Insulating a 1925 Montgomery Ward Prefab Catalog House

Kevin N Kathie Martin asks,

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 blown-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?

Our Answer

How cool that you have a true catalog house! Original prefab homes like yours were state-of-the-art in the 1920s. Manufacturers of these homes had access to the best lumber and had the tightest quality controls, courtesy of their great buying power. You'd be hard-pressed to find a house like yours that isn't very well built.
But the insulation issue: It's likely you do have a traditional stud bay of 3 to 4 inches between the inside and outside walls. This 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's 19 to 22 inches of fiberglass, because this is where most heat loss would take place. Another way to examine the walls for insulation would be to have an energy auditor scan them with an infrared camera. These cameras detect temperature differences, and can highlight hollow wall cavities that need to have insulation added.