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Where should the air barrier go on non-vented cathedral roof?

I am building a small house with a vaulted ceiling and a non-vented roof. I plan to use 2x10 rafters with fiberglass batts and 2 inches of foam on top of the roof sheeting. An asphalt shingle roof will go on top. I'm confused about where the air barrier should go. Drawings I have seen put it right under the roofing shingles and others have it under the foam. Also, what are my options for material for the air barrier. Do I need to do the entire roof with peel-and-stick or is there a cheaper way. I'm in Northern California just 2 miles from the ocean.


Asked by Stephen Houlihan
Posted Dec 12, 2012 4:44 PM ET
Edited Dec 14, 2012 7:58 AM ET


11 Answers

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The roof sheathing is your air barrier. Tape the seams of the roof sheathing. If you have plywood sheathing, tape the seams with 3M All Weather Flashing Tape or Siga Wigluv tape. If you have OSB sheathing, tape the seams with Siga Wigluv tape.

If you have Zip System sheathing, tape the seams with Zip System tape.

Answered by Martin Holladay
Posted Dec 12, 2012 8:22 PM ET


In taping the sheathing you should also consider Pro Clima tapes: TESCON Vana or TESCON No. 1 for both plywood and OSB. You can find them at www.foursevenfive.com

Answered by Ken Levenson
Posted Dec 13, 2012 11:13 AM ET

Answered by John Brooks
Posted Dec 13, 2012 11:21 AM ET


Wouldn't an air barrier also be required on the warm side of the insulation?

Answered by Ron Keagle
Posted Dec 13, 2012 1:15 PM ET


It's not required, but it's certainly a good idea.

Answered by Martin Holladay
Posted Dec 13, 2012 1:43 PM ET


It may not be required by code, but batts need air barriers on both sides of the fiber to resemble their ASTM rated performance both winter & summer. The "warm" side of the insulation isn't the same side all the time and convective loss of performance WOULD occur in winter if the bottom side is all fluff, no facer or other air barrier. The northern CA maritime zone is still a heating dominated climate.

Answered by Dana Dorsett
Posted Dec 13, 2012 3:42 PM ET


I'm confused, Martin. In your article "How to Build an Insulated Cathedral Ceiling" you state in the text that an air barrier is required under any fluffy insulation, but in the drawing thart goes along with the article there is a note "Do not use an interior vapor barrier. Is this one of those "a vapor barrier is not an air barrier" confusions?

In my case I will have sheet rock directly below the insulation but do I need to make it "air tight"?

Answered by Stephen Houlihan
Posted Dec 13, 2012 3:56 PM ET


You are correct.
1. There is a difference between an air barrier and a vapor barrier.
2. You should make every effort to ensure that the drywall under your insulation is airtight.

Answered by Martin Holladay
Posted Dec 13, 2012 4:14 PM ET


I have a similar query. I m possessing home in Berkeley, CA. It has one room on the second floor designed as a "sleep porch" except that it as an actual room with a cathedral ceiling. It has several large sliding windows but no roof venting at all. The roof was replaced a few years ago with asphalt shingles directly on plywood. I assume it had cedar shingles originally. The room is almost unusable since it is cold in the morning and by noon is unbearably hot. I am told to re-install cedar shingles over plywood roof decking, use sleepers to raise the shingles. The big question is how can I improve ventilation? Is it even possible?

Answered by Jenny Belman
Posted Dec 14, 2012 5:54 AM ET


Your roof doesn't need ventilation, because (by the sounds of it) your roof doesn't even include any insulation.

If you have no plans to install insulation, stop worrying. Everything is fine -- you don't need ventilation. The roof sheathing dries inward.

However, if the fact that the room is "unbearably hot" during the summer bothers you, it may be time to insulate the roof assembly. If that's the case, you can create either a vented insulated roof assembly or an unvented insulated roof assembly -- your choice. To learn how to insulate a cathedral ceiling, read this article: How to Build an Insulated Cathedral Ceiling.

Answered by Martin Holladay
Posted Dec 14, 2012 7:48 AM ET


Hi Martin,
Thanks for your prompt response and helping me in resolving my issue. I have also gone through the article and found it very informative in guiding for removing and replacing insulation for cathedral ceiling.

Answered by Jenny Belman
Posted Dec 17, 2012 5:12 AM ET

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