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Air control layer: Sheathing, Sheetrock, or both?

My question on making in ceiling speakers air tight got me thinking more about my air control layer. We are building a new home in climate zone 6A. The insulation strategy for the walls is 2x6 framing on 16" centers. Stud cavity filled with netted and blown fiberglass and 2.5" of foil faced exterior polyisocyanurate foam. The roof strategy has yet to be determined (based on overall budget) but the two options for the 2.5/12 slope un-vented cathedral ceiling are:
A) 14" TJI rafters with 5.5" of ccSPF and 8.5" of netted and blown in insulation
B) 16" TJI rafters with 16" of netted and blown insulation but using a smart nylon vapor retarding membrane called Certainteed Membrain stapled to the underside of the rafters. (I'm going to post a Q on the membrain in another thread).
I've read a fair amount on air sealing and about how difficult it is to ensure an air control layer at the sheetrock which is why I originally planned for my sheathing to be my air control layer. Then I began reading the articles about OSB not being as air tight as everyone thought so I'm planning to specify AdvanTech as my sheathing mainly for it being proven to be airtight but for the other "claimed" benefits too.
However with my wall design I need to make sure my wall can dry to the inside so I was planning latex paint as my vapor retarder. With my vapor retarding layer being painted sheetrock is it true that I now essentially have to ensure an airtight seal on my sheetrock? If I don't have airtight sheetrock will the vapor just get carried through the gaps in the sheetrock bypassing my vapor control layer? Or will this not happen because of my air tight sheathing resulting in no pressure difference between the inside of the sheetrock and the inside of the sheathing so there is no driving force pushing the air past the gaps in the sheetrock?
Fundamentally my question is this: If you have an air control layer at the sheathing and a vapor retarding layer at the sheetrock (latex paint) does the sheetrock need to be air tight as well?

Steve

Asked by Steve Mackay
Posted May 7, 2018 1:01 PM ET
Edited May 7, 2018 1:48 PM ET

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1.

Steve,
The main reason to include an interior air barrier as well as an exterior air barrier is because workmanship is often imperfect, and redundancy helps you achieve your goal.

For example, if there is a crack or a hole in the top plate of a wall -- enough of a defect to allow air to rise into the attic -- and the wall lacks an interior air barrier, then moisture-laden interior air can enter the stud bays at an electrical outlet, and exit through the hole in the top plate. If the moisture encounters a cold surface during its journey through your wall, it can deposit moisture there.

Of course, if your air sealing work is perfect, an exterior air barrier may be enough.

For more information on this issue, see One Air Barrier or Two?

Answered by Martin Holladay
Posted May 7, 2018 1:45 PM ET

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