Insulating Against a Roof Deck
I know its been covered quite a bit but I can’t find a great answer to my situation. I have a 1970s cape with sloped ceilings upstairs. The whole upstairs is finished already. I had “smart vents” and a ridge vent installed in my roof to stop ice dams from occurring. I also air sealed and re-insulated the attic to R49 using batts. I have 1.5″ XPS screwed and taped to the bottom of the rafters inside the knee walls, R30 batts in the floor joists (plywood ontop to create storage space), then R15 batts the walls of the knee walls. With all this, I still get snow melt and ice dams. I realized that the sloped ceilings have NO insulation in them and thats probably where all the heat was escaping. I don’t want to lower the ceiling as that would require me to remodel the whole upstairs. The house has 2×6 rafters and I would like to space rigid foam 1.5″ from the roof deck (to maintain airflow) which would all me 4″ for insulation. With that dimension I can only get R20 using XPS. Would that work, or is it not worth my time?
I am trying to avoid raking my roof every time it snows and balance the 8deg temperature difference between the main floor and second floor (hotter in summer/colder in winter). My neighbor has the exact same house and they don’t have ice dam problems.
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