Fiberglass vs Mineral Wool (Rockwool): Complete Comparison (2026)
Fiberglass vs Mineral Wool (Rockwool): Complete Comparison
Fiberglass is the most-used insulation in America. Mineral wool (Rockwool) is the fastest-growing. They compete directly in every batt application — walls, floors, ceilings — and the comparison comes down to one question: is mineral wool's superior fire, sound, and moisture performance worth the 40–70% price premium?
Quick Answer: Mineral wool beats fiberglass on R-value per inch (R-3.8–4.3 vs R-3.0–3.7), fire resistance (survives 2,150°F vs melts at 1,300°F), sound absorption (NRC 1.0+ vs 0.85–0.95), moisture handling (hydrophobic vs traps moisture), and durability (50+ years vs 15–30 years effective). Fiberglass wins on cost ($0.30–$1.50 vs $1.00–$2.10/sq ft). For standard residential walls on a tight budget, fiberglass with good installation quality is fine. For fire safety, soundproofing, moisture-prone areas, or long-term durability, mineral wool is worth the premium.
Table of Contents
- Master Comparison Table
- R-Value Comparison
- Fire Resistance
- Sound Performance
- Moisture Handling
- Installation Experience
- Cost Analysis
- Durability & Lifespan
- Upgrade to Mineral Wool When / Fiberglass Is Fine When
- Real Project Cost Comparison
- Key Takeaways
- FAQ
Master Comparison Table
| Property | Fiberglass Batt | Mineral Wool (Rockwool) |
|---|---|---|
| R-Value/Inch | R-3.0–R-3.7 (std), R-3.7–R-4.3 (high-density) | R-3.8–R-4.3 |
| R-Value in 2×4 (3.5") | R-11 (std) or R-15 (high-density) | R-15 (Comfortbatt) |
| R-Value in 2×6 (5.5") | R-19 (std) or R-21 (high-density) | R-23 (Comfortbatt) |
| Installed $/sq ft | $0.30–$1.50 | $1.00–$2.10 |
| Cost Premium | Baseline | 40–70% more |
| Density | 0.5–1.8 lb/ft³ | ~8 lb/ft³ (3× denser) |
| Fire: Combustible? | No | No |
| Fire: Temperature Rating | Melts at 1,300–1,500°F | Survives >2,150°F |
| Fire: FSI / SDI | ≤25 / ≤50 | 0 / 0 |
| Sound (NRC) | 0.85–0.95 | 1.00–1.05 |
| Sound (STC, standard wall) | 36–39 | 43 (Comfortbatt), 45–52 (Safe'n'Sound) |
| Moisture | Traps in air spaces, loses R when wet | Hydrophobic — water beads off, retains R |
| Vapor Permeability | ~30+ perms (unfaced) | ~30+ perms (unfaced) |
| Mold | Inorganic, no food source — but trapped moisture risks mold on adjacent surfaces | Zero fungal growth per ASTM C1338 |
| Installation | Flexible, itchy, requires PPE | Friction-fit, no-itch, cuts with bread knife |
| Settling | None (batts) | None |
| Recycled Content | 40–60% | ~37%+ |
| GREENGUARD Gold | Some products | Yes (Rockwool) |
| Effective Lifespan | 15–30 years | 50+ years |
R-Value Comparison
In the same cavity depth, mineral wool delivers more R-value:
| Cavity | Fiberglass (Standard) | Fiberglass (High-Density) | Mineral Wool | MW Advantage vs Standard FG |
|---|---|---|---|---|
| 2×4 (3.5") | R-11 (R-13 common) | R-15 | R-15 | +15% (vs R-13) |
| 2×6 (5.5") | R-19 | R-21 | R-23 | +21% |
| 2×8 (7.25") | R-25 | — | R-30 | +20% |
Mineral wool's R-3.8–4.3/inch consistently outperforms standard fiberglass (R-3.0–3.7/inch). High-density fiberglass closes the gap — R-15 in a 2×4 matches mineral wool exactly — but high-density fiberglass costs more than standard, narrowing the price advantage over mineral wool.
In a 2×6 wall, the difference between R-19 (standard fiberglass) and R-23 (mineral wool) is meaningful. For climate zones 3–4 where code requires R-20 walls (per 2021 IECC Table R402.1.3), mineral wool's R-23 exceeds code without exterior continuous insulation — potentially saving $1,500–$3,000 on rigid foam. The R-value per inch chart ranks every insulation material.
