Insulating open walls with glass fiber or mineral wool batts is the most economical. Adding additional strapping to open stud walls (horizontally is best) will give you more wall space that can be economically insulated. Double strapping will provide room for a good air/vapour barrier (polyethylene sheeting before the strapping) but be sure there is always twice as much insulation outside the vapour barrier as inside. (search keyword "vapour barrier" for the title "WHERE IS A VAPOUR BARRIER TO BE PLACED?")
Fill the entire space in the wall snugly with the batts. Don't compress them, though a tight fit is better than an air space. (search keyword "walls" for the title "MY CONTRACTOR SAYS I MUST HAVE AN AIR SPACE BETWEEN THE INSULATION AND THE SHEATHING INSIDE THE WALL CAVITY. IS HE RIGHT?")
Many specialty products exist for insulating open walls, from wet cellulose blankets sprayed into place, to polyurethane foams. The first is an exceptionally good sound barrier and the second an exceptionally good air barrier. Study well why you want a non-standard insulation, and its total cost (material and labour) before going against tradition.