Soundproofing with Insulation: What Actually Works

How sound moves between rooms

Sound transfers two ways: airborne (voices, TV) and structure-borne (footsteps, slamming doors). Insulation helps with airborne sound by absorbing energy inside the wall cavity. It does very little for structure-borne sound — that needs decoupling.

Best insulation choices for sound

Mineral wool batts (Rockwool Safe'n'Sound): top performer for STC, dense and inherently sound-absorbent

Dense-packed cellulose: nearly as good as mineral wool and seals cracks better

Open-cell spray foam: better than empty cavity, worse than mineral wool

Fiberglass batts: budget option, ~3-4 STC points less than mineral wool

What insulation can't do alone

An empty wall is around STC 33. Adding mineral wool gets you to STC 39 — noticeable but not dramatic.

Combine insulation with double drywall, Green Glue between layers, or a staggered-stud wall to hit STC 50+ (where conversations become barely intelligible through the wall).