Mineral Wool (Rockwool) Insulation: R-Value, Cost & Complete Guide

Quick Answer: Mineral wool (Rockwool) delivers R-3.8 to R-4.3 per inch, won't burn until 2,150°F, and provides the best sound absorption of any standard insulation (NRC 1.00–1.05). It costs 40–70% more than fiberglass at $1.00–$2.10/sq ft installed versus $0.30–$1.50 for fiberglass. It's hydrophobic, mold-proof, and won't settle or sag. If fire safety, soundproofing, or moisture performance matters for your project, mineral wool is the material to reach for.

Table of Contents

• What Is Mineral Wool?
• R-Value Performance
• Rockwool Product Line
• Fire Resistance — The Headline Advantage
• Sound Performance
• Moisture & Mold Resistance
• Installation Experience
• Cost Analysis
• Mineral Wool vs Fiberglass
• Mineral Wool vs Cellulose
• Best Applications for Mineral Wool
• Common Mistakes
• Key Takeaways
• FAQ

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What Is Mineral Wool?

Mineral wool insulation is made from basalt rock and recycled steel slag, melted at approximately 3,000°F and spun into dense fibers — the same basic process as making cotton candy, but with molten rock instead of sugar. The resulting product is roughly 3× denser than fiberglass (~8 lb/ft³ versus 0.5–1.8 lb/ft³ for fiberglass), which gives it its distinctive stiffness and superior fire, sound, and moisture properties.

Rockwool (the company, formerly Roxul) is the dominant brand name — so dominant that "Rockwool" and "mineral wool" are used interchangeably in the industry, the way "Kleenex" stands in for tissue. Other manufacturers produce mineral wool, but Rockwool controls the North American market for residential products.

The material contains a minimum of ~37% recycled content (recycled steel slag), with some production lines reaching up to 75% including post-industrial recycled content. It's GREENGUARD Gold certified for low VOC emissions — one of the strictest indoor air quality certifications available.

Mineral wool is not the same as fiberglass. Both are fibrous insulations, but the raw materials, manufacturing process, density, and performance characteristics are fundamentally different. The distinction matters — and it's why mineral wool commands a premium.

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R-Value Performance

Mineral wool delivers R-3.8 to R-4.3 per inch, with Rockwool Comfortbatt averaging approximately R-4.2 per inch. That's 10–35% higher per inch than standard fiberglass (R-3.0–R-3.7/inch), though comparable to high-density fiberglass.

In standard wall cavities, the difference is meaningful:

In a 2×6 wall, mineral wool's R-23 versus standard fiberglass R-19 is a 21% improvement in the same cavity — without changing the framing or adding exterior insulation. For climate zones 3–4 where the 2021 IECC (Table R402.1.3) requires R-20 walls, R-23 mineral wool in a 2×6 cavity exceeds code without continuous exterior insulation.

Mineral wool's R-value is also temperature-stable — it performs consistently across the full range of temperatures your home will experience. Unlike polyiso (which loses up to 25% of R-value in cold per Building Science Corporation research), mineral wool delivers its rated R-value regardless of climate. The DOE's insulation guide lists mineral wool alongside fiberglass and cellulose as recommended insulation for all climate zones. See our R-value per inch ranking and R-value insulation chart for full comparisons.

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Rockwool Product Line

Source: Rockwool product catalog.

About Safe'n'Sound's R-value: Rockwool does not publish an official R-value for Safe'n'Sound. Their FAQ confirms: they do not brand interior-only insulation with R-value information because R-value is not required for products used solely in interior partitions (these walls have no thermal requirement in the energy code). The material composition is identical to Comfortbatt, so the estimated R-value is ~R-4.3 per inch — but do not specify Safe'n'Sound by R-value. If you need a rated R-value for an interior wall (uncommon), use Comfortbatt instead.

Comfortboard 80 is mineral wool in rigid board form, designed for continuous exterior insulation over wall sheathing. At R-4.2 per inch, a 1.5-inch board delivers R-6.3 — enough to satisfy the "5ci" requirement in climate zones 3–4. It's an alternative to rigid foam boards (polyiso, XPS, EPS) with the added benefit of non-combustibility and vapor permeability.

Pro Tip: Comfortboard 80 is one of the most underused products in residential construction. It provides continuous exterior insulation that breaks thermal bridging — just like rigid foam — but it's non-combustible (FSI 0, SDI 0), fully vapor-permeable (~30+ perms), and allows the wall to dry in both directions. We've been specifying it on every new-construction project in fire-prone areas and moisture-sensitive climates.

