Soundproofing
Mineral Wool for Soundproofing: What It Can and Can't Do
How mineral wool insulation reduces noise between rooms, studios and ADUs, why it's noncombustible, and what else a wall needs to actually block sound.
Short answer: mineral wool is one of the best insulations to put inside a wall for sound. It's dense, absorbs sound well and is noncombustible. But insulation is only one part of a quiet wall. To really cut noise you also need mass (drywall), separation between the two sides, and every gap sealed.
What mineral wool is
Mineral wool (often called rock wool) is made from stone and slag spun into fibers. A common product for interior walls, ROCKWOOL Safe'n'Sound, comes in 3-inch batts with a nominal density of about 2.5 pounds per cubic foot, denser than typical fiberglass.
Why it helps with sound
- Absorption: ROCKWOOL lists a noise reduction coefficient of 1.10 for the 3-inch product, so it absorbs almost all of the sound energy bouncing around inside the wall cavity.
- Density: denser insulation does a better job damping mid and low frequencies than light, fluffy insulation.
- Fire safety: it's rated noncombustible under ASTM E136, with flame spread and smoke indexes of 0.
What insulation can't do on its own
Sound travels through the wall framing and through any gap, however small. A wall full of insulation with an outlet back-to-back, a gap under the door or a single layer of drywall will still leak noise. For real results we combine:
- Mineral wool in the cavity
- Mass: extra or heavier drywall
- Decoupling: resilient channel, sound clips or staggered studs, so the two sides of the wall aren't rigidly connected
- Sealing: acoustic sealant at edges, outlets and penetrations
Where we use it
- Home studios, podcast rooms and home theaters
- Bedrooms next to living areas, and between floors
- ADUs and units sharing a wall
- Commercial spaces, like a sound-isolation job of ours in West Hollywood
- Exterior walls on fire rebuilds, where a noncombustible material is a plus
Mineral wool or spray foam?
Open-cell spray foam also absorbs sound and adds an air seal, which helps with noise leaking through gaps. Mineral wool is noncombustible and usually the better pick between rooms. On many jobs we use both. See our spray foam page.
Common questions
Is mineral wool better than fiberglass for soundproofing?
It's denser, which helps it absorb more sound inside the wall, and it's noncombustible. But insulation alone won't soundproof a wall; mass, decoupling and sealing gaps matter just as much.
Is mineral wool fireproof?
Products like ROCKWOOL Safe'n'Sound are rated noncombustible under ASTM E136, with flame spread and smoke developed indexes of 0.
Can you add mineral wool to an existing wall?
The best results come when the wall is open, during a remodel or new build. For finished walls, we'll look at what's possible before recommending opening them.