It does not matter what the R value of a stone wall is in a 200 year old home. You really need to be concerned about how moisture and air are moving (or not moving) between the interior and exterior of the home. Just focusing on insulation is short sighted. Controlling air infiltration, exfiltration and moisture movement is, in my mind, more important. What insulation? It's a stone wall, there is no insulation outside of the stone acting as it will. This is premilary as I have only been on the property for about 60 minutes, that includes seeing the house, barn and walking 4 plus acres of wooded lot in the snow, uphill, both ways. I intend to take all of the steps needed to preserve an old building while staying as comfortable as possible and that would include moisture intrusion and elimination. As far as I can tell the house shows no signs of any moisture problems in the living space, basement or attic but a second visit will be accomponied by a more thorough look.