Batts

• Place batts of insulation between the rafters of the roof if you are undertaking major renovations and removing the ceiling so that you have access to the roof. Removing the ceiling solely for the purpose of insulating is unnecessary, because you can also insulate using blown-in insulation. However, batts provide superior insulation because they aren't prone to settling and leaving gaps where heat can escape.
  
  

Blown-In Cellulose

• Inject blown-in insulation into ceiling and wall cavities by drilling a small hole in the drywall to gain access to each space that exists between rafters and studs. You can do this from inside the house or outside. If you do it from the inside you will need to plug, spackle and sand the holes after you are finished. If you do it from the outside, the method of repairing the holes will vary depending on what type of siding is on your house. If you are planning to re-shingle the roof, blow cellulose insulation into the roof cavities after you have removed the old shingles but before you put on the new ones.
  
  

Windows

• Improve the heat retaining qualities of the house by replacing the windows with high-efficiency thermal pane windows. If a house from the 1920s still has its original windows, chances are that they are drafty and not very efficient. You can lose more heat through old windows than any other way in an old house. If thermal pane windows represent too large an investment, you can cut down heat loss through the windows by putting plastic over the window and taping it to the frame during cold weather.
  
  

Rigid Foam

• Rigid foam can be used to create extra insulation in either roof or walls, although it also represents extra work and expense. You can apply rigid foam to the inside of your ceiling and then drywall over it. As with batts of insulation, this idea makes the most sense if you are removing the ceilings anyway as part of a larger renovation.