Insulating roof under a roof – GreenBuildingAdvisor

Hi, I am a homeowner trying to figure out how to insulate the attic and room below of part of a old 1.5 story house in Minnesota, zone 6a on the most recent map (zone 7 previously). I have a general plan but still have a few questions that I have not been able to locate the answers to in any of the articles or previous question threads I’ve read. I would greatly appreciate any help or advice you can provide.

The house was built in the 1880s with balloon framing and dimensional lumber. In 1981 additions were built on three sides of the house during which a new roof was built above the existing one. The two roofs do not connect except for a small section at the bottom of a roof valley, one can stand in the attic over the attached garage and see right over the ridge of the old roof. The old asphalt shingle roof has just one gable vent with no soffit vents (the new roof over it has ridge, gable, and soffit vents). I’m planning on insulating the exterior walls of the room and the attic under the old roof.

The rafters are true 2x4s so I’m planning on furring them out to accommodate 5.5” batts of Rockwool. Since there are no soffit vents and the old roof has an vented attic above it, can I put the mineral wool touching the old roof’s sheathing or should I have an air gap/baffle to avoid moisture issues? I’m aware this is well below the R49 minimum to meet current code, but am trying to maintain headroom as the ceiling will already be 7’ sloping down to 5’. Would this be a good use for the Bonfiglioli method to use some 1.5” XPS boards I already have lying around to improve the overall R value of the sloped part of the ceiling?

There is also a small roof valley that extends into the upper corner of the room (where the new and old roofs meet). It’s about 3’x3’ at ceiling level narrowing to a point 2’ down with true 2×4 rafters without a soffit vent below. I think I have to treat it as a cathedral ceiling but would like to avoid foam. The roof was just redone so cannot put on exterior insulation either. Can I insulate it as a vented cathedral ceiling even if there is no soffit vent?

Thank you for reading, I apologize for the length and if I missed something important. I’m definitely not a builder and am just trying to improve comfort in the upstairs without causing harm to the building. I think getting a good air barrier is the most important part, but might as well insulate to the best of my ability while the walls are open.

Thanks, Heather