Zone 5 Envelope Enclosure / Parallel Chord Truss
I’m will be building a home this summer / fall and have started working out the details for my build.
I’m in zone 5, BC, Canada, and intend to build a simple shaped gable end structure with cathedral ceilings. I’ve located a truss manufacture who can build parallel chord scissor trusses for me that will meet my 20′ span without a ridge beam.
I intend to insulate well. I am leaning towards a 2×6 wall, sheathed, tyvek, with 2″ of roxul comfortboard on the outside. I could also consider a double wall if there were significant cost savings, but my research has informed me that it is a little riskier from a vapour diffusion perspective.
My question is in regards to the roof assembly that I am considering. Which is, ordering trusses with no overhang, and wrapping an airtight vapour permeable roof underlayment straight on top of the tyvek on my walls. This would create a simple plane to create an airtight enclosure for my roof. I would then strap the roof with 2x material on top of the membrane before roofing (and create my overhangs with 2x). The trusses would then be dense packed with cellulose.
I think this system will allow the sheathing to breath to the outside, but am I just adding cost and extra work for a little bit of air sealing convenience?
Should I just install ventilation baffles in the trusses to create the ventilation channel, and have poly on the inside to be my air barrier?
See the attached detail. Any comments are appreciated.
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