Is 5/8 drywall sufficient to support R-60 cellulose in a ceiling?

Michael,
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."