Vaulted ceiling insulation options
I have a 1974 Garrison Colonial with an original two-story family room/ two car garage ell which needs to be reroofed. I know from previous forays onto the roof that the roof sheathing is soft under foot. I’m assuming dry rot. I’m going to take this opportunity to address the sheathing and what I assume to be a woefully under insulated and poorly air sealed roof system while addressing the new roof. The parameters of the project are as follows.
· Zone 5.
· ~ 6 ½” pitch roof, sloped portion of vaulted ceiling is about 23 degrees.
· Original flat ceiling was vaulted sometime in the 90’s .
· Existing plaster is in good shape, therefore hoping to attack this entirely from the outside
· Original rafters and rafter ties are 2×6, assuming vaulted material is probably the same.
· Planning an unvented assembly as there are two skylights on the north side and the south facing portion of roof will eventually be getting a cross gable addition which will be “California”/over framed eliminating all but 3-4 feet of eave line.
· The limiting factor is the lack of depth in the rafter bays for proper amounts of insulation.
My ideas are as follows.
1. – Strip roof, sheathing and existing insulation
– air seal at drywall plane, remove poly if found
– Sister rafters if necessary
– fur down underside of rafters to accommodate 8″ of Rock Wool comfort batt- R-30. I can achieve full depth(7.25″) for all but the first 18″ of the rafter bays
– sheath rafters,(1/2 cdx) install SA air barrier
– install five inches of rigid insulation R- 20
– second layer of sheathing (3/4 cdx)and re-roof.(architectural shingles)
My questions are,
1. Should I use foam insulation or mineral wool as my outer layer of insulation? EPS, Poly Iso or Rock wool comfort board
For me the only downsides to mineral wool are weight and availability right now
2. Should the batt insulation follow the run of the rafters to the ridge or can I follow the ceiling plane which would create small unvented attic (see sketch)
The main concern is what I’m going to find when I open up the roof. Rotted rafters and or mold growth on drywall could completely change the scope of the job.
I’ve included a sketch. The colored rafters are the original 2×6’s. the white framing defines the vaulted space and interior drywall plane.
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