Winner: Mineral wool — 15–21% more R-value in the same cavity depth versus standard fiberglass.
Pro Tip: In zones 3–4, a 2×6 wall with R-23 mineral wool meets the R-20 code requirement without adding continuous exterior insulation. Standard R-19 fiberglass in the same wall doesn't — requiring either an upgrade to R-21 high-density or the addition of R-5 exterior foam. When you factor in the avoided cost of exterior foam, mineral wool's 40–70% premium over standard fiberglass often pays for itself in cold-climate wall assemblies.
Fire Resistance
This is the comparison's most dramatic differentiator.
| Property | Fiberglass | Mineral Wool |
|---|---|---|
| Flame Spread Index (FSI) | ≤25 | 0 |
| Smoke Development Index (SDI) | ≤50 | 0 |
| Temperature Rating | Melts at 1,300–1,500°F | Survives >2,150°F |
| Combustible? | No (non-combustible) | No (non-combustible) |
Both are non-combustible and Class A rated. But the gap in real-world fire performance is enormous. Fiberglass melts at 1,300–1,500°F — in a structural fire (typically 1,100–1,200°F), fiberglass insulation can lose its form and R-value. Mineral wool maintains structural integrity above 2,150°F — well above any temperature it would see in a residential fire.
Mineral wool's FSI/SDI of 0/0 is unmatched among common insulation materials. It contributes zero flame spread, zero smoke. Fiberglass at FSI ≤25/SDI ≤50 is good — but "zero" is in a different category.
Where this matters most: Fire-rated wall and floor assemblies, garage-to-house shared walls, wildfire-prone areas (WUI zones), around fireplaces and chimneys, and multi-family construction. For fire-rated assemblies, mineral wool is the default specification — there is no comparably priced substitute. Building Science Corporation recommends mineral wool for all fire-sensitive wall assemblies. Complete fire data at our insulation fire safety guide.
Winner: Mineral wool — decisively. The only common insulation with FSI 0 / SDI 0.
Sound Performance
Mineral wool provides measurably better sound absorption than fiberglass in every test metric.
| Metric | Fiberglass Batt | Mineral Wool (Comfortbatt) | Mineral Wool (Safe'n'Sound) |
|---|---|---|---|
| NRC | 0.85–0.95 | 1.00–1.05 | 1.00–1.05 |
| STC (standard wall) | 36–39 | 43 | 45–52 |
NRC 1.0+ means mineral wool absorbs essentially all sound energy that strikes it. Fiberglass at 0.85–0.95 is decent — but the difference is audible to most people, particularly with mid-frequency sounds like voices and TV audio.
The STC numbers are more telling: in a standard 2×4 stud wall, mineral wool delivers STC 43 versus fiberglass at STC 36–39. That 4–7 point gap translates to a noticeable reduction in sound transmission. Safe'n'Sound in an optimized assembly (resilient channel, double drywall) achieves STC 45–52 — enough to block normal conversation.
Where sound matters: Home theaters, shared walls between bedrooms and living spaces, bathrooms, home offices, and especially multi-family shared walls and floor/ceiling assemblies. The soundproofing guide covers optimal assembly designs.
Winner: Mineral wool — measurably superior NRC and STC ratings.
Moisture Handling
Mineral wool is hydrophobic. Water beads off the fibers and drains through rather than being absorbed. If mineral wool gets wet (construction exposure, a leak), it dries quickly and retains its R-value once dry. Per ASTM C1338 testing, mineral wool supports zero fungal growth — it's inorganic with no food source for mold.
Fiberglass is not hydrophobic. It traps moisture in the air spaces between fibers. Wet fiberglass can lose 40%+ of its R-value until it dries. The material itself is inorganic (won't support mold growth), but trapped moisture against wood framing or paper-faced drywall absolutely will.
Both materials are highly vapor-permeable (~30+ perms unfaced), allowing moisture to move through the assembly. This is good — it means neither traps moisture vapor in a properly designed wall.
Winner: Mineral wool — hydrophobic behavior, maintains R-value when wet, zero mold risk on the material itself.
