Fiberglass and mineral wool are the two dominant acoustic insulation materials in construction. Together they account for the majority of cavity insulation, acoustic panel cores, and absorptive lining installations worldwide. Yet specifiers frequently treat them as interchangeable when they are not — and choose between them based on brand familiarity rather than performance data. This comparison examines both materials head-to-head across the dimensions that matter for acoustic applications.
What Are These Materials?
Fiberglass (also: glass wool, glass fibre) is manufactured by melting silica sand, soda ash, limestone, and recycled glass, then spinning the molten material into thin fibres typically 3–9 microns in diameter. Key manufacturers include Owens Corning (US), Saint-Gobain ISOVER (Europe), Knauf Insulation (Europe/global), and CSR Bradford (Australia). Fiberglass is available as loose-fill, flexible batts, and rigid boards. For acoustic applications, the rigid board products — Owens Corning 703 and 705, ISOVER Party Wall Roll, Knauf Earthwool — are most commonly specified.
Mineral wool (also: rock wool, stone wool, slag wool) is manufactured from basalt rock or industrial slag, melted at higher temperatures than glass (1,400–1,600°C versus 1,200°C for glass), and spun into fibres typically 4–10 microns in diameter. Key manufacturers include Rockwool (global), Knauf Insulation (their "Rocksilk" range), Paroc (Nordic), and USG Thermafiber (US). Mineral wool is inherently non-combustible and achieves Euroclass A1 fire reaction without additives.
Absorption Performance: Octave-Band Data
Acoustic absorption performance is primarily determined by three physical parameters: fibre diameter, bulk density, and thickness. The material type (glass or mineral) matters less than these three variables, which is why comparing products at equal density and thickness is essential.
50mm Boards at Quoted Density
| Frequency (Hz) | OC 703 (24 kg/m³) | Rockwool RWA45 (45 kg/m³) | Rockwool RW3 (25 kg/m³) | ISOVER APR 1200 (15 kg/m³) |
|---|---|---|---|---|
| 125 | 0.29 | 0.35 | 0.22 | 0.18 |
| 250 | 0.72 | 0.82 | 0.68 | 0.55 |
| 500 | 0.97 | 0.99 | 0.95 | 0.87 |
| 1000 | 0.99 | 0.99 | 0.99 | 0.97 |
| 2000 | 0.99 | 0.99 | 0.99 | 0.99 |
| 4000 | 0.98 | 0.99 | 0.98 | 0.98 |
| NRC | 0.95 | 0.99 | 0.90 | 0.82 |
Key finding: at equivalent thickness, higher density provides better low-frequency absorption. Rockwool RWA45 at 45 kg/m³ outperforms OC 703 at 24 kg/m³ in the 125–250 Hz bands — the frequencies that are hardest to treat and most problematic in music rooms, broadcast studios, and large conference spaces.
At 25mm Thickness
| Frequency (Hz) | OC 703 25mm | Rockwool RWA45 25mm |
|---|---|---|
| 125 | 0.09 | 0.14 |
| 250 | 0.35 | 0.51 |
| 500 | 0.76 | 0.88 |
| 1000 | 0.94 | 0.97 |
| 2000 | 0.98 | 0.99 |
| 4000 | 0.97 | 0.98 |
| NRC | 0.76 | 0.87 |
At 25mm, the denser mineral wool product shows a meaningful advantage. For standard acoustic panel construction — typically 50–100mm deep — the performance gap narrows at 50mm and is negligible at 100mm.
