Ceiling gypsum wallboard can sag under heavy loads, especially those sometimes created by insulation. One gypsum wallboard manufacturer recommends loads of no more than 1.3 lb/ft2 for V2" ceiling gypsum wallboard with framing spaced 24" on center. The limit increases to 2.2 lb/ft2 for framing spaced 16" on center and for %" gypsum wallboard.2 Cellulose and rock wool insulation manu­facturers usually include weight-limit information on the bag because the thickness required may cause excessive weight limits.

Because fiberglass is much less dense, its weight on ceiling gypsum wallboard may not be a concern.