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Community and Q&A

Vented cathedral ceiling and vapor barrier

PeteMossPNW | Posted in Energy Efficiency and Durability on

I am renovating a small cabin in northwest Oregon (climate zone 4 marine) which has two intersecting cathedral ceilings. One is exposed T&G to which I will eventually add insulation above the roof, but the other is a vented 2×12 rafter with a continuous ridge vent (standing seam metal roof). I was planning on making a 1-1/2″ ventilation air gap with a site-built baffle from 1-1/2″ foil-faced polyiso foamed in place, furring down the rafters with strips of 1-1/2 polyiso and 1×3 strapping to make room for R38 batt (as well as insulate the rafters). One question is whether there should be or not be a vapor barrier at the ceiling. My thinking is that the ventilation baffle is both vapor impermeable and a good air barrier so keeping the batt-filled cavity ‘vapor open’ would be better, and with a 1-1/2″ thick foam ventilation baffle the inside surface should not be a condensing surface, not often anyway – winters are long and dreary but not super cold.

I have read the many good articles and discussion on GBA and see some feel the ventilation baffle itself should not be completely vapor impermeable and others approach this by adding a full layer of foam board across the underside of the rafters, effectively sealing in the batt insulation in a vapor barrier – not just an air barrier as is recommended. And this begs the question is it better to add a complete layer of foam at the ceiling level rather than at the ventilation baffle – both might be nice, but at some point “this is just a cabin”. I like the well-sealed ventilation baffle for its being a good air barrier on the top side of the batt.

One other issue with this structure is the T&G roof is an addition that intersects and crosses over the rafter roof, so on one side of the rafter roof the eave vents are no longer there – though there is a bit of ‘attic’ above that roof so much of the sheathing is not exposed. The 2×12 rafters are on hangers but sit above their ridge beam by about 2 inches and the metal roof provides a large continuous ridge vent so the rafters are fully vented at the top, but only at the top. For this reason too I thought a good vapor impermeable ventilation baffle would be a good idea. If venting low on the roof is really necessary I suppose I could add holes in the sheathing that is now in an attic space and make sure the knee wall is well insulated and sealed and the attic space has some ventilation. But is this really necessary?

I look forward to any feedback and recommendations on this situation. Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, you should read this article and follow its advice: How to Build an Insulated Cathedral Ceiling.

    Second, you might want to read this article: Vapor Retarders and Vapor Barriers. The brief version: In your climate zone, there is no code requirement for an interior vapor retarder. But it's always a good idea to have an interior air barrier (which usually means drywall detailed for airtightness).

    I think it's impossible for readers of your question to be able to visualize how all of your various roofs intersect. You'll have to draw a sketch and post it so we can visualize what you are talking about.

    Q. "The metal roof provides a large continuous ridge vent so the rafters are fully vented at the top, but only at the top. ... If venting low on the roof is really necessary I suppose I could add holes in the sheathing."

    A. Vented roof assemblies need a soffit vent as well as a ridge vent -- in every single rafter bay.

  2. PeteMossPNW | | #2

    Thanks Martin, yes your article is very thorough. I will look up the air barrier/retarder article.
    I've attached some elevations. The lower roof with the triangular gable end windows is the original building with the 2x12 vented ceiling. The addition, built ten years ago by others, is a non-vented roof with some insulation above the sheathing and will be left alone for now. This newer roof covers the eaves on the east side of the older building. The hip for the addition was framed on top of the old roof sheathing. The ridge beam of the addition runs over top of the older building ridge beam, so a vent in the small gable (seen in west elevation) could vent that attic space.
    I hope that clarifies things a little.

  3. GBA Editor
    Martin Holladay | | #3

    It's hard for me to determine whether you can get an air intake at the bottom of each rafter bay for the section of your house which has a vented cathedral ceiling.

    If you can determine that there is some type of air intake at the bottom of every rafter bay -- either a soffit vent or a vent opening connected to a vented attic -- then the roof assembly will work.

    If some of the rafter bays can't have any air inlet, the approach is risky -- and it would be better to make all of these cathedral ceilings unvented.

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