I just purchased an old home (~1910) that was converted to a duplex in the 70's that needed some major cosmetic work. When I purchased it the previous owner performed very little in maintenance and upgrades of the property, he apparently did not see the potential for return on investment. However, I am not a slum lord and removed all of the 70's style wall paneling and intend to replace it with drywall. After removing the panels I discovered that the exterior wall in the living room was fitted with fiberglass (that appears fairly old) while the bedrooms had the old slats and plaster. Being from Northern Minnesota where temperatures in the winter can reach -30 F for weeks in the peak of winter, I am considering making some minor improvements to the insulation and was wondering if putting up XPS polystyrene over both the fiberglass and the slats before putting up the drywall would be an appropriate way to go, or is there another option? The studs are 2X4s and with so much other work to do I want to keep the cost low and finish it quickly.