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Community and Q&A

Enabling sheathing to dry on a low-slope unvented cathedral ceiling

Nils Koons | Posted in Energy Efficiency and Durability on
I’m working on a project where I have an unvented low-slope cathedral ceiling and am worried I’m walking right into the rotten OSB SIPs trap. The building is in climate zone 6. The roof pitch is 2-12 and there is currently triflex installed on top of taped osb sheathing. On top of the installed sheathing there will be 5 inches of rigid foam, followed by an additional layer of osb sheathing wrapped in a peal and stick membrane followed by a standing seam metal roof.
This layup seems risky as the outermost layer of sheathing is trapped between impermeable layers. Most of the advice I have dug up recommends adding ventilation channels between metal roofing and sheathing. The problem I see with this is that with a 2-12 roof I feel I need to use something like Ice and Water on the whole roof, thus preventing drying towards the metal roofing. Is there a way to add ventilation channels underneath the top layer of sheathing? I was wondering if I could use strapping between the foam and sheathing layers to create ventilation channels. It seems like this would create some choice critter habitat along with a tricky fascia/drip edge detail. Is there a simpler way to do this? Should I just skip the ventilation and double down on air sealing in the ceiling/lower sheathing?
Many thanks for the help and invaluable resource.
Cheers,
Nils

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Replies

  1. Nils Koons | | #1

    Think I may have come up with a solution - ventilation channel tied into some sort of soffit vent a la cor-a-vent ps-400. Not sure why I was struggling with that so much.

  2. GBA Editor
    Martin Holladay | | #2

    Nils,
    Lots of unvented roofs include a layer of sheathing that is unable to dry in either direction. As long as the roof is otherwise well-detailed, this type of roof works fine.

    The key point: The weather has to be dry -- ideally, dry and sunny -- on the day that the sheathing is installed. You don't want to cover the sheathing will peel-and-stick if the sheathing is damp. As long as the sheathing is dry when it is encapsulated, you can sleep well at night and stop worrying.

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