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Fire Resistance — The Headline Advantage

Mineral wool's fire performance isn't just "good" — it's in a different category from every other common insulation material.

Mineral wool won't ignite, won't contribute fuel to a fire, and produces zero smoke. It maintains structural integrity above 2,150°F — well above the typical residential fire temperature of 1,100–1,200°F. Fiberglass is also non-combustible but melts at 1,300–1,500°F, losing its form and R-value in a fire. Cellulose is technically combustible and depends entirely on its borate treatment for fire resistance. Spray foam is combustible and requires a thermal barrier (½" drywall) per building code.

Where fire resistance matters most:

• Garage-to-house shared walls (fire separation assemblies)
• Multi-family shared walls and floor/ceiling assemblies
• Wildfire-prone areas (WUI zones)
• Around fireplaces, chimneys, and flue pipes
• Any UL-rated fire wall assembly

We specify mineral wool in every fire-rated wall and floor assembly. There is no substitute at this price point. See our insulation fire safety guide for complete fire rating data.

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Sound Performance

Mineral wool is the default choice for sound control in residential construction, and the data backs it up.

NRC (Noise Reduction Coefficient) of 1.00+ means the material absorbs essentially all sound energy that hits it. Fiberglass at 0.85–0.95 is good; mineral wool is measurably better. The STC (Sound Transmission Class) numbers tell the story in real assemblies — a standard 2×4 wall with Safe'n'Sound achieves STC 45–52 depending on the complete assembly, versus STC 36–39 for fiberglass.

Interestingly, closed-cell spray foam is one of the worst materials for soundproofing despite being the most expensive. Its rigid, dense structure transmits vibration rather than absorbing it. We've had homeowners spend $5,000+ on closed-cell spray foam in shared walls expecting better sound isolation and getting worse results than a $1,200 mineral wool install.

Where sound performance matters:

• Shared walls in multi-family buildings
• Home theater rooms
• Bedrooms adjacent to living spaces or mechanical rooms
• Home offices
• Between floors in multi-story homes

For complete sound isolation strategies, see our soundproofing guide.

Pro Tip: For maximum sound isolation, don't just swap insulation — build a proper sound-rated assembly. The best residential approach: staggered-stud or double-stud wall, mineral wool in the cavity, resilient channel on one side, two layers of drywall on at least one side. This can achieve STC 55+ — enough to block normal conversation completely. The insulation is one piece of the puzzle, but it's the piece most people can easily upgrade.

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Moisture & Mold Resistance

Mineral wool is hydrophobic — water beads off the fibers and drains through rather than being absorbed. If the material does get wet (construction exposure, a roof leak), it dries quickly and retains its R-value once dry. Compare this to fiberglass, which is not hydrophobic, traps moisture in its air spaces, and can lose 40%+ of its R-value when wet.

Mineral wool is also highly vapor-permeable at ~30+ perms (unfaced), allowing moisture to pass through freely. This means it doesn't trap moisture in wall assemblies — it allows drying in both directions. In wall science terms, mineral wool is a "reservoir-safe" material: it won't hold water and it won't block drying.

Per ASTM C1338 testing, mineral wool supports zero fungal growth. It's inorganic (rock and slag) with no food source for mold. Even when wet, mineral wool itself won't grow mold — though any organic material adjacent to it (wood framing, drywall paper) can still develop mold if moisture persists.

This moisture profile makes mineral wool a strong choice for:

• Bathroom walls (steam exposure)
• Exterior walls in all climates (allows drying)
• Applications where construction moisture is a concern
• As exterior continuous insulation (Comfortboard 80) — fully drainable

For more on moisture management in wall assemblies, see our vapor barrier guide.

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Installation Experience

Mineral wool is DIY-friendly — arguably more so than fiberglass for someone who's never installed insulation before.

Friction-fit: Mineral wool's density and stiffness means it holds itself in the cavity without fasteners or staples. Cut it slightly oversized (¼ inch wider than the cavity), press it in, and it stays. No sagging, no slumping — even in ceiling cavities where gravity would pull fiberglass batts out.

Cutting: Use a serrated bread knife or a purpose-built insulation knife. The material scores and snaps cleanly. It's denser than fiberglass, so cutting around electrical boxes and plumbing takes more effort, but the cuts are cleaner.

No-itch: This is a real quality-of-life difference versus fiberglass. Mineral wool fibers are larger and less prone to skin penetration. We still recommend long sleeves and an N95 respirator (any fibrous material is a lung irritant), but the intense skin itching that fiberglass causes is largely absent with mineral wool.