Pro Tip: For bathroom walls (interior partition between bathroom and adjacent room), we've switched entirely to mineral wool. Steam from showers periodically wets the insulation — mineral wool sheds it and dries within hours. Fiberglass in the same application holds moisture for days, promoting mold growth on the drywall paper facing. The $30–$50 upgrade per bathroom is worth it for moisture and sound performance alone.
Installation Experience
Fiberglass: Flexible blankets that cut easily with a utility knife. Light weight (~15 lbs per bundle). The flexibility is a double-edged sword — easy to stuff into cavities, but also easy to stuff poorly. Fiberglass causes intense skin itching and requires full PPE (long sleeves, gloves, N95 mask, safety glasses). Installation quality varies enormously — RESNET Grade I vs Grade III is a 30%+ performance gap.
Mineral wool: Dense, rigid batts that friction-fit into cavities without fasteners. Heavier (~40 lbs per bundle). Cuts with a serrated bread knife or insulation knife — cleaner cuts than fiberglass. Significantly less itchy than fiberglass (larger fibers, less skin penetration). The rigidity means batts hold themselves in place — even in ceiling cavities — but also makes fitting around wiring and plumbing harder. You have to cut precise notches rather than flexing the material around obstructions.
Both are DIY-friendly. Mineral wool's friction-fit and reduced itch make it arguably easier for first-time installers, despite the heavier weight and less forgiving fitting around obstructions.
Winner: Tie — different tradeoffs. Mineral wool is more comfortable to work with; fiberglass is more forgiving around obstructions.
Cost Analysis
Fiberglass is the cheaper material. The question is whether mineral wool's advantages justify the premium.
| Material | Installed $/sq ft | Cost for 1,000 sq ft of wall |
|---|---|---|
| Fiberglass R-13 (2×4, standard) | $0.30–$1.50 | $300–$1,500 |
| Fiberglass R-15 (2×4, high-density) | $0.40–$1.70 | $400–$1,700 |
| Mineral wool R-15 (2×4, Comfortbatt) | $1.00–$2.10 | $1,000–$2,100 |
| Fiberglass R-19 (2×6, standard) | $0.30–$1.50 | $300–$1,500 |
| Mineral wool R-23 (2×6, Comfortbatt) | $1.00–$2.10 | $1,000–$2,100 |
The premium ranges from 40% (mineral wool vs high-density fiberglass) to 70%+ (vs standard fiberglass). On a 1,500 sq ft home with ~3,000 sq ft of wall area (exterior + interior), the total difference is approximately $2,000–$5,000. Project-specific numbers at our insulation cost calculator.
Winner: Fiberglass — significantly cheaper. The value question is whether mineral wool's benefits (fire, sound, moisture, durability) justify the premium for your specific project.
Durability & Lifespan
Mineral wool's density and inorganic composition give it a significant durability advantage.
Mineral wool: 50+ years effective performance. It doesn't settle, sag, absorb moisture, or compress over time. Rodents avoid it (inorganic, dense — no food source, difficult to nest in). We've pulled mineral wool out of 40-year-old walls and found it performing like the day it was installed.
Fiberglass: 15–30 years effective performance. The glass fibers themselves last 80–100 years, but real-world performance degrades faster. Batts sag over time (especially in ceiling applications), rodents nest in it and displace it, moisture degrades cavity performance, and installation defects worsen as the material ages. Blown-in fiberglass fares better (20–30 years) but can experience wind washing in vented attics.
Winner: Mineral wool — dramatically better long-term performance retention.
Upgrade to Mineral Wool When / Fiberglass Is Fine When
Upgrade to Mineral Wool When:
- Fire-rated assemblies — garage walls, multi-family shared walls, any UL-listed assembly. Non-negotiable.
- Soundproofing — home theaters, shared walls between rooms, bedrooms, multi-family. NRC 1.0+ makes a measurable difference.
- Moisture-prone areas — bathrooms, kitchens, exterior walls in rain-heavy climates. Hydrophobic behavior matters.
- Long-term durability matters — if you're building once and want insulation that lasts 50+ years without maintenance.
- Wildfire zones (WUI) — non-combustible to 2,150°F, FSI 0 / SDI 0.
- Personal preference — no-itch installation, less PPE needed, peace of mind.
Fiberglass Is Fine When:
- Budget is the primary constraint — saving $2,000–$5,000 on a whole house matters.
- Hidden cavities where fire/sound don't matter — interior walls of rooms without acoustic requirements.
