You drill smallish holes in the drywall in each stud bay, blow the insulation in through the holes, and patch them. This is much less expensive and less messy t

You drill smallish holes in the drywall in each stud bay, blow the insulation in through the holes, and patch them. This is much less expensive and less messy than removing and replacing drywall. It can be done with cellulose insulation, with fiberglass (the loose fluffy kind, not batts), and possibly some other products.