AFB which gives r 4.1/" but costs about 50 % more.  AFB comes in 1", 1 1/2",2" 2 1/2", 3", 4",5",6" thickness and either 24" or 16" width.  Being a bit on the cheap side I'll use a 5" space, 2 layers of 2 1/2" comfort batts and try to get a match of Lowes prices per r sq ft.