I'm hoping you'll weigh in on a problem I'm trying to solve. I have a 90 year old Toronto (Ontario) home in great need of insulation. The upper floor has sloping ceilings that are actually the roofline of the building (about 12:12 slope). The rafters are common to the roof and interior walls. Sheathing is planking. Given that the interior finish is wood lath and plaster, what is the least destructive method of adding insulation?
I have been considering dense-pack cellulose applied from the interior but am concerned that this may be problematic should the outer, shingled surface fail, allowing water infiltration or there are condensation issues in the winter.
Your thoughts and recommendations??