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Community and Q&A

Cathedral ceiling rebuild

clarklewis | Posted in Green Building Techniques on

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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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Clark,
    You wrote, "I don't think it makes sense anyway to have a tight ceiling with leaky walls and no HRV system."

    That's where you're mistaken. The reason that you need an airtight ceiling has nothing to do with HRVs or indoor air quality. The reason that you need an airtight ceiling is to prevent warm indoor air from entering your rafter bays. (The leaky ceiling is the reason for your ice dams.)

    No solution will work unless you start with an airtight ceiling.

    You have two choices: You can install drywall with taped seams on the interior side of the ceiling boards, or you can install a layer of spray foam on the exterior side of the ceiling boards.

    One of these steps is Step One. Once you have established a ceiling that is more or less airtight, you can take one of several approaches to insulating your roof assembly.

    For more information, see How to Build an Insulated Cathedral Ceiling.

  2. clarklewis | | #2

    i know i need an air barrier, but couldn't it go on top of the rafters if it includes the open ends of the rafter bays? my builder friends up here don't like ceilings drying towards the interior, so trying to avoid that.

  3. GBA Editor
    Martin Holladay | | #3

    Clark,
    With fluffy insulation like fiberglass or mineral wool, you need an air barrier on all six sides of the insulation.

    If you omit the interior air barrier, it's highly likely that small exit cracks in your roof assembly (near the beam or near the ridge) -- the flaws that exist in most assemblies, are are always missed in spite of efforts not to miss them -- will pull warm interior air into the roof assembly (due to the stack effect).

    When it comes to detailing this roof assembly, nothing is more important than an airtight ceiling.

  4. clarklewis | | #4

    ok thank you.
    i know we're not supposed to get "creative", but is my "double-roof" plan reasonable with mineral wool boards between the cross-strapping? i know you'd rather see continuous rigid, but i don't think that's approved/proven yet for mineral wool boards, especially with our snow loads - if it stays cold the roof sometimes doesn't shed the snow right away, so lots of weight and shear force.

  5. GBA Editor
    Martin Holladay | | #5

    Clark,
    If you are able to create an air barrier at the ceiling, you can certainly insulate the rest of your roof assembly with mineral wool if you want, as long as you have a ventilation channel between the top layer of mineral wool and the top layer of sheathing. This ventilation channel is required by code and recommended by building scientists.

    To allow for thick layers of mineral wool, you can add several layers of framing if you want, with mineral wool inserted between 2x6s on edge -- as long as you obey the basic rules (an airtight ceiling and a ventilation channel between the top of the mineral wool and the topmost layer of sheathing).

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