A.Contributing editor Paul Fisette responds: Although the energy crisis hit during the '70s, most homes built during that decade were still under-insulated, so it's a good idea to upgrade. Adding an exterior layer of insulation to the walls is easy to do while you're residing. It creates a continuous layer of protection that mutes through-conduction of the framing members. The installation of a "winter coat" also keeps walls warmer, reducing the likelihood that condensation will form within the wall cavities. In your climate, it's best to select insulating sheathing that is somewhat permeable to vapor. Northern building codes require the installation of interior vapor barriers. Installing an impermeable barrier on the exterior of the wall can trap moisture that reaches wall cavities, creating conditions that are ripe for mold, mildew, and rot. So it's best to avoid the use of foil-faced foam panels in cold climates. Using unfaced polystyrene or other semipermeable foams is a reasonable choice in your cold location. Dow Sturdy-R is urethane foam board with a perm rating of 3 and an R rating of 5 per inch, a good choice.