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Community and Q&A

Ceiling Service Cavity and Mositure

MarkJameson | Posted in Energy Efficiency and Durability on

Hello my name is Mark and I live in eastern Tennessee (4A). I will be building a new home soon and I’m working through a few last details. One concern that I have is that I will have a continuous air barrier on the exterior sheathing (zip sheathing) and will build a service cavity for the ceiling, which will be 2×8’s in most rooms, with zip installed above that, with all seams taped including the top corner of the walls as it transitions from the wall to the attic floor/ top of service cavity. My concern is that of permeability of the zip in the ceiling service cavity in my humid climate and wether plywood, with its somewhat more forgiving management of moisture, would be a better choice?

Also, would it be worth the effort to try and follow a more air tight drywall approach for any penetrations in the ceiling, such as using gasketed electrical boxes to limit any household moisture from accumulating in the cavity?

lastly, above the zip/ plywood air barrier I do plan to use blown in cellulose insulation somewhere in the tune to around r60 and I also plan to have a variable speed ducted HVAC system and balanced ventilation.

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  1. user-2310254 | | #1


    You don't want to put your HVAC and ducts in an unconditioned attic. Is that your plan?

    1. MarkJameson | | #2

      Hey Steve, thanks for your reply. No, i will have a conditioned crawl space where my heating and cooling system will be ducted. The attic will have no storage space, living space or equipment.

      I guess, to rephrase my concern, is having zip sheathing or plywood at the top of a service cavity or the attic floor, whichever way you want to look at it, becoming a vapor barrier in a climate that is best served with no vapor barrier there.

  2. MarkJameson | | #3

    Here is a drawing of the detail in question.

    1. Expert Member
      MALCOLM TAYLOR | | #4


      I'm afraid I don't know enough about building in your climate to c0mment usefully, but the notes on your section caught my eye.

      You should consider omitting both "consult the owner for details" and "flashing as necessary". Both are very problematic notes to have on permit or contract drawings.

      1. MarkJameson | | #5


        I am actually the GC in this case and all of the exterior details will be handled and installed by me but thank you for your feed back.

        1. Expert Member
          MALCOLM TAYLOR | | #6


          I'm not much help at all I'm afraid!

          Good luck with your build.

        2. Expert Member
          BILL WICHERS | | #8

          I would consider adding a callout or detail page for each of those “consult owner for details” details. Even if you’re doing the work yourself, it’s still a good idea to have a complete set of plans with all details included for future reference.


  3. the74impala | | #7

    There isn't a big benefit to the zip over plywood since you shouldn't need to shed water in this area. I don't believe Zip is a vapor barrier, in fact when I asked about something along these lines Huber said don't do it.

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