I just spent a fair amount of money having a frozen pipe repaired in a the basement of a 90 year old house. The plumber said that the air in the space between the inner and outer walls has no insulation and is about the same temperature as the outside air. I don't want to add insulation to the area without knowing whether it will just trap the cold air. The basement walls are drywall, but the upper three floors are plaster. None of the walls in this place have insulation.