- Large-volume new construction — the cost premium multiplied across hundreds of units adds up.
- Competent installers are available — fiberglass at Grade I installation performs within 15–20% of mineral wool's R-value.
- Attic floors — for blown-in attic applications, cellulose or blown fiberglass are cheaper than stacking mineral wool batts.
Real Project Cost Comparison
Scenario: 1,500 sq ft two-story home in climate zone 5, 2×6 exterior walls (~1,800 sq ft wall area), interior sound walls (~600 sq ft between bedrooms/bathrooms).
| Component | Fiberglass | Mineral Wool | Difference |
|---|---|---|---|
| Exterior walls (1,800 sq ft, 2×6) | R-19 at $0.50–$1.00/sq ft = $900–$1,800 | R-23 at $1.20–$1.80/sq ft = $2,160–$3,240 | +$1,260–$1,440 |
| Sound walls (600 sq ft, 2×4) | R-11 at $0.30–$0.80/sq ft = $180–$480 | R-15 at $1.00–$1.60/sq ft = $600–$960 | +$420–$480 |
| Total | $1,080–$2,280 | $2,760–$4,200 | +$1,680–$1,920 |
The mineral wool upgrade costs approximately $1,700–$1,900 more for this home — buying you R-23 instead of R-19 on exterior walls (21% more R-value in the same cavity), dramatically better fire and sound performance, and 50+ year durability. Whether that's worthwhile depends on your priorities and budget. We think it's one of the best upgrades in residential construction for the money.
Key Takeaways
- Mineral wool delivers 15–21% more R-value than standard fiberglass in the same cavity: R-15 vs R-13 (2×4), R-23 vs R-19 (2×6).
- Fire resistance is mineral wool's headline advantage: survives 2,150°F vs fiberglass melting at 1,300°F, with FSI/SDI of 0/0 (unmatched).
- Sound absorption: mineral wool NRC 1.00–1.05 vs fiberglass 0.85–0.95 — audibly better.
- Moisture: mineral wool is hydrophobic (water beads off); fiberglass traps moisture and loses R-value when wet.
- Fiberglass costs 40–70% less ($0.30–$1.50/sq ft vs $1.00–$2.10/sq ft). For a typical home, the mineral wool upgrade adds $1,500–$3,000 total.
- Mineral wool lasts 50+ years at full performance; fiberglass effective lifespan is 15–30 years.
- For fire-rated assemblies, soundproofing, and moisture-prone applications, mineral wool is the default recommendation. For budget-driven projects where these factors aren't critical, fiberglass is perfectly adequate.
FAQ
Is Rockwool the same as mineral wool?
Rockwool is a brand name — the largest mineral wool manufacturer in North America. "Mineral wool" is the generic product category. All Rockwool products are mineral wool, but other manufacturers also produce mineral wool (Thermafiber by Owens Corning, Knauf, etc.). In practice, the terms are used interchangeably because Rockwool dominates the residential market.
Is mineral wool worth the extra cost over fiberglass?
For fire-rated assemblies — absolutely, there's no comparable alternative. For soundproofing — 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 15–21% more R-value in the same cavity plus fire, sound, and moisture benefits. On a typical home, that's $1,500–$3,000 extra. For many homeowners, the combined benefits justify the cost; for strict budget projects, fiberglass with Grade I installation is adequate.
Which is better for soundproofing — fiberglass or mineral wool?
Mineral wool, conclusively. NRC 1.00–1.05 vs 0.85–0.95, STC 43 vs 36–39 in a standard wall. For dedicated sound applications, Rockwool Safe'n'Sound achieves STC 45–52 in optimized assemblies. If soundproofing is a priority, mineral wool is the right material. Pair it with resilient channel and double drywall for maximum effect. More at our soundproofing guide.
Which is easier to install — fiberglass or mineral wool?
Both are DIY-friendly with different tradeoffs. Mineral wool: heavier (~40 lbs vs ~15 lbs per bundle), but friction-fits without staples, holds itself in ceiling cavities, and causes significantly less skin itching. Fiberglass: lighter and more flexible (easier to work around obstructions), but requires full PPE due to intense skin and lung irritation. First-time installers often find mineral wool easier to work with despite the weight — the friction-fit and no-itch properties compensate for the denser cuts needed around plumbing and wiring.