Vented Cathedral Roof With Dense-Pack Cellulose and Smart Membrane
I have been flipping between roof designs and am hoping to come to a decision as my plans need to start taking shape. Zone 6, 1000 square foot, cathedral roof, coastal Maine, owner/builder.
I switched from a vented truss design (because of cost) to an unvented outsulation design but am uncomfortable with the use of any rigid, including EPS and Polyiso, because of cost and GWP. So, i am back to a vented design, as described below, and would appreciate feedback on it.
Its a 32 foot square shed cathedral roof. I am going to bump up the slope to 2.5/12. I will probably be framing with 2×10’s or 12’s and will hang gussets down to increase the depth to 16 inches or so to get my r-60 using dense pack cellulose. I am interested in using the “vented, with vapor variable membrane” roof described on page 124 of PGH. Basically, its tyvek over framing, strapping in line with the rafters on top, and then sheathing, creating a vented channel. Strip vents at soffit. Here are my questions.
1. Will the tyvek, supported by the strapping above, be strong enough to resist the pressure of the dense pack without tearing?
2. If I rip a 2×4 in half, I could create a vented space of 1 and 5/8″. Would this be sufficient?
3. I would like to use zip all the way around as my air barrier. Will the tyvek, then, also need to be taped and sealed, as a belt and suspenders approach?
4. At my “gable’ ends, I will be cantilevering framing members over the gable end wall in a 1:1 ratio, with a 2 foot overhang. Would I simply continue my above tyvek strapping at 16 oc to catch my sheathing, or is there a shear consideration here that needs to be made?
5. I will be blocking with wood between rafters. is it advisable to cut rigid and install it with spray foam on the inside of the blocks (as one would insulate a rim) or will the dense pack be sufficient in this space?
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