I recently purchased a 1200 square foot, 1910 bungalow in St. Paul, MN. The house has a 15+ year-old gas furnace (forced air). It also has a combination of the original single-pane windows (many of which are missing storms) and cheap vinyl replacement windows (most of which have broken latches).
Over the winter, I put plastic over the windows, installed thermal curtains, and kept the thermostat at or below 65 degrees, but my energy bill was still around $300/month and most of the house (especially upstairs) wasn't particularly comfortable.
I am already planning to add insulation to my attic (there's very little/none there at the moment), but I am wondering whether I should consider replacing the windows and/or the furnace. If yes, which should be higher on my priority list? Or is there something else I should consider doing instead (like insulating the exterior walls)?
Thanks very much!