ADU Sound Control in San Diego: STC 50, IIC 50, and What It Takes to Get There
California requires ADUs to meet STC 50 and IIC 50 sound ratings. Here's what those numbers mean, which assemblies achieve them, and what owner-builders need to verify before cover inspection.
Why Sound Control Matters for ADUs
When an ADU is attached to or located above/below a primary dwelling, California Civil Code §1932 and CBC Section 1207 require that separation assemblies meet minimum sound isolation ratings. The two key metrics are: STC 50 for airborne sound (voices, TV, music) and IIC 50 for impact sound (footsteps, dropped objects) in floor-ceiling assemblies.
STC vs IIC — What Each Measures
| Rating | Measures | Typical Sources | Min. Required |
|---|---|---|---|
| STC | Airborne sound through walls and floors | Voices, TV, music, HVAC | STC 50 |
| IIC | Structure-borne impact noise through floors | Footsteps, furniture, dropped objects | IIC 50 |
Important: STC and IIC are independent ratings. A floor assembly can achieve STC 50 but fail IIC 50. Both must be met for floor-ceiling assemblies between ADU units. Wall assemblies only need STC 50.
What Achieves STC 50 in a Wall Assembly
A standard single-layer drywall partition (STC ~33) falls far short of STC 50. Reaching STC 50+ requires one or more of these strategies:
✅ RC Channel (Resilient Channel)
RC channel decouples drywall from framing, breaking the direct sound vibration path. Typical assembly: 2x4 or 2x6 framing at 16" o.c. | RC-1 resilient channel at 24" o.c. | 5/8" Type X gypsum on resilient side | R-13 batt insulation | 1/2" or 5/8" gypsum on primary dwelling side. Properly installed: STC 52–58. Critical: any short-circuit (screw through channel into stud, drywall touching framing at perimeter) collapses isolation. Full guide: What Is RC Channel and What Does It Actually Do?
✅ Double-Stud Wall
Two separate stud walls with an air gap — no shared framing. Highest performance but requires 4–6" additional wall thickness. Typical STC 58–65 depending on insulation and drywall layers.
✅ Staggered Stud Wall
Single 2x6 plate, alternating 2x4 studs on opposite edges. Moderate performance: STC 45–52. May not achieve STC 50 consistently without additional layers or insulation.
⚠️ Common Failure: Short-Circuiting RC Channel
The most common reason RC channel assemblies fail to achieve their rated STC is a rigid connection between drywall and framing that bypasses the channel — screws missing the channel, drywall contacting framing at plates, electrical boxes nailed through to studs, or pipes rigidly attached to drywall. Every point of rigid contact significantly reduces STC performance.
What Achieves IIC 50 in a Floor-Ceiling Assembly
- Carpet and pad: Most effective single upgrade — adds 20–30 IIC points. Hard floors (tile, hardwood, LVP) transfer much more impact noise.
- Resilient underlayment: Acoustic underlayment beneath hard flooring (cork, rubber, specialty foam) significantly improves IIC vs. bare subfloor installation.
- RC channel on ceiling below: Decouples drywall from floor joists, reducing structure-borne impact transmission.
- Additional mass: Extra layers of gypsum on the ceiling below improve both STC and IIC.
- Insulation: Batt insulation in the joist cavity improves STC more than IIC, but contributes to overall assembly performance.
Ref: 2022 CBC 1207.3
Airborne vs. Impact Sound — Quick Reference
| Problem | Type | Primary Fix |
|---|---|---|
| Hearing neighbor's TV through the wall | Airborne (STC) | RC channel, double drywall, double-stud wall |
| Hearing footsteps from unit above | Impact (IIC) | Carpet/pad, resilient underlayment, RC channel ceiling |
| Hearing music through the floor | Airborne (STC) | Additional ceiling mass, insulation, RC channel |
| Hearing children running above | Impact (IIC) | Underlayment + RC channel ceiling combination |
Owner-Builder Note: Verify Before Cover
⚠️ Plan Compliance Is Your Responsibility
San Diego inspectors verify assemblies match approved plans during the cover inspection — they do not field-test STC or IIC. Install the assembly specified in your approved plans. If your plans specify RC channel at 24" o.c. with 5/8" Type X and R-13 insulation, that is what must be installed before sign-off. If you are unsure what your approved plans specify, review your permit documents before framing and before insulation. Changes to separation assemblies typically require a plan revision. Our full ADU checklist: ADU Drywall Owner-Builder Checklist
Related Resources
Building an ADU in San Diego? Get the Separation Wall Right.
SGP Drywall installs STC and IIC-compliant separation assemblies for ADUs throughout San Diego County — including RC channel, double-layer, and fire-rated configurations.
Call (619) 806-2169