How do I keep thick ceiling batts from sagging before drywall goes up?

You definitely need mechanical support for R-49 batts overhead—the kraft paper flanges simply aren't engineered to carry that much weight. Either pick up a box of **wire insulation supports** (often called tiger teeth or lightning rods) that wedge between the joists every 16 to 24 inches, or staple up heavy-duty strapping across the joist bays as you go. Trying to rely on kraft paper staples alone on 15-inch-thick fiberglass will tear out every single time.

For the Romex and junction boxes, the pro trick is splitting the batt. Don't try to shove the full thickness over a wire. Take a serrated insulation knife and slice the fiberglass batt horizontally right down the middle, parallel to the paper. Slip the back layer behind the wire, then fold the front half right over it. For junction boxes, actually cut out a plug of insulation to match the box size. Compressing thick fiberglass destroys its thermal performance, so carving it cleanly around obstacles is the only proper way to handle it. Also, face-staple the paper flaps over the joist edges instead of inset-stapling inside the bays. Inset stapling crushes the sides of the batt, which loses you R-value right at the framing, and the paper rips much easier when you're stapling at an angle. Stapling flat across the joist face gives you way more holding power and keeps your vapor barrier continuous. If you've still got half the ceiling left, ditch the ladder and grab a basic rolling scaffold. My brother and I do big garage ceilings like this, and staging heavy R-49 bundles up on a platform where you can stand flat-footed saves your shoulders and cuts your install time in half.

Man this gets me so pumped to finally start my own garage build, seeing R-49 in a ceiling is wild! Quick question though while everyone is talking about garage ceilings—what kind of heating setup are you actualy putting in there once it's all sealed up? Are you doing a mini-split, or just running one of those 240V electric ceiling-mount heaters when you're out there working? I'm planning my layout right now and dying to know if heating a garage with that much overhead insulation ends up being super cheap to run!

Are your ceiling joists actualy 14 to 16 inches deep, or are you attempting to hang a full 15-inch nominal batt inside standard 2x6 or 2x8 dimensional lumber? If the framing depth is substantially shallower than the uncompressed loft of the fiberglass, the batts are going to protrude past the bottom plane of the joists anyway, wich will make hanging drywall later a nightmare unless you intend to compress the material and destroy the stated thermal resistance. Also, if you aren't sheeting the ceiling immediatly, bear in mind that exposed asphalt-saturated kraft paper carries a flame-spread classification well above 25, meaning it technically violates residential code to leave it uncovered without a 15-minute thermal barrier.