Cathedral ceiling rebuild
i live in whistler, canada (zone 6, tons of snow) in a 1970 cabin with recurrent ice damming issues.
we have 3 simple 12/12-pitch shed roofs (no valleys or hips) with no overhangs which i plan to rebuild from the outside (i’m a handy homeowner).
i have thoroughly read and understand all the pertinent articles on this site, and others.
i’m hoping for some feedback to make sure i don’t waste time/money/materials, but cost and simplicity are priorities.
roofs are currently constructed: cedar shakes, tar paper, sheathing, R22 kraft-faced fiberglass batts between 2×10 rafters, poly, gaping T&G cedar ceiling boards.
the batts are installed tight to the poly, leaving a 2″ vent space above them, yet there are neither soffit nor ridge vents! snows melts quickly along a mid-span beam, indicating a major air leak (i suspect the batts have settled down away from the beam, plus the poly is not sealed to the beam). this is where the giant ice damns occur.
goals: leave ceiling boards intact, improve air seal and insulation, add 2-foot overhangs all around.
i can’t afford spray foam, and don’t think it makes sense anyway to have a tight ceiling with leaky walls and no HRV system.
i prefer to avoid a continuous layer of rigid above the rafters because of the hassle of fastening sheathing blindly through a thick foam layer, but also the hassle of trying to frame overhangs over/around the foam.
due to interior height restrictions and interior walls, etc its not feasable to add insulation or air barrier below rafters, must all be above.
plan A: remove everything from outside, leaving ceiling boards +/- poly. R32 roxul batts between rafters, air barrier (taped plywood vs house wrap over rafters vs nothing), 2-foot rafter extensions, cross-strapped 2×6 on edge (extended 2 feet over sides for overhang), fill space between 2×6 with roxul comfortboard vs comfortbatts, then vertical 2×4 on flat for vent channels, plywood sheathing sealed with titanium psu30 membrane, 1×4 horizontal purlins, metal.
questions: any point in a good air barrier and so much insulation if i have leaky R10 walls which i may or may not improve in the future? should i leave the poly intact (its not well sealed)? should i just leave the kraft batts and fill the 2″ rafter space above them with insulation? should i use roxul batts or comfortboard between the cross-strapped 2×6? should i leave an air gap above this layer (ie. not fill the entire 2×6 space) for horizontal cross-ventilation (in addition to the soffit-ridge ventiliation)?
i realize there will be some thermal bridging where the old rafters and new cross-strapped ones meet, but it’ll be a helluva lot better than the current setup!
option b: keep it simple, just use R32 batts between the rafters and leave a vent gap, build the overhangs off the current rafters. maybe use tyvek atticwrap/raftercaps for an air barrier, for what its worth: http://www2.dupont.com/Tyvek_Weatherization/en_US/assets/downloads/Master%20Architect%20Binder/Weather%20Resistive%20Barriers/AtticWrap/AtticWrap%20Building%20Science%20Bulletin.pdf
thanks for your thoughts.
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