Fix a pillowed ceiling
If your ceiling drywall is sagging between joists, sometimes called “pillowing,” it’s probably on the top floor and probably 1/2-in. drywall instead of 5/8-in. Half-inch drywall can sag if it’s hung on framing spaced every 24 in. It isn’t strong enough to handle the span, and the weight of the attic insulation just makes the sagging worse.
You have two choices: Rip out and replace the sagging ceiling, or add spacers and new drywall below it. In either case, you can use 5/8-in. drywall or lightweight, sagresistant 1/2-in. drywall. Ripping out the old stuff might seem like the right approach, but it’s a nightmare job. You have to pry out the ceiling drywall along the edges and pluck out all the old drywall screws. If you’ve ever torn out drywall, you know that without taking extreme measures, the dust goes all over the house. As if that weren’t enough fun, the attic insulation will likely collapse into the room. That in itself can make the decision easy. If the attic insulation contains vermiculite, there’s no question. As noted in the asbestos discussion on p. 80, you don’t want vermiculite floating around your house.
If you can live with a slightly lower ceiling height, you can save a lot of time by installing a new ceiling below the old one. Add furring strips on the trusses, as shown. To make the drywall easier to hang, use 1x3s rather than 1x2s. If your ceiling sags more than 3/4 in., use 2x2s. Screw the 1x3s to the truss framing with 2-1/2-in. drywall screws.
Next, install, tape and finish new 5/8-in. drywall. If you have ceiling fixtures, you’ll have to extend all the boxes to make them flush with the new ceiling. It’s OK to use standard 1/2-in. drywall on ceilings as long as it meets building code in your area. A better option is USG UltraLight Gypsum Board. It’s stronger and lighter and it meets fire code.