All of our homes have exterior walls framed with 2×12 walls. With 12 inch thick walls the best way to insulate them is with spray foam. It fills every nook and cranny and has teh best R value. We also spray the underside of the roof with at least 12 inches of foam so the attic area stays at a much cooler temperature. Your air conditioning ducks run through this space and the cooler it is, the less demand on your AC system.