Climate zone 6B, 1890 home renovation with mansard roof. The focus is the steep section of roof, an appx 10′ stretch from soffit to under the drip edge of the flat roof. Construction from the interior is lath and plaster, 2×6 rafters with dense fill cellulose in the bays, 1” rough sawn skip sheathing, with original paper underlayment and tin roofing. The sheathing is in good condition based on limited evaluation. The new roofing material will be steel shingles with ice & water shield on the bottom 3’ and a vapor permeable synthetic membrane above. Without a change in insulation or venting this new roof should retain the function of the original construction but doesn’t help with increasing insulation.

Researching all the insulation and venting articles it seems we could add rigid exterior insulation without venting or add a vent channel over the existing sheathing. I’m leaning against rigid insulation as we cannot get to appropriate level of R value on the exterior, and plaster removal and adding closed cell insulation in some rooms could pose a vapor permeability issue. We prefer to leave the existing assembly in some rooms, and will try to add rigid insulation to the interior of the plaster walls in others. If material removal waste, labor and cost weren’t issues, I know we should just remove all lath and plaster and spray foam below the existing sheathing. Given this I had decided the #2 vented assembly with interior rigid insulation from “5 Cathedral Ceilings that Work” gives us flexibility to insulate differently depending on the room. However, after reading “All About Roof Venting” I hesitate to implement venting on a convoluted roof. The dormers sit right at the lower drip edge so there are no rafter bays below and it seems we can ensure air flow by staggering the 2x4s supporting the new topmost sheathing at the hips and above the dormers. Am I right to think the vented assembly gives us flexibility or is it likely to cause greater problems?