GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Another try at a ceiling service cavity framing

user-1137156 | Posted in Energy Efficiency and Durability on

I now want the duct ed mini split IDU(s) and ducts within the ceiling therefore what I thought was an economical construction (lattice of flat wise 2x4s) really is not, especially if I’m to have code acceptable ducts. I simply need a taller cavity, 3″ isn’t enough, 6″ is probably minimum. My latest thought is to use a variation of a Larsen truss (2×2 chords and OSB strips on alternating sides forming the “web”) installed horizontally. The “Larsen’s” top chord is fastened through the upper air barrier material (probably drywall) with screws into the bottom chords of the roof trusses. With this construction the height of the cavity has practically no impact on cost and virtually any height presents virtually identical complexity. Different ceiling heights in different rooms, zones or areas can easily be accommodated with one “framing” system by simply using segments with differing web length. Even coffered ceilings. Top chord gaps allow ducts 1 1/2″ less tall than the cavity to run parallel to the ridge line and be supported by the bottom chord,simplifying duct installation. My “inventions” often have unintended consequences, I am far from an objective critic. What do you think? .

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. Expert Member


    That sounds promising. You could leave gaps in the OSB web to allow lateral runs too.

  2. user-1137156 | | #2

    My description was evidently poor, my intention is the web consists of 4"--6" wide strips 48" OC on each side, interleaved so there is a web segment 24"OC on alternating sides. so indeed there is ample place to cross from one to the adjacent. but if the truss has continuous chords top and bottom there is only a 3" gap.However with transitions to and from oval duct such as 6" round, 30 deg el,6" round to 7" oval, oval duct,to next cavity,7"oval to 6" round, 30 deg el. a crossing could be made and should add only 20' of equivalent 6" duct length. It would, however be far less costly to just have gaps in the trusses, Menard's price for the round to oval transitions is about $14 each. BTW sticking with 6" round is very economical at the register end as well, a ceiling boot is $9.50 and a 12x6 register is $8.59. I'm tinking my "extended plenum" will be made of 1" duct board with 8"x26" outside dimensions.

  3. GBA Editor
    Martin Holladay | | #3

    Make a sketch. Photograph your sketch with your cell phone camera, conventional digital camera, or scanner/printer. Upload the photo to GBA.

    Saves a lot of words.

  4. Expert Member
    Dana Dorsett | | #4

    IDU==??? (" InDoor Unit" ??) Why not use the conventions of most manufacturers and call wall coils "heads", and other types "cassettes" (ceilng cassette, slim-duct cassette, etc.)

    "indoor" is one word, not two, which makes it even more confusing.

  5. Jon_Lawrence | | #5


    I don't know if you have started construction yet, but I used floor trusses ( so I can keep my ductwork, plumbing, electrical and ventilation runs in the ceilings. I am very happy with them and I don't think I will ever go back to TJI's. There is one section (under a low-slope roof) where I had to use LVL's, so I will need to build a ceiling service cavity for lighting as well as a soffit for a few branches in that area.

  6. jberks | | #6


    thanks for mentioning the Open Joist triforce trusses. I've been telling myself lately that next house I build I'll use open web joists for easier mechanical work.

    Only thing I noticed is the the Triforce 9.5" joists can only accept a 5" round duct. compared to a 9.5" TJI will do a 6" round duct.

    However, there are only so many holes you can cut in a TJI based on their specs. I suppose if you're using square ducts or many ducts, the triforce looks really good for it.

  7. Expert Member


    I'd avoid trusses if your design can only accommodate a floor system of that depth. The advantages of trusses largely disappear. Their shape is a physical manifestation of the forces that are at work on the upper and lower chords, and the most efficient way to connect them. With a 9.5" structural member you might as well join the chords with OSB or use solid lumber. Trusses gain their advantage by being deep.

