Insulating an Old Farmhouse

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Kevin N Kathie ...
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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?