Mineral wool and fiberglass batts perform equally well for soundproofing when installed in standard wall cavities, both achieving an STC rating of 36 in a typical single-drywall, 2×4 stud wall. The insulation you choose matters less than most people think. What surrounds the insulation, specifically the number of drywall layers, the framing method, and whether you seal air gaps, has a far greater impact on how much sound gets through.
How Soundproofing Is Measured
Two ratings matter when you’re comparing insulation for sound control. Sound Transmission Class (STC) measures how well an entire wall or ceiling assembly blocks airborne noise from passing through to the other side. It’s rated on a scale from 0 to 100, with higher numbers meaning better isolation. An STC of 50 means the assembly reduces sound by about 50 decibels.
Noise Reduction Coefficient (NRC) measures how much sound a material absorbs rather than reflects. It’s expressed as a decimal between 0 and 1. An NRC of 0.80 means the material absorbs 80% of the sound energy hitting it. NRC matters more for controlling echo and reverb inside a room, while STC is the number you care about when your goal is stopping sound from traveling between rooms.
Mineral Wool vs. Fiberglass
These are the two most common choices for soundproofing wall and ceiling cavities, and testing shows they deliver identical STC performance. Johns Manville’s acoustic data puts both at STC 36 in a single-drywall wall with standard 16-inch stud spacing, and both at STC 53 when you double the drywall. The difference between those two numbers isn’t the insulation. It’s the drywall.
Mineral wool is denser and stiffer, which makes it easier to friction-fit into stud bays without sagging. It also has a higher fire resistance rating and slightly better performance at low frequencies due to that added density. Fiberglass is lighter, less expensive, and easier to cut. For pure sound blocking in a wall cavity, the performance gap between them is negligible.
Where mineral wool may have a slight edge is in NRC, particularly at lower frequencies like bass and traffic rumble. Its higher density (typically around 2.5 to 4 pounds per cubic foot compared to fiberglass at 0.5 to 1.5) helps it absorb more low-frequency energy. If your noise problem involves deep bass or heavy footfall, mineral wool is the better pick. For general voice and TV noise, either works. Mineral wool batts run about $2 to $3 per square foot, while fiberglass batts typically cost less.
Blown-In Cellulose
Cellulose insulation, made from recycled paper fiber, offers sound absorption comparable to mineral wool. Research published in BioResources found that loose-fill cellulose achieves absorption coefficients similar to mineral-fiber products, with particularly strong performance at higher frequencies. Its absorption also improves as you increase thickness, and slightly higher moisture content can nudge performance up a bit further.
Cellulose’s main advantage is coverage. Because it’s blown in, it fills irregular cavities, gaps around wiring, and odd-shaped spaces that batts can’t reach. Air gaps are one of the biggest enemies of soundproofing, and cellulose’s ability to conform to every corner of a cavity helps eliminate them. It’s a strong choice for retrofitting existing walls where you can’t remove drywall to install batts. Blown-in insulation costs vary widely depending on the space, with professional installation running roughly $40 to $200 per job area.
The tradeoff is that cellulose absorbs low-frequency sound less effectively than it handles mid and high frequencies. If deep bass is your concern, mineral wool batts in an accessible cavity will outperform blown cellulose.
Open-Cell Spray Foam
Spray foam comes in two types, and only one is useful for soundproofing. Open-cell foam absorbs sound effectively because air can flow through its cell structure, allowing sound energy to dissipate as it passes through the material. It’s softer, thicker, and less dense than closed-cell foam, all properties that help with acoustic absorption.
Closed-cell foam is rigid and airtight, which makes it excellent for thermal insulation and moisture barriers but poor at absorbing sound. Its hard surface reflects sound waves rather than absorbing them. If a contractor suggests spray foam for a soundproofing project, make sure it’s open-cell.
Open-cell foam costs more than batts and isn’t dramatically better at sound absorption. Its real value is in sealing air leaks while also providing some acoustic benefit, making it useful in situations where air sealing and sound control are both priorities, like basement ceilings or exterior walls.
Why the Wall Assembly Matters More
The jump from STC 36 to STC 53 in the Johns Manville data came entirely from adding a second layer of drywall, not from changing the insulation. This highlights the single most important principle of soundproofing: mass and decoupling matter more than what’s inside the cavity.
Adding mass means more drywall, or adding a dense barrier layer. Mass loaded vinyl (MLV), a thin, heavy sheet material weighing about one pound per square foot, adds an STC rating of 27 on its own and is designed to be layered into wall assemblies alongside insulation. At just 1/8 inch thick, it adds significant sound-blocking mass without eating up much space. The ideal installation is directly over studs and insulation, underneath the final layer of drywall.
Decoupling means separating the two sides of a wall so vibrations can’t travel directly through the studs. Staggered stud walls, resilient channel (metal strips that suspend drywall off the framing), and double-stud walls all accomplish this. A decoupled wall with basic fiberglass insulation will outperform a standard wall with premium mineral wool every time.
Choosing the Right Insulation for Your Situation
For new construction or a wall you’re building from scratch, mineral wool batts are the most popular choice among soundproofing professionals. They’re easy to install, stay in place without fasteners, and handle a broad frequency range. Pair them with double drywall or resilient channel for the best results.
For retrofitting an existing wall without tearing it open, blown-in cellulose is your best option. A contractor can drill small holes and fill the cavity, giving you meaningful sound reduction without a full renovation. It won’t match the performance of a properly built new wall, but it’s a significant improvement over an empty cavity.
For a budget project where you want decent sound control without premium pricing, fiberglass batts deliver the same STC performance as mineral wool at a lower material cost. The money you save on insulation is better spent on a second layer of drywall or acoustic caulk to seal the perimeter.
- Best overall for new walls: Mineral wool batts ($2 to $3 per square foot), paired with double drywall
- Best for retrofits: Blown-in cellulose, filling existing cavities without demolition
- Best on a budget: Fiberglass batts, with savings redirected to additional drywall layers
- Best add-on layer: Mass loaded vinyl (1 lb/sq ft), installed between framing and drywall
No insulation alone will make a wall soundproof. The cavity fill absorbs sound energy and prevents the wall from acting like a drum, but the real performance gains come from the full assembly: mass, decoupling, and airtight sealing. Filling a wall cavity with any insulation typically adds only 3 to 5 STC points. Doubling the drywall can add 10 or more. Spend your time and budget on the complete system, not just the insulation inside it.