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Will R-70 cellulose sag ceiling drywall 24" o.c.?

My plans call for an R-70 ceiling of loose-pack cellulose in double-netted bays between trusses 24" o.c., with 5/8" Type X drywall holding it up as my air barrier. My installer says this will weigh about 2.55 lbs/sq/ft. Is this too much? Will it sag over time or pop screws? Do I need to use some special (expensive) low-sag drywall? I don't want to go to the expense and lost clearance of, e.g., an OSB layer above the drywall.

Anyone have experience with this?



Asked by Randy Jorgen
Posted Jul 16, 2017 12:17 AM ET


6 Answers

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I'd strap the ceiling with 1x4 16" o/c and use 1/2" drywall. That's what we did, although we used a membrane under the bottom chord as our air barrier. We blew the cellulose from a port in the gable ends.

Answered by stephen sheehy
Posted Jul 16, 2017 7:04 AM ET


This question comes up often on GBA. Here is a link to a previous thread on the topic:
Is 5/8 drywall sufficient to support R-60 cellulose in a ceiling?

My response to that question back in 2012 was fairly complete, so I will copy and paste that answer below.

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."

Answered by Martin Holladay
Posted Jul 16, 2017 7:46 AM ET
Edited Aug 16, 2017 2:37 PM ET.


Martin, thanks for the link. I read that post a long time ago and have been trying to relocate it. However, it is so far still the ONLY reference I have found to the question anywhere, so I was hoping to find something more recent or confirmation from other builders.

Stephen, I've been hoping to avoid the extra expense of strapping or wood sheathing, and, believe it or not, the loss of 3/4" height may cause us a problem in a couple of places.

Does anyone know about types of low-sag drywall and what their extra expense might be?

Answered by Randy Jorgen
Posted Jul 16, 2017 9:41 AM ET


Hi Randy, we've loose-filled dozens of 24" OC attics to R-60 over a period of 10 years and can report zero issues related to the weight of the cellulose. This includes both new construction and retrofits with older, nailed sheetrock.

Answered by Jon Harrod
Posted Jul 16, 2017 2:57 PM ET


Thanks for your comment, Jon. Your experience has been confirmed by a tech representative at American Gypsum (Bob Ek?): the published 2.2 lbs figures is outdated and they are talking about revising it. He says R-70 will be fine with 5/8 type X 24 o.c., without even increasing screws.

Answered by Randy Jorgen
Posted Jul 21, 2017 12:30 AM ET


We typically use ceiling drywall rather than 5/8". It is as strong, but much easier to install, holds screws better and is only 1/2".

Answered by Malcolm Taylor
Posted Jul 21, 2017 11:36 AM ET

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