Why would you insulate the outside walls. They are only so thick. You may get R-2 out of all that work.
No, an uninsulated 2x4 wall (with good air sealing and no windows) is R-2. Wall cavity insulation is R13 to R15. Exterior foam is a different story altogether as it's real purpose is to minimize the risk of condensation inside the wall with that R4/inch.
The plaster walls have about R-7 resistance.
No, plaster and lathe layer has an R value of 0.25, so your R-7 magic plaster would need to be about 14 inches thick to achieve R7.
Fill the gaps around cheap vinyl windows, insulate the attic really well. Seal all openings in the attic. Seal well around all the doors.
Great advice. Air sealing can make a big difference in comfort. I'd agree that it's more important than just insulating, especially if the insulation is fiberglass. Air seal in the attic first.
But adding insulation to vertical walls is a waste. Warm and cool air move vertically, not horizontally.
Well, we should have fixed the air movement with the air sealing, but heat radiates and will go through an uninsulated wall with little resistance (the R in R value!). With a big temperature differential the wall insulation will keep a lot more heat in the house. On a cold day that uninsulated wall will feel cold. That's why the building code mandates insulation in the walls of new construction.
If you have wood windows, rebuild them and use bronze guides.
Single pane wood windows can be improved to slow or stop drafts, but the temperature differential will make that whole area around the window feel cold and can even create convective loops that feel like a draft.
This house stays a comfortable 68 average year around without heat or cooling.
IN NORTH CAROLINA!! Try that in Minnesota in winter. Also, most houses probably aren't as well sited and designed as yours.