- Q.I am planning to build with
12 inches or more of blown-in cellulose insulation in
the ceilings. How much insulation can you put in a
ceiling before the unsupported drywall between joists
sags? I usually use 1/2-inch drywall with trusses 24
inches on-center.
A.> According to the USG
Gypsum Construction Handbook, cellulose
and rock wool insulation weigh about 2.5 to 3
pounds per cubic foot. Fiberglass weighs about 1
pound per cubic foot. With 1/2-inch drywall applied
perpendicular to the trusses, the allowable weight
load is 1.3 pounds per square foot. This is equal
to about 6 inches of cellulose insulation. With
5/8-inch drywall, the allowable load is 2.2 pounds
per square foot, or about 10 inches of
cellulose.
I have seen many ceilings with 5/8-inch drywall
over a 24-inch on-center framing with 12 inches of
loose fill insulation that did not have problems.
But if you are planning to use much more than this,
the drywall should be hung on strapping spaced 16
inches on-center.
Most sagging in drywall ceilings is caused by
improper nailing or moisture. The primary strength
of drywall is in the paper facing. As the paper
becomes wet, the strength is drastically reduced.
Even condensation on the back of the sheet or
excessive use of water-based texture finishes can
cause problems.