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My thoughts for a ceiling air barrier and service cavity

Jerry Liebler | Posted in Energy Efficiency and Durability on

I’ve struggled to find a reasonable cost way to have a good air barrier in the ceiling and really want to protect it with a service cavity. I’ll share my current thoughts and ask for suggestions. I can find nothing better than OSB hung from the bottom chords of deep “energy”heel t trusses, Then, after taped, caulked and otherwise sealed. Followed by screwing 2×4 s cross wise to the trusses 4′ OC. (1 1/2” vertical through the OBB into the to the trusses), then screwing another “layer” of flat wise 2×4’s running parallel to the trusses on 2′ centers, offset from the trusses so more screws can e added if needed. Drywall hangs from the lower layer of flat-wise 2x4s. The 3″ service cavity with no drilling wire in any direction 2 dimensional grid will minimize the trauma to the air barrier and support 18″ of cellulose. A final touch is to glue a strip of foam around the perimeter to dramatically reduce the thermal bridge at the top plates of the perimeter walls. FWIW if I opt to use SIP walls I’ll use a similar “lattice” service cavity on the walls.

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jerry,
    You didn't ask a question, so maybe you are happy with your plan.

    Personally, I think that it would be easier to hang 2x4s on edge from the ceiling air barrier, using Simpson clips. (See photo below).

    But your way would work (as long as the cavity has no ventilation ducts).

    More information here: Service Cavities for Wiring and Plumbing.

    .

  2. Jerry Liebler | | #2

    No ventilation ducts but I'm thinking that it'll serve well as a cold air return "plenum". I was thinking of ducted mini-split head(s) along the mid-line in the basement's ceiling with supply ducts to floor outlets near north and south walls., "Cold air return" through grills into the service cavity and vertical columns in central walls near the head(s). located below the basement ceiling one head for the north half of the house and another for the south (which will get more sun and need more cooling.

  3. GBA Editor
    Martin Holladay | | #3

    Jerry,
    Using a framing cavity as a return-air plenum is strongly discouraged by building science experts. If you depressurize your service cavity, you might be pulling outdoor air into the service cavity through cracks near the rim joist.

    Some building codes, and most green building programs, prohibit the use of framing cavities as return air plenums. Use ducts instead.

  4. Keith H | | #4

    Jerry,

    I'm a DIYer and not an electrican but as a previous owner of a trussed return plenum, I have a few thoughts for you:
    - forget about sound insulation because fibers will end up in the return air (yuck)
    - better hope you never get rodents in there or your return air will both across the relics of their existence for the life of your house
    - if your wire isn't plenum rated, you could create a serious smoke hazard (code violation?)
    - If your wire is plenum rated, it is full of really toxic stuff.
    - your draw is likely to be really subject to shortest path shortcircuiting

    Again, I'm not a pro but I wouldn't build my return that way for my own home.

    Maybe I'm wrong and people commonly do a return of this nature but I'd never want to own it.

  5. Jerry Liebler | | #5

    THANKS! I'll put the return grills low on central walls and use proper metal or "ducktboard" ducts. Essentially all the ducts will be in the basement ceiling with supply runs between the I joist and returns directly below the I joist which is where the ducted indoor units are anyway.

  6. Expert Member
    Malcolm Taylor | | #6

    Jerry, If all the service cavity contains is a couple of fixture boxes and some wire, is it really necessary?

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