I have the same dilemna with my 140 year old house and another lil one on othe property. They still have their clapboard siding underneath insulbrick. I would love to redo the siding someday however, I worry of upkeep and the insulation factor too. The only thing from experience I can tell you is that in one room I redid I tore down the walls and insulated. It is one of the warmest rooms in the house. In the other room I figured I would tear down the outside walls someday but in the meantime I enclosed everything real tightly. There is no place for the cold to come through now with just painting and caulking. That now too is one of the warmest rooms in this house(I don't think I will tear down the walls and insulate in there now). In the livingroom which needs done desparately, they cheaply paneled and every nook and cranny and cut hole for cable the air comes running in through. In the kitchen, on the outside walls they were damaged by jacking the house up to put in a better foundation, I will have no remorse of tearing down those walls and insulating. When I can leave original I try to but if its damaged beyond repair then I wouldn't feel bad. I think too if you make the siding tight on the outside of your house it would help too.