The downside: Mineral wool's density and rigidity make it harder to fit around irregular obstructions. Fiberglass batts are flexible — you can (imperfectly) push them around a wire. Mineral wool requires you to cut a precise notch. This takes more time but produces a better result.

Weight: Mineral wool is significantly heavier. A bundle of R-23 Comfortbatt weighs about 40 lbs versus about 15 lbs for an equivalent bundle of fiberglass. Over a full day of installation, the weight difference is noticeable.

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Cost Analysis

Mineral wool costs 40–70% more than fiberglass. Here's the honest math on whether the premium is justified.

On a 1,500 sq ft home with ~1,000 sq ft of exterior wall, switching from fiberglass to mineral wool adds roughly $500–$1,500 to the insulation budget (depending on regional pricing and installer). That buys you higher R-value (+15–21%), dramatically better fire and sound performance, and superior moisture handling.

When the premium is justified:

• Fire-rated assemblies (no alternative at this price point)
• Soundproofing applications (measurably superior NRC/STC)
• Moisture-prone installations (bathrooms, exterior walls, coastal climates)
• Personal preference (no-itch installation, peace of mind)
• Wildfire zones (non-combustible exterior cladding system with Comfortboard 80)

When fiberglass is fine:

• Pure budget projects where every dollar counts
• Hidden cavities where fire and sound performance don't matter
• Interior partition walls without acoustic requirements
• Large-scale new construction where the cost premium multiplied across the whole house exceeds the value

Use our insulation cost calculator for a project-specific comparison and see insulation cost per square foot for regional pricing.

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Mineral Wool vs Fiberglass

The most common comparison we get asked about. Here's the summary:

For the full comparison with specific application recommendations, see fiberglass vs. mineral wool.

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Mineral Wool vs Cellulose

This comparison matters most for wall applications, where both materials compete:

Cellulose delivers R-3.5–3.8 per inch (dense-pack) and is the standard for retrofitting existing walls via drill-and-fill at $1.50–$3.00/sq ft. Mineral wool batts deliver R-3.8–4.3/inch but require open cavities — they're not an option for retrofit without removing drywall.

For new construction walls with open cavities, mineral wool offers better fire resistance (non-combustible vs treated-combustible), better sound (NRC 1.0+ vs 0.80–0.90), and hydrophobic moisture handling versus cellulose's hygroscopic behavior. Cellulose offers higher recycled content (80% vs 37%) and better air sealing properties (dense-pack fills around obstructions completely).

For the detailed comparison, see cellulose vs. mineral wool.

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Best Applications for Mineral Wool

Where We Recommend Mineral Wool

Exterior walls (new construction) — Our default recommendation for 2×4 and 2×6 walls when budget allows the premium. R-15 and R-23 Comfortbatt products exceed standard fiberglass R-values in the same cavity, plus you get fire, sound, and moisture benefits.

Fire-rated assemblies — Garage-to-house walls, multi-family shared walls, any UL-listed fire assembly. Non-negotiable choice — the fire performance is unmatched.

Between floors and ceilings — Impact noise (footsteps) and airborne noise (voices, TV). Mineral wool's density absorbs both better than any alternative in this price range.

Shared walls / home theaters — Safe'n'Sound in the cavity, combined with resilient channel and double drywall, achieves STC 50+.

Bathrooms — Hydrophobic material that won't hold moisture from steam or occasional splashing. Much better than fiberglass in this application.

Exterior continuous insulation — Comfortboard 80 on the exterior of wall sheathing breaks thermal bridging while remaining non-combustible and vapor-permeable.

Where Other Materials Might Be Better

Open attics — Blown-in cellulose or fiberglass at $0.60–$2.00/sq ft is significantly cheaper for achieving R-49–R-60. Mineral wool batts would work but at 11.4–12.9 inches for R-49, the material cost is substantially higher.

Budget-constrained projects — If the extra $500–$1,500 per 1,000 sq ft pushes the project over budget, fiberglass gets the job done.

Retrofit walls (without removing drywall) — Dense-pack cellulose is blown through small holes without demolition. Mineral wool batts require open cavities. See insulate walls without removing drywall.

Irregular cavities with lots of obstructions — Mineral wool's rigidity makes it harder to fit around clustered wiring and plumbing. Dense-pack cellulose flows into every gap.

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Common Mistakes

1. Specifying Safe'n'Sound for thermal applications. Safe'n'Sound is designed and marketed for interior acoustic walls. It has no official R-value rating. For exterior walls where you need a rated R-value for code compliance, use Comfortbatt. The materials are similar, but code officials want to see rated products in the thermal envelope.

