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.