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How do you get the details right in a roof assembly with an external vent?

John Huber | Posted in Green Building Techniques on

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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  1. GBA Editor
    Martin Holladay | | #1

    You have a low-slope ("flat") roof. So you can't use conventional venting details.

    Here is a link to an article that describes all options for insulating or venting this type of roof: Insulating Low-Slope Residential Roofs.

  2. John Huber | | #2

    Thanks, Martin. Given the roof framing, there is no way to have a vented assembly as described in that post, which leaves the unvented options, all of which require some foam. Is it hard to explain why the external vent is a bad option for the low-slope roof?

  3. GBA Editor
    Martin Holladay | | #3

    It's pretty easy to explain. Without a significant difference in elevation between the intake vents and the outlet vents, there isn't much chance of stack-effect air movement.

    You need a large volume of air (8 inches of clear space above the top of the insulation, plus one or more big doghouse vents) to make a vented approach work on a flat roof.

  4. John Huber | | #4

    Thanks! And happy holidays. John

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