For economy and ease of contruction, this is my prefered procedure for raising the insulation value of a residential exterior wall to R=35. Some use a double stud wall. Some use 8 inch studs. Both have their purposes in some circumstances, but are usually substantially more expensive. Basically, to conventional stick frame walls, add 2 inches of rigid foam to the exterior, extension jambs on the windows, and strapping on the outside for fastening the finish siding, and you have brought the wall up to the new standard for a Maine climate. Watch this video to see how it is done.