  8. user-1137156 | | #8

    I really appreciate the helpful feed back here and I sincerely apologize for using "mystery" shorthand. I will make a sincere effort at clarity.
    Encouraged by Malcolm's positive reception of the thought of using site built parallel chord trusses to form a ceiling service cavity I have " incorporated" them in the " Sketchup" model of my, under construction, house. If Sketchup is not in your vocabulary, it is a FREE 3D CAD program that is very capable but not easily mastered. I'm trying to use Sketchup to do a "virtual" build where mistakes can easily be corrected with a few keystrokes and only cost some of my abundant time. I've been working on "fitting in" HVAC and re-doing my service cavity using the proposed trusses allows it all to fit. Some observations from this effort include: My selected 18kBTU ceiling slim duct ed "heads" are BIG roughly 8 1/4" tall and 25" wide, perpendicular to output stream and 36.3" along the air stream direction with the electrical and plumbing connections on a 36.3"x8.25" side the right side if one is facing in the direction of the air stream. Another observation they must output a really high velocity "jet" The air outlet opening is tiny, only 3 3/16"x 7 7/8" and they deliver over 400CFM with no back pressure. I may be wrong but, to me, the high air velocity of that output "jet" means it'll be noisy and I need to be careful in my "processing" it into duct flow or I well might have noise "issues" in operation.
    My planis to mount the head in a 10 1/2" tall 3'x3' 2" metal box, a return air plenum, open on the underside. The metal box will be surrounded on 3 sides by 3/4" plywood, the 4th side of the metal box abuts the end of a "duct board" extended plenum output duct. the bottom of the metal plenum will be aligned with the bottom of the ceiling drywall. the top of the plywood above the return plenum is attached to the top chord of the "ceiling truss". The bottom chords of two of the trusses are removed so that the bottom of the return plenum can align with the drywall The lower surface of the extended plenum is 2" above the top of the ceiling drywall and supported by the lower chord of the " truss" , in it's "dropped ceiling portion. The extended Plenum is 20"Wx10 1/2"Hx20'long To summarize it all works out, 6" metal branch ducts, about 19''long for heating & cooling and 5 " branches for the ERV collector ducts, which must cross some of the 6" ducts all fit in a 6" deep service cavity deepened to 12" deep over the central hallway. Footnote, I can print a PDF of anything I can see with Sketchup

  9. user-1137156 | | #9

    While I've called them trusses their loading is very different. Their function here is to serve as "" a hanger for the drywall below. Floor trusses or I joist are designed to support loads over a span not transfer a load vertically as in this situation, they may well work but how cost effective?

  10. CMObuilds | | #10

    Instead of using wood, using a metal drywall grid will give you much more room. Armstrong or USG have different systems which can give you a drywall mounting surface at whatever depth "plenum" or service cavity you want. Armstrong Shortspan might be something to look at.

    Installation labor savings will make this a competitive option. Also Menards duct components are very expensive over what an HVAC sub would charge.

  11. Jon_Lawrence | | #11


    We used the 11 7/8" depth trusses so we are able to get up to 7 1/4" round ducts which covers all of our branches. I also have a lot of 4" ducts for the ERV and lots of cabling that I need to install myself and I was not looking forward to drilling all those holes. If you can use 5" ducts, then the trusses can be a real time-saver.

    11 7/8" is always spec'd where I am and the trusses were actually a few cents cheaper than TJI's, and lighter/easier to carry, but probably more expensive then other brands of I-Joists. There are also cheaper trusses that use metal plates, but I like the glued finger-joints of the Triforce trusses. There is a noticeable difference between the vibration in the trusses (less/better) vs the few areas we used TJI's. I think that has a lot to do with the strong backs. My framer was skeptical at first, but now he prefers them to I-Joists.

    Jerry - If this is new construction, then floor trusses can be cost-effective.

