Ceiling joists in this addition are 24" on center and the builder wants to blow in R-38 cellulose after we hang.

No, 1/2" drywall on 24" o.c. joists is not adequate for this application.

Code Requirements:

• IBC Table R302.3(1) requires 5/8" gypsum wallboard for ceilings on 24" o.c. framing (vs. 1/2" allowed at 16" o.c.)
• IRC R302.3 has the same requirement

Why 5/8" is needed:

• 1/2" drywall will sag between 24" spans, especially with the weight of dense-pack cellulose (typically 3.5-4 lbs/cu ft)
• Cellulose adds approximately 1.5-2 lbs/sq ft of additional dead load
• Sagging creates uneven surfaces and potential joint failures

Additional considerations:

• Fire safety: Check local fire codes - some jurisdictions require 5/8" Type X on habitable space ceilings
• Vapor control: Install a smart vapor retarder (like MemBrain) on the warm side if required by your climate zone
• Air sealing: Ensure all penetrations are properly sealed before insulation installation

Recommendation: Use 5/8" drywall minimum. Consider 3/4" if spans exceed 25'-0" or if local codes require it.

This is a structural/fire safety issue, not just a finishing detail. The builder's plan to use 1/2" will likely fail inspection and create long-term problems.

No. 24" OC joists require 5/8" Type X (or Type C) gypsum board for ceilings per IRC Table R702.3.5. 1/2" board is only permitted up to 16" OC (or 24" OC only if perpendicular to joists with specific blocking/backing per footnote 'c', which is impractical for blown insulation).

Why this matters for your assembly:

1. Sag: 1/2" board on 24" OC will sag between joists under the weight of R-38 cellulose (~1.5–2.0 psf) + board weight. IRC R702.3.5 limits deflection to L/240; 1/2" fails this.
2. Fire Rating: If this addition requires a fire-resistance rating (e.g., garage ceiling, townhouse separation), 1/2" fails the membrane protection requirement.
3. Insulation Contact: Blown cellulose settles and exerts outward pressure. 5/8" board resists pillowing/popping screws better.

Install correctly:

• Hang 5/8" Type X perpendicular to joists.
• Use 1-5/8" Type S/W screws (not nails) @ 12" OC in field, 8" OC at edges (IRC R702.3.5).
• Stagger butt joints; back-block unsupported ends.
• Vapor Retarder: If Climate Zone 4–8, install Class II vapor retarder (kraft facing on batts or smart membrane under the drywall) before blowing cellulose. Cellulose alone is not a vapor retarder.

Bottom line: Material cost difference is ~$0.15/sq ft. Liability for sag repair or fire code failure is massive. Use 5/8".