Soundproofing is one of the most misunderstood topics in construction. Insulation helps — but it's not the whole story. Here's an honest breakdown of what works, what doesn't, and what to actually specify.
Insulation alone won't soundproof a wall. Sound travels primarily through mass (denser = better) and structure-borne paths (studs, joists, flanking). Insulation fills the air cavity and absorbs some airborne sound energy — but a standard 2×4 wall with R-13 and no other treatment still achieves only STC 34–38. That's not great. What insulation can do is meaningfully improve STC in a well-designed assembly — and mineral wool is significantly better than standard fiberglass for this purpose.
STC (Sound Transmission Class) measures how much sound a wall, floor, or ceiling reduces. Higher is better.
| STC Rating | What You Hear | Typical Application |
|---|---|---|
| 33–39 | Loud speech audible | Basic interior partition |
| 40–44 | Loud speech heard as murmur | Standard office partition |
| 45–49 | Loud speech not audible | Better hotel / apartment walls |
| 50–54 | Very loud sounds barely heard | Code-compliant multifamily |
| 55+ | Most sounds inaudible | Recording studios, luxury hotels |
Mineral wool AFB (Acoustical Fire Batts) outperforms standard fiberglass in the same wall cavity. The higher density and open-cell structure absorb more mid-frequency sound energy. In a standard wood-frame wall:
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For floor/ceiling assemblies, IIC (Impact Insulation Class) measures impact sound (footfalls) while STC measures airborne sound. Mineral wool in the joist cavity plus a floating floor system is the standard approach for achieving IIC 50+ in multifamily. Standard fiberglass performs worse in joist cavities for both STC and IIC.
Adding insulation to a wall that has electrical outlets, recessed lights, or other penetrations without sealing those penetrations will dramatically underperform. Air gaps flanking through the structure carry sound regardless of how good the insulation is. Insulation is one layer of a system — not a substitute for good assembly design.
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