2. Expecting mineral wool to air seal. Like fiberglass, mineral wool batts do not create an air barrier. You still need to air seal penetrations, top plates, and gaps before installing mineral wool. The material's density is better at resisting air movement (wind washing) than fiberglass, but it is not an air seal.

3. Paying the premium where it doesn't matter. Installing mineral wool in a hidden interior wall that doesn't need fire rating or sound control is spending 40–70% more for benefits you'll never use. Save mineral wool for applications where its fire, sound, or moisture advantages matter, and use fiberglass where they don't.

4. Not cutting around obstructions. Mineral wool's stiffness makes it tempting to force-fit around wiring and plumbing rather than taking the time to cut precise notches. Force-fitting creates voids and compression — the same Grade III installation problems that plague fiberglass. Take the extra 30 seconds per obstruction to cut the batt properly.

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Key Takeaways

• Mineral wool delivers R-3.8–4.3 per inch (Comfortbatt ~R-4.2/inch) — 10–21% better than standard fiberglass in the same cavity depth.
• Fire resistance is mineral wool's standout feature: non-combustible to 2,150°F, FSI 0, SDI 0 — the only common insulation with a perfect fire rating.
• Sound absorption (NRC 1.00–1.05) leads all standard insulation types. Safe'n'Sound achieves STC 45–52 in optimized assemblies.
• Hydrophobic — water beads off, dries quickly, retains R-value when wet. Zero fungal growth per ASTM C1338.
• Costs 40–70% more than fiberglass ($1.00–$2.10/sq ft vs $0.30–$1.50). The premium is justified for fire-rated assemblies, soundproofing, and moisture-prone applications.
• Comfortboard 80 (rigid board form) provides non-combustible, vapor-permeable continuous exterior insulation at R-4.2/inch.
• Safe'n'Sound has no official R-value rating — use Comfortbatt for any thermal envelope application.
• Mineral wool does NOT air seal. Always pair with air sealing at penetrations and top plates.
• DIY-friendly: friction-fit, no-itch, cuts with a serrated knife. Heavier and stiffer than fiberglass.

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FAQ

Is Rockwool the same as mineral wool?

Rockwool is a brand name — the largest manufacturer of mineral wool insulation in North America. "Mineral wool" is the generic product category, which also includes products from other manufacturers. All Rockwool products are mineral wool, but not all mineral wool is Rockwool. In practice, the terms are used interchangeably in the US market because Rockwool dominates residential availability. Other mineral wool producers include Thermafiber (Owens Corning) and Knauf.

Does Safe'n'Sound have an R-value?

No official R-value. Rockwool does not rate Safe'n'Sound for thermal performance because it's designed and marketed for interior acoustic applications, and building codes don't require R-value ratings for interior-only products. The material composition is identical to Comfortbatt, so the estimated performance is approximately R-4.3 per inch — but you cannot use Safe'n'Sound to meet energy code R-value requirements. If you need a rated product for an exterior wall or any thermal envelope application, use Comfortbatt instead.

Is mineral wool worth the extra cost over fiberglass?

It depends on the application. For fire-rated assemblies — absolutely, there's no comparable alternative. For soundproofing between rooms — yes, the NRC difference (1.0+ vs 0.85) is audible. For standard exterior walls — it's a judgment call. The 40–70% cost premium buys you 10–21% more R-value in the same cavity, plus fire, sound, and moisture benefits. On a typical home, that's an extra $500–$1,500 total. Many homeowners find that worth it for the combined benefits; others don't. If budget is tight, fiberglass with good installation quality is perfectly adequate for thermal performance alone.

Can I use mineral wool in my attic?

Yes — Comfortbatt R-30 (7.25" thick) fits between attic joists. For higher R-values, you'd stack layers: two layers of R-30 gives R-60. However, blown-in cellulose or fiberglass achieves R-49–R-60 at lower cost for attic floor applications because you're filling an open horizontal space where depth is unlimited. Mineral wool batts make more sense in attics when you're insulating the rafter bays (for a conditioned attic) rather than the attic floor, since their stiffness holds them in place between rafters without support. For attic floor insulation on a budget, blown-in materials are more cost-effective. See our attic insulation guide.

Is mineral wool safe to install?

Mineral wool is one of the safest insulation materials to handle. It does not contain formaldehyde binders (Rockwool products are GREENGUARD Gold certified). The fibers are larger than fiberglass and cause significantly less skin irritation — most installers work with mineral wool without the intense itching fiberglass causes. That said, any fibrous insulation produces airborne particles when cut, so we still recommend an N95 respirator and safety glasses during installation. Long sleeves and gloves are good practice but less critical than with fiberglass. It's the most comfortable insulation material to work with.