There are plenty of insulation firms that specialize in insulating old homes without ruining the finishes. Typically you cut a small plug, either from the outsi

There are plenty of insulation firms that specialize in insulating old homes without ruining the finishes. Typically you cut a small plug, either from the outside or inside depending on the construction method and mechanically force (typically blow) cellulose fibres into the wall cavity. You need one plug per stud bay, but repairing those is a pretty simple job compared with tearing down all the finishes. Air sealing is tougher, but techniques like aerobarrier where an aerosolized polymer is sprayed throughout the home while it's under positive pressure has made air sealing fairly simple. Stuff like that can plug up to multiple inch gaps