My husband and I just bought a farmhouse that was a Montgomery Ward catalog house…built in 1925. Everything we have read says that houses built before 1930 did not have insulation. The house has the outside wall, then slats, then about 3/4 in of plaster, a gap, then the layers repeat before ending in the inner walls of plaster. We are wondering if the house would have an R rating? If so, what might that possibly be? And we know that blow in insulation won't work, it would be very time consuming and difficult to take down all the plaster and start fresh….would the styrofoam insulation boards and then 1/2 inch drywall be enough to insulate the house?
How cool that you have true catalog house! Original prefab homes were state-of-the-art in the 1920s. Their manufacturers had access to the best lumber, and also had the tightest quality controls. You'd be hard-pressed to find a prefab home from this era that issn't well built.
Likely you do have traditional stud bays of 3 to 4 inches between your inside and outside walls. These could be filled with blown-in insulation. Before tackling that tough-to-insulate area, though, I recommend looking cafefully at your attic. Make sure there are 19 to 22 inches of fiberglass, because that's where most heat loss will take place.
Another option is hiring an energy auditor to scan walls using an infrared camera. These camera detect temperature differences, and can highlight hollw wall cavities that need to have insulation added.