Sound-rated ceiling: crew installed resilient channel across the joists at 24" o.c., then hung double 5/8" with 2-1/2" screws so they'd 'get good bite,' and the recessed can lights were cut in afterward.

Performance Assessment: Marginal to Poor

Your installation has several critical flaws that will significantly reduce sound isolation performance:

Major Issues

1. Resilient Channel Misuse

• Installing at 24" o.c. is acceptable, but the screws penetrating too deeply defeats the resilient channel's purpose. The 2-1/2" screws likely hit the joists, creating a direct structural path that bypasses the resilient channel entirely.
• Resilient channel requires 1-1/4" to 1-1/2" screws to maintain the isolation gap.

2. Recessed Can Lights

• Cutting holes after installation creates air gaps and potential flanking paths. Recessed cans require proper acoustic sealing and IC-rated housings for fire safety.
• This is a significant weak point in any sound-rated assembly.

3. Wall Perimeter

• "Tight to walls" without resilient channel or acoustic caulk creates flanking transmission paths.

Code Compliance

Per IBC Section 1207 and ASTM E90 testing standards, your assembly likely won't meet the STC rating required for sound-rated ceilings. Most jurisdictions require STC-45 minimum for residential and STC-50 for commercial.

Recommendations

• Remove and reinstall with proper screw length (1-1/2" max)
• Seal all penetrations with acoustic caulk
• Consider adding hat channel perpendicular to joists for better isolation
• Use IC-rated airtight recessed housings with proper gaskets

Bottom line: This installation will provide minimal sound reduction beyond a standard ceiling. The resilient channel is effectively bypassed, making it equivalent to direct-mounted drywall.

No. It will fail. You have three critical errors that destroy the assembly's STC rating:

1. Screw Length (Fatal): 2-1/2" screws penetrate the resilient channel, the furring, and the joist. This creates a rigid structural bridge ("short-circuit") bypassing the isolation. Code (IRC R702.3.3 / ASTM C754) requires screws to penetrate the channel flange only—typically 1" to 1-1/4" Type S/W. You must remove all long screws and re-hang with correct length.
2. Perimeter Isolation Missing: Drywall "tight to walls" couples the ceiling diaphragm to the flanking walls. You need 1/4"–1/2" perimeter gap sealed with acoustic caulk (ASTM C919) on all four sides.
3. Recessed Can Lights: Standard IC-rated cans are massive leaks. You must use airtight, IC-rated, STC-rated backer boxes (e.g., Tenmat, Putty Pads) sealed to the drywall before the second layer goes up. Cutting holes after hanging destroys the air seal and mass continuity.

Fix Sequence:

1. Demo ceiling.
2. Verify channel installation (legs down, 24" o.c. max, 6" from walls, splices staggered 4' min).
3. Hang first layer 5/8" Type X with 1-1/4" Type S screws (12" o.c. field, 8" o.c. edges). Do not hit joists.
4. Seal perimeter & all penetrations (cans, boxes, pipes) with acoustic sealant/backer boxes.
5. Hang second layer 5/8" Type X with 1-5/8" Type S screws (12" o.c. field, 8" o.c. edges), staggering seams 24" from first layer.
6. Tape/finish.

Verify: UL Design L500 series or GA File RC-1. If the crew argues, show them the screw tip protruding below the joist in a test bay—that’s the proof of the short.