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Community and Q&A

Assembly for Ceiling Service Cavity

Joseph Dziedzic | Posted in Energy Efficiency and Durability on

I’m considering a ceiling service cavity created by nailing 2x strapping perpendicular to the joist/truss bottom (CZ5).  Intello Plus would be applied to the joist/truss bottoms first as an air barrier, then the 2x strapping would be installed on the joist/truss bottoms, nailed through the Intello.

I see a couple of potential issues: 1) electricians will probably nail wire staples to the joist/truss bottom through the Intello, puncturing the air seal in lots of places, and 2) electricians will try to snake wires under the strapping, potentially damaging the Intello.  (I don’t know if the strapping nailed through the Intello also potentially introduces an air leak.)

I’ve seen some images on the Web where 1x strapping was installed along the joist/truss bottoms first, with 2x strapping installed perpendicular to that.  That approach seems to provide better protection to the Intello along the bottom of the joist/truss, provides a clear path under the 2x strapping, and provides a taller service channel so deeper (2″) electrical boxes can be used.

R-60 loose fill cellulose will be used to insulate the attic, and it seems the 1x strapping along the joist/truss bottoms would provide better protection against the Intello possibly separating from or tearing at the staples.

The alternative of using taped OSB sheets as the air barrier is probably out due to cost.

Does this seem like an appropriate assembly?  Can Intello Plus handle that weight of insulation?  Any guidance would be appreciated!

(FWIW, I live in New England where it’s common practice to install 1×3 strapping across the joist/truss bottom as a drywall attachment surface, so either I substitute 2x material for that strapping OR add 2x material.)

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Replies

  1. Expert Member
    Zephyr7 | | #1

    I think you'd find that the Intello would be prone to tearing, and you'd run that risk every time anything was added to or changed in the service cavity. You'd be much better off using OSB or plywood instead.

    You could also look at insulation support netting, but I don't think any of that can be an air barrier.

    Personally, I wouldn't bother with a service cavity. People ask about service cavities like this on GBA from time to time, and at least some responses always mention that it's not really worth the effort.

    Bill

  2. Expert Member
    Malcolm Taylor | | #2

    Joseph,

    I'm one of the posters Bill refers to as typically responding to threads on service cavities by saying that in most situations I don't see the point. They often end up having nothing or next to nothing in them when used indiscriminately on walls or ceilings.

    However as you are already planning to strap the ceiling, I don't see why increasing the depth of that strapping to hold electrical services wouldn't be a good idea.

    1. DCContrarian | | #3

      In New England it's customary to strap the ceiling with 1x3 or 1x4 and run electric between the joists and the strapping. It probably wouldn't pass code in a lot of the rest of the country but that's the way it's done. I don't see any point in having a deeper service cavity.

      I would do the strapping first, then the wiring, then the intello, then drywall and finally insulation. Intello is between drywall and strapping. Drywall holds the weight of insulation. The only penetrations in the intello are at ceiling lights.

      In the future if you want to add wiring in the ceiling either push the insulation aside, or just run it over the insulation in the attic.

  3. Joseph Dziedzic | | #4

    Thanks for the replies, appreciate the feedback!

    I'm not looking at the "service cavity" as something to facilitate future remodeling, but as more of a means of minimizing protrusions through the ceiling air barrier that will need to be sealed by tradespeople who probably don't go in for detail work.

    If the Intello Plus is installed before the "traditional" 1x strapping and interior partitions that approach seems more likely to result in a reliable air barrier at the ceiling, as compared to trying to create an effective seal at the top plate of interior partition walls when the Intello is installed later. For example, where the partitions are nailed to the 1x ceiling strapping there will be lots of gaps between the partition top plate and the strapping where the Intello will need to be carefully fitted and sealed to the top plate with tape. I'm sure that's possible if the installer takes great care, but I'm not sure I can depend on that level of care being expended.

    OSB does provide a sturdier air barrier, but I still have the issue with fitting ceiling electrical boxes between the air barrier and the drywall, so some sort of strapping is still required: probably 2x to allow space for a 1-1/2" deep box. Either that or recess the boxes into the OSB, but that adds yet another big hole per box to be sealed. (And while cost isn't a primary concern, the material and labor cost to hang ~100 sheets of OSB on the ceiling does factor in to the equation.)

    I do appreciate the feedback, lots to think about going forward.

    1. Expert Member
      Zephyr7 | | #5

      You could recess the electrical boxes into the OSB and then use putty pads behind them to seal them. That should be pretty reliable, and it's easy to do well.

      Your idea with the Intello should work, but it's relatively easy to damage, and other trades probably won't go around taping tears. It's much more difficult to damage a layer of plywood or OSB, so you'll be more likely to maintain that as an intact air barrer.

      It's also desireable to minimize services in the attic space regardless, including wiring. If you don't have anything up there, then you won't have any need for penetrations, and thus nothing to have to seal.

      Bill

  4. Tom Wheeler | | #6

    I have intello under trusses and then box out areaswith 2x and 1/2" plywood I plan to have fixtures at. I am doing my own wiring though, so sealing the penetrations is just normal for me. If any changes come up, it won't involve revealing anything. After all the intello is up, I plan to use 1x to avoid holes going into the membrane. I do not intend to insulate until drywall is up and any holes can be found easier.

    Using 2x would be easier to keep the wires inside except at the partition wall junctions which I have failed to determine a good way to do it without very sharp bends. I will tape the wires from the top for the few wires coming from up there to the wall.

    I could take more pictures tomorrow with more details since these were taken.

    1. Tom Wheeler | | #7

      I still need to caulk some corners. The strapping will go perpendicular, and since I am using puck lights, I will be able to have significant wiggle room where I cut the drywall holes to mount them.

      1. Expert Member
        Zephyr7 | | #9

        Note that you might need nail plates over the wire through holes in some of those ceiling joists (which look like 2x4 truss members?).

        The purpose of the nail plates is to prevent pointy things like drywall screws from pentrating the wood and damaging the insulation of any wiring passing through in shallow holes. Wire damaged this way can be a hidden danger, waiting to surprise you some day. Nail plates are required whenever wiring (or plumbing) passes within 1-1/4" of the edge of a framing member.

        Bill

  5. Deleted | | #8

    Deleted

  6. Randy Williams | | #10

    I've used the 2x strapping on a few projects, I personally like the detail. The homes are designed so that no there are no interior load bearing walls. This way all the strapping can be completed, then partition wall built. The electricians I work with understand what we are trying to achieve and are careful not to poke holes when drilling. There are a few small holes from the staples, but I haven't been able to detect any air leakage around them, I'm sure there is some leakage, just not enough to detect. The only holes in the ceiling air control layer have been plumbing/radon vents and bath fans/HRV ducts. All the homes have slab on grade foundations. We used a plenum truss for any ductwork, heat pump line sets that were needed to be run across the ceilings. Last house ended up at .75ACH50. You can read more here https://www.greenbuildingadvisor.com/article/air-sealing-an-attic-in-a-cold-climate

  7. Jonathan Lawrence CZ 4A New Jersey | | #11

    I have some flat roof sections with exterior insulation and dense pack on the interior. I used Siga Majrex as added insurance to minimize moisture migration into the cellulose. Since I needed to add lights and did not want to have any penetrations into the Siga, I added strapping in a grid pattern using 2x3's. I used the double-sided tape to adhere the Siga to the bottom of the LVL's (due to load issues we had to use LVL's here) then I attached the first layer of 2x along the bottom of the LVL's and the second layer perpendicular. My electrician attached his wiring to the first layer and I used housing free LED's that fit within that service cavity and a center light fixture with a ceiling box attached to the strapping.

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