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Air vapor barrier ceiling

Smokey059| Posted inGeneral Questionson

My name is Tony I m building in zone 6a.
I want to put plywood on the underside of the bottom cord of the trusses to create a ceiling and an air barrier tied into the wall plywood air barrier. The walls will have taped foil faced polyiso behind the plywood air barrier for the vapor wall barrier. But do I also need to put some taped foil faced polyiso up before installing the plywood air barrier on the ceilng. Or is there some other way to get a vapor barrier for the ceiling. Below the plywood on the ceiling will be a service cavity just like the walls have a service cavity inside the plywood air barrier.
The attic is ventilated with soffits and gable vents. I would like to just blow loose cellulose into the attic between the trusses and on top of the plywood (or polyiso if needed) to R60. I will put sealed vent channels between the rafter bays before blowing in cellulose.
How do I get an effective vapor barrier on ceiling or is it not necessary with a caulked and taped plywood air barrier on the ceiling. Thanks

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Replies

  1. Expert Member
    MALCOLM TAYLOR||#1

    Smokey,

    My understanding is the only place code requires an in interior VB is Canada, and that can be met with a vb paint on the plywood. Code aside, your plywood ceiling is a variable-perm vapour-resistor. Something that performs better thann a vapour barrier on a ceiling. it allows an appropriate amount of moisture to pass through depending on how high the interior humidity level is.

  2. Expert Member
    Michael Maines||#2

    Smokey, to meet the 2015 IRC building code for a vented roof assembly in your climate zone, if the area of your roof vents is at least 1/150th of the area being vented, there is no vapor retarder requirement at the ceiling level. If you use a class 1 or 2 vapor retarder, or if 40% to 50% of the venting are is near the ridge, you can reduce the venting area to 1/300th of the area being vented. You probably won't meet the second option using gable vents, but dry plywood is a class 2 vapor retarder, at roughly 0.5 perms, so you can use it on your ceiling without another vapor-retarding layer.

    1. 用户- 7948890||#9

      Can I sue 3/8" plywood to achieve this ? Seams taped with Wigluv tape.
      I am ASHRAE climate zone 4. I have a well vented low slope 1:12 roof with greater than 1/150th of roof area venting.
      I was going for 1/2" but prices are crazy high these days.

      1. Expert Member
        MALCOLM TAYLOR||#10

        David,

        The only difficulty you may find is that 3/8" plywood is a bit wavy and so may not stay in plane with the adjacent sheets in between the supports - which would mean the tape may not stay adhered. You might need either H-clips or blocking to keep things straight.

  3. Smokey059||#3

    Thanks Malcolm. So should I paint the plywood with a vapor retarder paint or just leave unpainted. The interior side of the service cavity (2x4 uninsulated) below the plywood ceiling will have finished drywall.

  4. Smokey059||#4

    Thanks Michael for that explanation. So I guess I can skip the vr paint

  5. Expert Member
    MALCOLM TAYLOR||#5

    Smokey,

    I think Michael and I both agree you are best to leave it alone.

    I'm not generally a fan of ceiling service cavities. i don't see they offer much when you look at the additional work involved and what they will actually contain. That said, they don't do any harm either.

  6. Expert Member
    Michael Maines||#6

    Correct, you could leave the plywood exposed and unfinished if you want. Or paint or cover with pretty much anything. Vented attics are very forgiving, by code and in real life. Just make sure any penetrations are air-sealed and you should be fine.

  7. Smokey059||#7

    是的,我同意服务蛀牙不提供much but in this case I have high sidewalls 12ft and ill be doing the labor myself and I have to have my home theater speakers somewhere without cutting holes in my air barrier

  8. Expert Member
    MALCOLM TAYLOR||#8

    When you have things like speakers to mount that makes a lot of sense. To me service cavities are often incorporated as an idea with little thought to what they will actually contain.

    Good luck with your build!

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