A.Corresponding Editor Paul Fisette responds: According to drywall manufacturers, 1/2-inch drywall can be installed on ceiling framing spaced at 24 inches on-center. However, I have known 1/2-inch drywall to sag when installed on 2-foot centers. For a better installation, either use 5/8-inch drywall, or first strap the ceiling with 1x3s installed at 16 inches on-center.
I don’t think that the difference in density between cellulose and fiberglass is enough to make any difference in the drywall sagging.
to read the full article.