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Rigid air barrier options below scissor trusses?

aaronbeckworth | Posted in General Questions on

Which of the following would be the best option for between T&G pine and bottom chord of scissor trusses? The house will be a simple ~1000 ft^2 cottage with a vented gable roof located in CZ4B mixed/dry.

1) Zip sheathing
Taped Zip sheathing will be used for the exterior walls as the primary air barrier. I doubt there will be many arguments in favor of Zip sheathing on the ceiling.

2) 1/4 in plywood
Would 1/4 in plywood make a durable air barrier above T&G pine? Would the Zip system tape adhere as well to plywood as to Zip sheathing?

3) drywall
My concern with drywall is that the ceiling air barrier needs to be installed for the initial blower door test, prior to wall insulation and drywall installation.

Are there other options?


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  1. Expert Member
    Peter Engle | | #1

    Any of those approaches would work. But first, why are you so concerned about it being rigid? There are plenty of air barrier film materials on the market for just this sort of situation. Many of them are tough enough to stay in place during a blower door test. Once the T&G ceiling is on, it will support the weight of the insulation, and the insulation will hold the membrane in place against the ceiling. This is done all the time.

    If you really need a rigid material, drywall is the cheapest form of sheet goods. It is a very good air barrier if properly detailed, and the details are easy to do. Add a coat or two of vapor barrier latex paint and it also provides vapor control. Plus, you get some additional fire resistance for the ceiling assembly, which is not a bad thing.

    Most inspectors (public and private) will work with you on phasing materials in order to do your testing. While it does potentially mean two trips from your drywall contractor, you could install the drywall and make it airtight as a part of your building envelope work prior to installing insulation and the rest of the drywall. Once the blower door testing and rough inspections are done, you can go ahead and insulate and install the rest of the drywall.

    That said, there are plenty of examples on this site and others from people using plywood and/or OSB for interior air/vapor control. ZIP would probably be better than regular OSB - some OSB has been found to not be tight enough to pass stringent blower door testing.

  2. aaronbeckworth | | #2


    I realize drywall would be a good, cheap option. However, the drywall must be mudded and tapedwhich would require two visits from the drywall contractor. Definitely an option, but 1/4 in ply is fairly inexpensive and can be easily installed by the framing crew.

    I also like the idea of 1/4 in plywood baffles for attic ventilation.

    I have no arguments against sheet air barriers, but I’m not interested in those options for this project.

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