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?