re; 2×4 vs. 2×6 stud walls. In our part of Canada r22 wall insulation is minimum. Our most cost effective method is 2×6 studs, with r22 fiberglass insulation. We have also used 2×4 walls with r 12 fiberglass batts, and r10 (1 1/2″) SM  over the sheathing to acheive code requirements, but also minimize thermal bridging through studs. It is quite effective, but material and labor add up to substantial cost increases, debate able whether there is a reasonable energy payback. If we go 24″ o.c, we use 5/8″ gypsum. Also, any walls 10′ or over we go 16″o.c., with blocking.