  12. user-1137156 | | #12

    Thank You,
    I'll check out both options.
    And Jonathan It is New construction, are you saying that the finger jointed open web joist system is competitive with 11 7/8 I-Joist , which I currently have spec'd for my basement ceiling/main floor & I sure would prefer Open Joist there as well! No dropped ceilings needed with a cavity that is 11 7/8 tall. With a Quasi second floor floor system I would stick with OSB, forget drywall eliminate tapping, instead lay house wrap over that deck prior to setting the trusses, Tape the horizontal house wrap as the air barrier, for a much quicker, simpler,cheaper build. My inner exterior wall would use pre cut studs, maybe even pre cut OSB the outer wall with single top plate and inter wall mineral wool would surround the rim boards FWIW I'm P---D at Microsoft!!! My internet connection has been gone for the past six or more hours, apparently due to a "windows update" shutting off my computer's WIFI adapter..
    Edit: Maybe I can get code approval for the ceiling truss system to be 32"OC or even 48"OC and use flat wise 2x4 strapping on the underside, either 16" OC or 24"OC to hang the drywall, the attic floor is only carrying cellulose.
    Edit 2: The winner is truss system 24" OC with NO STRAPPING labor tips the balance!
    Edit 3: I'm working on exterior double walls that are 10'2" tall over all, including the 1/2" tie plates at the top and bottom, the outer wall uses custom cut studs @ 9'8" tall 16" OC. + a single top and bottom plate The inner wall uses 9' pre cuts 24" O.C. and 9' OSB + 1'1/2" OSB The horizontal OSB joint line is "blocked by" one of the upper plates and the bottom of the sheathing sits upon the lower 1/2" tie plate. The inner wall is broken into 12' segments each segment has:a single 12' bottom plate and two top plates (11'10 1/2 long),6, pre cut studs, a single 9'8 stud at one end, an 11 7/8" wide x 11'10 1/2"long OSB rim board, 3 full sheets of 4'x9' OSB and 3ea. OSB cuts (4'x1'1/2"). The rim board serves the "second" purpose as a header over header less windows etc. The ceiling joist, 24" OC each sit on a stud (cripples over openings) with one per panel next to a 9/8" stud. The "attic floor" air barrier as it's top surface aligned with the top of the upper tie plate because it is 2 3/8" narrower than the lower tie plate and it is made of the same thickness as the attic floor. The interior partitions will use pre cut studs, single bottom plate double top plate PLUS the thickness used for the bottom tie plate.

  13. Jon_Lawrence | | #13


    The Triforce equivalent of the TJI 560 was cheaper for me. I don’t know if that would still apply to other variations of TJI and TJI’s are not the cheapest I-Joists out there. There is always something cheaper, but I would have gladly paid a premium for an open joist product because of the labor and time savings. Another thing I like is that combined with the clip mounted slim LED’s I am going to use (no more cans) I will have easy access to the ceilings in case I need to pull any additional wires/cables after construction.

  14. user-1137156 | | #14

    I sure thank you for the suggestion of the Open Joist. I'll switch over to them for my basement ceiling but stick with I joist for the main floor ceiling service cavity. My basement has a cetner wall so 2 pc of 20 footers work beautifully. However the ceiling service cavity is best done with I joists because What I've called a central hallway is actually a row of internal roomlets with walls that aren't in line including from left to right the foyer, the coat closet, the 1/2 bath, a passage way between kitchen and family room, a pantry, the stairway,the guest bath, the laundry room, the sitting room of the master suite, a walk in closet. With 40' I joist I can't span 40' but it can, equally well span 18',7',15' or 20',20' or 11' 4',10,15' or 13',8',4',15'',which is kinda what I have.

  15. lance_p | | #15

    If I remember correctly, the price difference to step up in thickness with the TriForce product was very reasonable, on the order of 10-12% between an 11-7/8" and 16" product. I'm planning to use either 14" or 16" joists as the cost increase is minimal, and the benefits in joist stiffness and cavity clearance are huge. The strongback construction seems like it would offer vastly increased joist-to-joist stiffness over the average (read: poor) blocking installation, with greatly reduced chances for squeaks to develop.

    Agreed (Malcolm T.) that the 9-1/2" TriForce product seems a little spendy for minimal benefits, but could still be useful for shorter spans where plumbing and electrical work needs to pass.

