Ceiling air barrier with suspended ceiling?
I’m designing a 1-story, wood frame building w/raised heel, wood roof trusses, slab on grade, in zone 5. Does this ceiling system work?
Attic insulation is R-38 kraft-faced fiberglass batt, extending over the exterior walls, with kraft facing down. (vapor retarder)
5/8″ GWB ceiling as air barrier applied to bottom chord of trusses. (also supports batts.)
12″-18″ high plenum space so all HVAC ducts can be inside thermal envelope and accessible. (The HVAC system will go in a mechanical closet on the slab.)
Suspended acoustic ceiling tile.
I’m questioning the cost of the double ceiling, but if I eliminate the GWB, what acts as the air barrier? The suspended ceiling is the desired ceiling finish. Is there a better way to do this?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
The truss can be engineered with a plenum for keeping the ducts inside the conditioned space. You have to use drywall (or another material) to create an air barrier inside the plenum, but it doesn't require any finishing if you intend to close up the plenum.
Laura,
In most Climate Zone 5 locations, building codes call for attic insulation to have a minimum R-value of R-49.
Fiberglass batts are the worst possible choice -- because they don't fill all the voids and are hard to install well. It's really worth switching your spec to R-49 of cellulose.
If the main purpose of the gypsum drywall is to provide an air barrier, 1/2-inch drywall is fine.
Laura,
You might want to read this article: How to Insulate an Attic Floor.
Going with 1/2" OSB rather than GWB for the air-barrier ceiling may be useful, since it supports more load, and is a better attachment surface for the wiring staples, plumbing hangers, duct hangers & other service plenum items. Attached to the truss chords ith ring shank nails OSB can take quite a bit of load without pulling.
Be sure to spec the truss chords for the intended loads. R49 of open blown cellulose will run 1.5-2lbs per square foot, it's denser than fiberglass, but also somewhat more effective. Blown fiberglass would run about half the weight, but lower density fiberglass needs a top-side air barrier to perform as well as cellulose with no top side air barrier. Spec the energy heel for an initial blow of at least 15" to allow for a settled depth of 13.5-14".
The marginal cost of blown R49 instead of blown R38 is remarkably small. It takes only very slightly longer during the installation phase, but the set up & break down time is identical. The total time the crew & equipment stays about the same, with only a marginal uptick in the material cost.
Based on the acoustic ceiling tile, this may be a commercial building, in which case the code requirement is indeed R-38 for a zone 5 building per IECC 2015.
That said, springing for the extra insulation may still be worth it, but it if it's a commercial building, it's probably not required.
Don't see many commercial buildings on here, so that's exciting to see! It's not just houses that we can do better on!
About thirty years ago I was in charge of a radio studio complex where the ceilings were standard "acoustic" drop in tiles. We had considerable problems with mechanical noise from the ducts above. That wasn't a problem when the radio station was a "pop" format where the announcers generally talked over the records, but it became a more significant issue when the station went "jazz".
I replaced those ceilings with drop in panels with real absorbent sound deadening material on the visible side, and sheetrock/OSB cores/backers towards the mechanical equipment above.
These panels had to be custom measured/fabricated where the grid spacing was shorter than the typical 2' at corners and wall edges, for diffusers, etc. And they were a pain to install in the grid as they fit quite tightly. However they did the trick as the sound transmission through those ceilings was significantly reduced. Essentially the same type goal as reducing air flow, we created a barrier and minimized holes.
I see that USG has a commercial line of sheetrock tiles: https://www.usg.com/content/usgcom/en/products-solutions/products/ceilings/acoustical-panels/usg-sheetrock-lay-in-acoustical-ceiling-panels.html Perhaps an option for you, depending on how code and fire officials feel about the level of sealing provided. (These particular tiles appear kind of "utility like" in appearance.)
Take a look here:suspended acoustical ceiling. It is a high quality, fabric covered sound absorbing board designed to reduce unwanted room reflection, flapping and fluttering echoes and providing a more pleasant and accurate listening environment.