How do you get the details right in a roof assembly with an external vent?
My questions concern Martin Holliday’s May 2016 blog post (“How to Build an Insulated Cathedral Ceiling…”). My home will have a flat shed roof that is one continuous plane (1.5:12) in weather zone 6. The roof will be built with 16″ TJIs and a metal roof. I am avoiding foam as much as possible, and plan to use dense-pack cellulose for roof insulation. This rules out the option of an unvented roof with some closed cell foam. The roof joists often run perpendicular to the slope of the roof, so an internal vent is impossible. So the plan is to use an external vent as described in Martin’s article. I have two questions.
First, I want to make sure and understand the details of the external vent assembly. Is this correct?
(1) Install fiberboard sheathing directly on top of the roof joists
(2) Install a vapor-permeable underlayment on the fiberboard
(3) Install the 2x4s that provide the vent channel on top of the underlayment
(4) Attach a second layer of roof sheathing on top of the 2x4s. Does this need to be sheathing (could you instead run 1x’s across the sheathing and attach the metal roof to these 1xs)? If it needs to be sheathing, can it be OSB with another layer of felt paper?
Second, does anyone have advice for the best way to detail the top of the shed roof to allow the air to exit the vent?
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