Flow Resistivity and Why It Matters
Flow resistivity (measured in Pa·s/m²) is the physical parameter that most directly predicts absorption performance. It describes resistance to airborne flow through the material. The optimal flow resistivity for a single-layer absorber is approximately 10,000–50,000 Pa·s/m² — below this range, sound passes through without being absorbed; above it, sound reflects off the surface.
| Material | Typical Flow Resistivity (Pa·s/m²) |
|---|---|
| OC 703 (24 kg/m³) | 12,000–18,000 |
| OC 705 (32 kg/m³) | 25,000–35,000 |
| Rockwool RWA45 (45 kg/m³) | 40,000–60,000 |
| Rockwool RW3 (25 kg/m³) | 15,000–22,000 |
| Acoustic foam (open-cell PU) | 3,000–8,000 |
| Dense mineral wool (100 kg/m³) | 80,000–150,000 |
Dense mineral wool boards (100+ kg/m³, flow resistivity > 80,000 Pa·s/m²) start to reflect sound at mid frequencies and must be used with an air gap or at greater thickness to remain effective. This is why very high-density products are better for soundproofing (mass) than absorption.
Fire Ratings
This is where mineral wool has an unambiguous advantage.
Mineral wool achieves Euroclass A1 (non-combustible, no smoke, no burning droplets) and ASTM E136 non-combustible without any additives. At 1,000°C it remains structurally intact. This makes it suitable for fire-rated wall assemblies, ceiling plenums, and applications where the product may be exposed to ignition. It is also Approved Document B (ADB) compliant in the UK for all fire-risk locations.
Fiberglass achieves Euroclass A2 or Euroclass E depending on the specific product and any facing. Fiberglass melts at approximately 700°C — it does not burn in the conventional sense but loses structural integrity at lower temperatures than mineral wool. ISOVER and Knauf products with specific facings achieve A2-s1,d0, while unfaced batts typically rate A2-s1,d1 or E. For fire compartment walls and fire-rated ceiling systems, mineral wool is the standard specification.
US context: Under ASTM E119 (fire-resistance rating), both materials are used in rated assemblies, but mineral wool is preferred for fire-stopping and area separation walls due to its higher melting point and ability to maintain integrity during fire growth.
Health and Safety Considerations
Both fiberglass and mineral wool underwent significant reformulation beginning in the 1990s following concerns about fibre biopersistence. The key questions are: do fibres reach the deep lung, and if they do, how long do they persist before dissolving?
Modern fiberglass: Bio-soluble glass fibres (those used in insulation products since ~1993 in Europe and ~2000 in the US) dissolve in lung fluid within 10–40 days. The fibres used in thermal and acoustic insulation are classified as bio-soluble under MMVF (Man-Made Vitreous Fibre) regulations. IARC (International Agency for Research on Cancer) removed glass wool from Group 2B (possible carcinogen) in 2001. Current products carry GHS hazard statements for respiratory and skin irritation, but not carcinogenicity.
Modern mineral wool: Rockwool and similar products are bio-soluble, dissolving in 10–30 days. Rockwool products carry the EUCEB (European Certification Board for Mineral Wool Products) bio-solubility mark. Similarly removed from IARC Group 2B.
Practical installation precautions (both materials):
- P2/FFP2 respirator during cutting and handling
- Safety glasses and gloves
- Long sleeves and neck coverage
- Ventilate the working area during installation
- Wash work clothing separately
Moisture and Mould Resistance
Mineral wool is inherently hydrophobic in most product ranges — Rockwool adds a water-repellent treatment during manufacture. When mineral wool gets wet, it retains structural integrity and dries without deformation. Mould cannot grow on mineral wool because there is no organic nutrient content. This makes mineral wool well-suited to high-humidity environments: sports halls, swimming pool halls, commercial kitchens, and below-grade applications.
Fiberglass is also inorganic but typically has lower water-repellency without treatment. When wet, fiberglass batts can mat, reducing their absorption coefficient significantly. Rigid fiberglass boards (OC 703) are more resistant than batts. Fiberglass should not be used as the sole material in persistently damp environments.
