R-value doesn’t measure soundproofing at all. It measures thermal resistance, meaning how well a material blocks heat transfer. The confusion is understandable because insulation products are marketed primarily by R-value, and many of those same products also reduce noise. But chasing a higher R-value won’t necessarily give you better sound control, and in some cases, the highest R-value option is actually one of the worst for blocking noise.
The ratings you actually want to look at are STC (Sound Transmission Class) for blocking sound through walls and floors, and NRC (Noise Reduction Coefficient) for absorbing sound within a room. Here’s how those work and which insulation materials genuinely perform best for noise reduction.
Why R-Value Doesn’t Predict Sound Performance
R-value tells you how much a material resists the flow of heat. Sound is mechanical vibration, not thermal energy, so it travels through materials in a completely different way. A material can have an excellent R-value and do almost nothing for sound, or it can have a modest R-value and absorb noise effectively.
Closed-cell spray foam is the clearest example. It delivers some of the highest R-values per inch of any insulation product. But it hardens after application, filling the wall cavity with a rigid material that sound vibrations pass through easily. For STC ratings, spray foam provides little value. Open-cell foam, which has a lower R-value, actually absorbs sound better because air can move through its softer, less dense structure, dissipating sound energy along the way.
The takeaway: if you’re shopping for insulation and soundproofing matters to you, ignore R-value as a proxy for noise control. Look at the acoustic ratings instead.
STC and NRC: The Ratings That Actually Matter
STC measures how well a complete wall or floor assembly blocks airborne sound from passing through to the other side. It’s tested by playing sounds across 16 frequencies (125 Hz to 4,000 Hz, roughly the range of human speech) in one room and measuring how much quieter those sounds are in the adjacent room. A standard interior wall with no insulation typically scores around STC 33 to 36. Adding insulation to that same wall can boost the rating by up to 10 points, which represents a noticeable reduction in how much conversation, TV audio, or music you hear through the wall.
NRC measures how well an individual material absorbs sound within a room, rated on a scale from 0.0 to 1.0. A material with an NRC of 0.80 absorbs 80% of the sound energy that hits it, reducing echo and reverberation. This rating matters most in open spaces like offices, restaurants, and home theaters where you want to control how sound bounces around.
STC applies to full assemblies (the combination of drywall, studs, insulation, and any other layers), while NRC applies to individual materials. Both are relevant to soundproofing, but STC is what most people care about when they want to stop noise from traveling between rooms.
Best Insulation Materials for Soundproofing
Mineral Wool (Rockwool)
Mineral wool is the top performer for combined thermal and acoustic insulation. Made from volcanic rock spun into dense fibers, it delivers roughly R-4 to R-4.2 per inch of thickness while also improving wall assembly STC ratings by up to 10 points. Its density is the key advantage. Denser materials resist sound vibration more effectively, and mineral wool is significantly heavier than fiberglass at the same thickness. It also offers excellent fire resistance, which is why products in this category are often marketed under names like “Sound & Fire Block.”
If you’re building or renovating walls specifically for noise control, mineral wool batts in the stud cavity are the standard recommendation among acoustic consultants.
Fiberglass
Fiberglass is the most common insulation in American homes, and it’s a solid acoustic performer, especially for mid- to high-frequency sounds like voices and television audio. It’s lighter and less dense than mineral wool, so it doesn’t block low-frequency bass and rumble as effectively. But it’s also cheaper and widely available, making it a reasonable choice if you’re balancing noise reduction with budget. Fiberglass and mineral wool both contribute the biggest STC gains among cavity insulation options.
Cellulose
Blown-in cellulose (made from recycled paper fibers) performs moderately for both thermal and acoustic insulation. It can achieve NRC ratings of 0.80 or higher depending on the installation and assembly design, meaning it absorbs a significant amount of sound energy. Its higher density compared to fiberglass batts helps it resist air movement, which contributes to both thermal and acoustic performance. Cellulose is a practical option for retrofitting existing walls because it can be blown into closed cavities without tearing out drywall.
Spray Foam
Closed-cell spray foam is the worst common insulation choice for soundproofing despite having the highest R-value per inch. Once it cures, it forms a rigid bridge between the wall surfaces, making it easy for vibrations to travel straight through. Open-cell spray foam performs better acoustically because of its softer, more porous structure, but it still doesn’t match mineral wool or fiberglass for noise reduction per dollar spent.
What Actually Improves Soundproofing
Insulation in the wall cavity is only one piece of the puzzle, and it’s not even the most impactful one. Several other factors affect STC ratings more dramatically.
- Mass: Adding a second layer of drywall to one or both sides of a wall increases STC far more than upgrading insulation alone. Heavier walls block more sound.
- Decoupling: Using staggered studs or resilient channels prevents sound vibrations from traveling directly through the framing from one side of the wall to the other. This breaks the mechanical path that sound follows.
- Deeper cavities: Using 2×6 studs instead of 2×4 studs creates space for thicker insulation, which boosts both thermal and acoustic performance.
- Air sealing: Sound leaks through any gap. Electrical outlets, gaps around door frames, and unsealed penetrations can undermine an otherwise well-built wall. Even small openings dramatically reduce real-world performance.
A standard 2×4 wall with one layer of drywall on each side and no insulation scores around STC 33 to 36. Adding mineral wool insulation can push that toward STC 43 to 46. But combining insulation with a second layer of drywall and resilient channels can reach STC 50 or higher, which is enough to make loud speech nearly inaudible from the next room.
Choosing Insulation When You Want Both
If you need both thermal and acoustic performance from the same wall, mineral wool is the most efficient single material. It delivers competitive R-values alongside the best acoustic performance of any standard cavity insulation. Fiberglass is the budget-friendly alternative that still performs well on both fronts.
The one thing to avoid is choosing insulation based solely on R-value and assuming the noise reduction will follow. A wall packed with closed-cell spray foam might hit R-25 or higher, but it will let more sound through than a wall with R-15 mineral wool batts. For soundproofing, density and flexibility matter more than thermal resistance. Soft, fibrous materials that trap air and absorb vibration will always outperform rigid materials that conduct it.