Is 5/8 drywall sufficient to support R-60 cellulose in a ceiling?
user-1140356 | Posted in Green Building Techniques on
Is 5/8 drywall screwed and glued 24″ on center sufficient to support R-60 cellulose in an attic ceiling?
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Here's what U.S. Gypsum, the manufacturer of Sheetrock, has to say about this issue (see page 4 of the document): "To prevent objectionable sag in new gypsum panel ceilings, the weight of overlaid unsupported insulation should not exceed: 1.3 psf for 1/2"-thick panels with frame spacing 24" o.c.; 2.4 psf for 1/2" panels on 16" o.c. framing (or 1/2" Sheetrock brand interior gypsum ceiling board, sag-resistant on 24" o.c. framing); 2.2 psf for 5/8" panels on 24" o.c. framing."
So how much does settled cellulose weigh? It depends on who you talk to. It used to be said that the old hammer-mill cellulose settled to a density of 2.3 pounds per cubic foot. But Bill Hulstrunk from National Fiber claims that newer types of cellulose settle to about 1.5 pound per cubic foot.
So, how much cellulose will a drywall ceiling support if we follow the USG recommendation? Let's do the math:
1/2" Sheetrock ceiling, 24" o.c. - Max. weight of insulation, 1.3 psf (6.75 inches of hammer-mill cellulose, or maybe 10.25 inches of newer varieties of cellulose)
1/2" Sheetrock ceiling, 16" o.c. - Max. weight of insulation, 2.4 psf (12.5 inches of hammer-mill cellulose, or maybe 19 inches or newer varieties of cellulose)
5/8" Sheetrock ceiling, 24" o.c. - Max. weight of insulation, 2.2 psf (11.5 inches of hammer-mill cellulose, or maybe or maybe 17.5 inches of newer varieties of cellulose).
Here's the next question: should we follow the USG guidelines? Bill Hulstrunk doesn't think so. He says, "We have never seen a sagging issue due to the weight of the cellulose installed above a ceiling. That may be because some of the weight of the cellulose is being redistributed onto the ceiling joists. We have blown very high R-values, up to R-100, and never had any issues with the ceiling sagging."
I am with Martin, and if concerned just strap your ceiling to 16 or even 12 inches on center and double screw. And the wider the strapping the more load carried by the strapping even.
Thanks for the information. I think "never an issue" trumps the math in this case.
Michael Arnold: How did it work out? I'm having trouble finding installers for dense pack in my 24" o.c. 12 " deep i-joist ceiling. Im using 5/8 drywall..but i want to insulate first...then install the drywall.
The usual method for cathedral ceilings is to install drywall (or continuous rigid foam) on the interior side of the ceiling before installing the cellulose.
For more information, see How to Install Cellulose Insulation.
In that article, Bill Hulstrunk advised: "If you have a 12-foot ceiling span, you’ll be accessing the ceiling from one 3-inch-diameter hole in the center of the room, so you can blow 6 feet in either direction. Once you reach the desired density, 3½ pounds per cubic foot, you can’t push a tube back through the material any more, because it’s too dense. That’s one way to tell if you’ve reached the density you’re aiming for. But if there is a weaker area, you will be able to move the tube toward that low-density area."
I've dense-packed cathedral ceilings through sheetrock and pre-sheetrock. My personal preference is pre-sheetrock because it takes away the stress of popped screws and bowed panels.
If you want to go the pre-sheetrock route, I'd recommend installing an air barrier membrane (Intello Plus, MemBrain, etc.) to the bottom on the I-joists to contain the cellulose. I'd also recommend 1x or 2x strapping at right angles to the joists. This supports the membrane and provides a flat surface for sheetrock, since the dense pack will tend to bulge out.
It can also be done with an air-porous net product such as InsulWeb, though at 24" o.c. gravity will be working against you; unless you install strapping the sheetrockers may have some trouble getting the panels in place. If your climate requires a vapor retarder, you can use a vapor retarding primer on the sheetrock.
My drywall ceilings have been up for 5 years and there is no sag. I think strapping is a good idea to alleviate problems with bulging if you use netting to hold the cellulose before installing drywall but I think that some professionals use a big metal rolling pin to push the cellulose back into place. I wish my installers had done this as they had trouble pushing the bulge back to get drywall into place. I also think that the cellulose gains some rigidity as it coalesces thus supporting itself between the joists.
I am very happy with my cellulose insulation.
When you glued and screwed your ceiling drywall, did you omit a poly ceiling vapor barrier/retarder? Did you use vapor retarder paint on the ceiling drywall?
I ask because I'm trying to figure out this detail myself in my current new construction house.
I was always planning on stapling up vapor retarder (Certainteed Membrain) to the underside of the ceiling studs prior to gluing and screwing up the drywall. When I took a moment to think, I realized gluing would be a waste in this scenario since it would only be glued to the surface of the vapor retarder.
I too will be adding R-60 cellulose to the ceiling, and I wonder if Bill Hulstrunk's successful experience of adding R100 to the ceiling without any sagging was always using glue with the screws.