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Community and Q&A

Sheathing Air Barrier and Vented Roof Assembly

Eric Colbeck | Posted in Green Building Techniques on

Sheathing air barriers and vented roof assemblies both sound like wonderful ideas and seem to have the blessing of many building professionals. Are they compatible with each other?

I want to build a cathedral ceiling insulated with cellulose. Some people say such a roof must be vented and carefully detailed with an interior air barrier. Others are building them unvented with sheathing air barriers and are publishing their methods. Do you agree with them in any way?

If the roof must be ventilated has anyone tried to build an air gap (like a rain screen) above the roof sheathing? Instead of fumbling around with site built air sealed baffles under the roof sheathing why not build in an air space above the roof sheathing to allow vapor to dissipate?

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Replies

  1. Expert Member
    ARMANDO COBO | | #1

    By code you have to vent the cavities under the roof decking, not above, which is a very labor intensive and high-quality control issue on itself. Typically, a true cathedral ceiling is best to condition it. The up-cost of the conditioning can be mitigated from the labor intensive ventilated cavities, and you'll end up with better results.

  2. EthanT | | #2

    I'm not sure what you mean by "a true cathedral ceiling is best to condition it." I have read a lot about cathedral ceilings but don't know what you mean by this sentence.

  3. charlie_sullivan | | #3

    user-6882359,

    Yes, sheathing air barriers and vented roof assemblies are compatible. If the top of the wall is air sealed across to the ceiling plane, you have a continuous air barrier. Add to that air sealing of the baffles and you are all set.

    People who are building cellulose insulated roofs without venting are building very risky assemblies that are likely to fail, perhaps with the exception of some hot/dry climates.

    Air gaps above the decking are also sometimes used and discussed here. I don't know much about them so you'd have better luck searching for previous discussions or waiting for someone else to comment.

    ps See https://www.greenbuildingadvisor.com/blogs/dept/green-building-news/how-gba-site-displays-readers-names for how to make your name appear as your real name.

  4. Eric Colbeck | | #4

    Hi Charlie,
    Thank you for your response. I am little unclear on the method you prescribe. Are you saying that the walls have an exterior air barrier that transitions to an interior air (drywall) air barrier at the top plate of the wall?

  5. charlie_sullivan | | #5

    Yes, that is what I mean, although I would include air-tight drywall on the interior wall as well. The outside air barrier transitions to the interior and joints the interior air barrier. But it could work without that.

  6. GBA Editor
    Martin Holladay | | #6

    Eric,
    I discuss vented roof assemblies that include ventilation channels above the roof sheathing in my article titled How to Build an Insulated Cathedral Ceiling.

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