Best way to suspend a ceiling for service cavity under air barrier?
I’m working on a single story house design. I would like to make a continuous taped osb (Zip likely) air barrier on the bottom side of the roof truss under my insulation. I would like to suspend the drywall 6+ inches below the air barrier to make a service cavity for sewer vents, ducts, and recessed lights. What would be the most economical way to construct this?
A couple ideas came to mind:
A: 2x lumber somehow attached to the trusses.
B: Ceiling joists supported by the interior walls.
C: Wide steel studs screwed to the trusses.
D: z-furring screwed to the trusses.
I don’t like option B because the joists are span limited and the interior walls become structural. (It does have the perk of decoupling the drywall from truss uplift, though.) Option A seems tough to hang. Options C and D look pretty easy. However, I haven’t heard of studs or z-furring being used to drop a ceiling, and I would be concerned about their strength and reliability.
Any input on these solutions? Is there a better (less expensive, lower environmental impact) way that I’m missing?
–John
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Replies
Presumably you've considered and discarded the conventional suspended ceiling tile option widely used in commercial construction. That would provide a true serviceable service cavity, but making it look good for residential use seems challenging.
One thing I don't like about permanent service cavities is that it's hard to find and seal air leaks in the buried air barrier with a blower door test. This problem can be mitigated by doing a blower door test after the air barrier is in and before the service cavity gets closed, which then imposes other requirements on the construction sequence, so that the building is at least somewhat ready to test at that point.
I agree that C and D sound good but I don't think my opinion about that is worth much; others will have more relevant expertise.
I don't like the look of most of the conventional suspended ceilings that I've seen. The ones that look good probably also cost a pretty penny. However, researching those ceilings let me to a few suspended gypsum systems.
The natual gypsum construction guide has some good info on suspended gypsum. It looks like I can hang the drywall from a true suspended ceiling, using the details on page 99. This would give a very usable service cavity.
https://nationalgypsum.com/resources/construction-guide/NGCConstGuide.pdf
USG and Certainteed also have drywall suspension systems that might be easier and take less space, but they probably cost more.
https://www.usg.com/content/usgcom/en/products-solutions/products/ceilings/drywall-suspension-systems/drywall-suspension-system.html
http://www.certainteed.com/products/ceilings/by-material/suspension-systems/344943
John,
That might be a good option to pursue. I'd be inclined to only drop the ceiling where it was needed. It could well turn out to be an architectural feature in some spaces. Varied ceiling heights add interest and complexity to houses. Remember that the interior walls that penetrate or surround the dropped ceiling have to be fire blocked, as according to the code it is a concealed space.