  16. user-1137156 | | #16

    The metal drywall grid systems are all "suspended ceilings , hung by wires to "structure" above. Typically: At the upper end of the wire it is looped through a "screw eye" and twisted around itself at the lower end the wire is passed through a hole in a "main tee" and secured by wrapping it around itself. the "main tee s are typically spaced 4' apart and run the long dimension of the room the cross tee s are typically spaced 2' OC giving a grid to which the drywall is fastened. at the "beginning and end of they main channels they are supported by angles mounted to the walls.
    Why Not a wood framed suspended ceiling??
    For example use 2x4 s in a similar wire suspended lattice? Flat wise 2x4 s replacing the "main tee" s and being suspended by wires but with screw eyes at both ends and 4' lengths of 2x4, notched at both ends ( a 1 1/2 " h,1 3/4" d notch) serving as "cross tee s" . easily fabricated " shiplap" joints could easily allow arbitrary extension of the " main tee members. Fastened together, after squaring and alignment with screws, driven upward, through the 3 1/2" w, 1 1/2" tall main tee into the 1 1/2" w 3 1/2" tall cross tee s at each end of all of the cross tee s.
    Material cost of this "scheme" is lowest of all options, labor is nearly adds fabricating the "parts" to the metal system,s labor cost, methinks wood wins the total cost comparison, the metal suppliers are pretty poud of their stuff!
    Will I have building code pain if I opt for the suspended wood lattice?

  17. CMObuilds | | #17

    Added weight is one problem of using wood over metal, you would have to account for this in truss design and supply truss designer with your hanger layout.

    Second issue is why reinvent the wheel? Their is no difficulty in attaching drywall to metal over wood, the system is designed to work together for speed of assembly and its straight.

    I dont know if I would trust a regular lag eye for drywall suspension, you may need to go with an alternate hanger attachment or check pullout specs. Only code issues an inspector should care about is weight and attachment method, safety is their concern.

    My ipad isnt letting me enter my name, but I'm a general contractor that builds/remods residential and I work in light gauge steel in commercial interior buildouts, there is no reason it wouldn't be a viable option in residential to accomplish what you're trying to do.

  18. Expert Member
    MALCOLM TAYLOR | | #18

    Depending on the sequence of construction, the suspended ceiling could end up being defined by the code as a concealed space. You would have to fire block on all the stud spaces of the interior walls.

  19. CMObuilds | | #19

    As long as the main ceilIng is drywalled, or fireblocked, that seperates floor or attic space you can have open wall sides above suspended. Silmilar to a kitchen soffit, you can frame a soffit but either the wall or ceiling inside the soffit needs fireblock, not both.

    Code is confusing in that it calls out suspended ceilings requiring blocking at its set height but its assuming open joists above and drywall not installed above suspended height which is rare anyways. Key code language is floor and ceiling level, your suspended isnt really the ceiling, the plane of your truss bottom chord could be considered your seperation.

    There are drawings out there that detail this, mostly soffit sectionals, some from municipalities. Im not aware of any volume restrictions, but for air sealing sake I would hang all rock except suspended, run your mechanicals, fire tape ceiling and walls to suspended height, seal around penetrations then install the ceiling and finish the living volume area. The ceiling wall lap isnt anything to worry about.

    As always check with your code official and make an argument if you have to.

  20. user-1137156 | | #20

    I'm almost convinced that it can be done most cost effectively with 11 7/8" I joist. Using I joist allows the air barrier to be installed from above so it can easily include taped "house wrap" instead of depending on taped seams of OSB or drywall. this should be a big cost saving.too. I will, however, need to put bigger holes in the web than would be allowed in actual flooring applications. Allowing some clearance, so that sections can be added and "slid" in means that for an 8"x?? rectangular metal duct, the " web" will be entirely removed where the duct will be. I'm thinking that it would be best to make, at least these web openings , "on the ground" prior to installing the I joist s, Where the fan coils are the bottom chord will need to be removed from at two least adjacent joist. Rather than cutting just the bottom chords, and depending on the truss above for support, the fan coil locations will use joists cut and terminated in headers. The joists next to the header locations will "pick up" some additional load but since the joist loading, is for an unoccupied " attic" floor (10 PSF live+ calculated dead) they can comfortably handle the load. even without being attached to the truss above them. . . .

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |