It’s fairly common for cold-climate builders to install a very thick layer of cellulose on an attic floor (that is, above the drywall ceiling of the floor below). Many green builders aim for a ceiling rated at R-60, R-70, or even R-100. When it comes installing cellulose, the usual advice is to “pile it on.”
Cellulose rated at R-60 is about 16 or 17 inches deep. Cellulose rated at R-100 is about 27 or 28 inches deep. That’s a lot of cellulose — so much, in fact, that some builders ask, “Is my drywall ceiling going to sag?”
Recommendations of drywall manufacturers
The biggest manufacturer of drywall in North America, U.S.G. Corporation, provides the following recommendation: “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.”
These recommendations are summarized in the table below.
Readers will note that is table has columns for two types of cellulose: hammer-mill cellulose that settles to a density of 2.3 lbs/ft³, and fiberized cellulose that settles to a density of 1.5 lbs/ft³. The higher density cellulose was sold in the 1960s, ’70s, and ’80s. Newer types of cellulose developed in the early 1990s (so-called fiberized cellulose) have an installed density (in attic installations) of 1.5 lbs/ft³ (shown in the last column of the table).
Should these recommendations be followed?
Many green builders ignore these recommendations from U.S.G., and routinely exceed the recommended weight limits.
In a post on the Q&A pages of GBA, a reader named Jon Harrod noted, “We’ve loose-filled dozens of 24-inch-on-center attics to R-60 over a period…
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