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Insulating sloped wall/ roof

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??

Re: Insulating sloped wall/ roof

a closed cell foam could applied through small holes drilled in each cavity. If you have a knee wall at the lower end of the slanted roof and an attic above, you could fish batt insulation up the cavity without out tearing out anything.

The best way is to tear out the lath and plaster, install a barrier against the underside of the roof, then install batts with a vapor barrier. The rafters will short circuit a lot of th eheat out of the room though. On a cold morning where frost forms on the roof, you will see every rafter because the frost will not form above them. To stop this heat loss, you can use foam boards, 1/2" to 1' thick between the sheetrock and the rafters. That will really help make the rooms more comfortable. It will also go a long way to preventing ice dams.

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