If your house is 100 years old, any existing insulation is recently installed. Prior to WWII insulation in residential construction was rare. It is a good bet that your walls have no insulation at all. The walls should be filled with blown-in or foam insulation. Not only will this insulate, it will decrease the infiltration (blowing air) from outside. This process is not easy or inexpensive since old houses have an unkown arrangement of wiring, blocking, and modifications that make filling the voids difficult. Holes must be drilled in the walls for installation and then plugged. Installers are clever about finding locations that hide the holes. If you are going from no wall insulation to R-13, you will notice a big improvement.

If the attic insulation is less than 8 inches deep, you will benefit greatly by adding to it. The attic should have a minimum rating of R-30. That translates to about 10 inches of insulation (depending on the type and brand). A total ceiling insulation value of R-40 or more is not uncommon in your area. You will see an improvement even if the attic is not properly ventilated. You can upgrade the ventilation at the same time. Ventilation will lessen the chance of wood rot, mold, and mildew.

These upgrades, in conjunction with the windows already replaced, should make your home "heatable" in 10 degree weather and decrease your heating bill noticably.

Regards