I suspect that any new work, such as replacing the roof, would be required to meet current, rather than 1920s, building standards. Consequently, you might get the roof insulated to modern standards whether you like it or not. I think the OC as a whole should pay for this. Arguably the entire roof only benefits the top floor but the OC as a whole has a maintenance responsibility here. Perhaps there is some other maintenance areas that would benefit other units to a greater extent and these also could be addressed while you have the builders in. Insulate the walls too?