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Constructing a ceiling service cavity?

Jerry Liebler | Posted in Energy Efficiency and Durability on

My plan includes a ceiling service cavity. The primary air barrier will be the top of the service cavity, the interior visible ceiling will be the bottom. Is there any reason that drywall can not be used both places? My 3″ tall service cavity will be framed by a lattice of flat wise 2x4s, consisting of an upper layer perpendicular to the trusses and a lower layer parallel to the trusses all screwed together..

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jerry,
    Q. "Is there any reason that drywall cannot be used both places?"

    A. No.

  2. David Goodyear | | #2

    Although I agree with Martins answer to a certain extent, the full picture may be necessary in order to assess the situation. How do you plan on tying the primary air barrier continuously to the walls? is there a vapor retarder above and how is it tied continuously through to the vapor retarding barrier in the walls? I have never dealt with the air tight drywall approach but I would think that relying on drywall attached to the bottom of trusses could lead to long term troubles. I assume the joins would be taped and plastered. I would expect, with effects such as truss uplift, that the taped joins could crack and open eventually. A solution which involves redundancy will likely provide better long term results. Should one assembly fail, the other is there to back it up. It is fairly common in passive house buildings to use OSB as an air barrier and a vapor retarder. The OSB is nailed to the underside of the trusses, the joins are sealed with acoustical sealant and covered with some sort of self adheared membrane or tape. I have built 2 houses (code built) and now working on #3 (passive house). My experience is that using polyethylene is a pain, sealing the joins is a pain, getting it flat is a pain. I hate using the stuff. Sealing sheet goods (OSB in my case) is much easier. Having a degree of flexibility built into the joins with redundancy will provide long term performance.

    I am not sure about your local code but our code only requries a 1.5" service cavity. Wiring was sufficienty easy and the electricians didn't require any more space. This would save half of the work involved as well as the materials.

    You will have to excuse my ignorance but I have very little knowledge of the air tight drywall approach if this is what is being proposed.

  3. GBA Editor
    Martin Holladay | | #3

    David,
    Neither OSB nor drywall is foolproof. When used as a ceiling air barrier, each has advantages and disadvantages. But I would rather use drywall rather than OSB.

    OSB is heavy; I think it's easier to install 1/2-inch drywall than OSB on a ceiling.

    It's easier (and cheaper) to tape the seams of drywall with paper tape than to tape the seams of OSB in an airtight manner.

    Finally, some brands of OSB aren't even airtight. For more on this last problem, see “Is OSB Airtight?”

  4. David Goodyear | | #4

    Martin, you're right! OSB is heavy, and was much easier to apply using a drywall lift! I have seen the article on the air tightness of OSB...and even though I did a lot of research about the brand available locally, there was a degree of uncertainty that plagued me through this project wondering and worrying about the day of the blower door test...luckily it was all good!

  5. Jerry Liebler | | #5

    All, THANK YOU!
    Martin, you didn't mention the cost of materials where drywall is about 1/3 the cost of OSB just for the sheets and way less expensive for the sealing materials. In addition there are way more competent dry wall contractors than trustworthy air sealers of OSB.
    David, The 3" , two layer lattice, allows low profile duct work in the cavity.

  6. Lance Peters | | #6

    1. 1/2" drywall is cheaper than OSB
    2. Typical mud/paper drywall joints can crack

    Given these two points along with the fact that Jerry is planning a service cavity where the upper layer will be an unseen air barrier, could the more flexible/durable joining method typically used on OSB (caulk and tape) be used on drywall panels to take advantage of drywall's lower cost without the risk of joints splitting and allowing air leakage in the future? These joints would be permanently hidden, unlike the joints in a regular ceiling, so any problems would go undetected.

    My concern over drywall joints splitting comes from the lack of flexibility in drywall compound combined with the potentially high loads involved with a deep cellulose installation. I know, I know... drywall doesn't sag under cellulose, but that doesn't mean it's not under stress that could potentially harm a compound and tape joint over time.

    I'm taking interest in this conversation as I'm planning a service cavity in my ceiling as well as R80 cellulose attic floor insulation. I was considering using the Zip sheething system for my ceiling air barrier due to its strength and high air tightness when installed, though that would come at a much higher cost. I would not consider 1/2" drywall for this application, so the cost comparison will be relative to 5/8" drywall, which is almost twice the cost of 1/2".

  7. Lance Peters | | #7

    David G., looking forward to the results of your blower door testing!

  8. Expert Member
    Malcolm Taylor | | #8

    Jerry,

    if you have "a lattice of flat wise 2x4s", don't you end up with a series of 20 1/2" squares 3" deep on the ceiling connected to one another by 1 1/2" gaps? Will your low profile ducts fit in that small a depth?

  9. Jerry Liebler | | #9

    Malcolm,
    The lattice is " interrupted", not installed, where there are ducts. The upper layer (perpendicular to the trusses ) is on 3' centers. Allowing a 32 x3 "extended plenum (parallel to the ridge line) and 3 x20" branches (parrallel to the truss chords). While the upper layer of lattice framing helps support the attic insulation it is not needed and can be removed without peril. Where the supply plenum is the ceiling drywall is dropped an additional 6 1/2" and supported by other framing attached to interior partitions" so the lattice supports nothing in this area it is occupied,however by, ERV ducts.

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