Cost Comparison
The cost of acoustic insulation materials varies significantly by region, project volume, and product specification. The figures below represent UK/EU mid-2026 supply-only pricing for standard acoustic-grade products at commercial volumes (>50 m²).
| Product | Thickness | Density | Approx. Cost (£/m²) | Approx. Cost ($/m²) |
|---|---|---|---|---|
| Rockwool RWA45 | 50mm | 45 kg/m³ | £4.50–£6.00 | $5.50–$7.50 |
| Rockwool RWA45 | 100mm | 45 kg/m³ | £8.50–£11.00 | $10.50–$14.00 |
| OC 703 Fiberglass | 50mm | 24 kg/m³ | £5.00–£7.00 | $6.00–$9.00 |
| OC 703 Fiberglass | 100mm | 24 kg/m³ | £9.00–£13.00 | $11.00–$16.00 |
| Knauf Acoustic Roll | 100mm | 20 kg/m³ | £3.50–£5.00 | $4.50–$6.50 |
| Paroc Pro Slab | 50mm | 50 kg/m³ | £5.50–£8.00 | $7.00–$10.00 |
At similar performance levels, fiberglass rigid boards (OC 703) are broadly price-competitive with mineral wool semi-rigid boards. Flexible fiberglass batts are cheaper than mineral wool, which accounts for their continued prevalence in residential cavity insulation — but flexible products perform poorly as acoustic panel cores without a rigid facing.
Installed cost premium for mineral wool: Mineral wool is heavier (higher density products), which increases transport and labour costs marginally. The fire-rating advantage of mineral wool often reduces the need for additional intumescent strips or compartmentation measures, partially offsetting material cost.
Application-by-Application Recommendation
Recording Studios and High-Performance Listening Rooms
Preferred: Mineral wool (Rockwool RWA45 or equivalent)
The low-frequency absorption advantage of denser mineral wool products is decisive for critical listening environments. At 125 Hz — the lower bound of the octave bands in ISO 3382 — mineral wool at 45 kg/m³ absorbs approximately 20% more energy than fiberglass at 24 kg/m³. For 100mm panel depth, this difference diminishes, but mineral wool remains the standard specification for bass traps and broadband absorber panels in professional studio design.
Office Open-Plan Ceilings and Wall Panels
Either suitable; fiberglass is cost-competitive
In office environments, the critical frequency range is 500–2,000 Hz (speech intelligibility). At 50mm or greater, both materials achieve α > 0.95 in this range. Fiberglass rigid boards in suspended ceiling baffles and fabric-wrapped wall panels are widely specified and perform equivalently to mineral wool for this application.
Party Walls and Floor/Ceiling Assemblies (Sound Insulation)
Preferred: Mineral wool
For sound insulation rather than absorption, the goal is to add decoupling and damping without transmitting vibration through the cavity. Mineral wool's higher density and flow resistivity provide marginally better sound isolation performance in standard cavity wall configurations. More importantly, mineral wool's fire rating makes it the only compliant specification for party walls in most building codes (ADE in UK, DIN 4109 in Germany, NCC Section F in Australia).
HVAC Duct Lining
Preferred: Fiberglass with factory-applied facing
Rigid fiberglass duct liner (ASTM C1071 compliant) is the standard specification for internal duct acoustics in HVAC systems. It has a long track record, is lightweight, and the factory-applied erosion-resistant facing prevents fibre shedding into airstreams. Mineral wool is used for duct insulation (external wrap) but is less common for internal lining due to weight and handling requirements.
Fire-Rated Assemblies
Required: Mineral wool
For fire-rated walls, fire barriers, and any application where non-combustibility is required by code, mineral wool is the only appropriate specification. Fiberglass achieves A2 but not A1 Euroclass — for many fire-compartmentation applications, A1 is the minimum requirement.
Common Specification Mistakes
Mistake 1: Specifying by generic name rather than density. "50mm mineral wool" is a meaningless specification unless density is stated. A 25 kg/m³ product and a 100 kg/m³ product at the same thickness have completely different absorption coefficients and flow resistivities.
Mistake 2: Using flexible batts as panel infill without rigid substrate. Both materials in flexible batt form sag and compress over time unless properly contained, degrading low-frequency absorption performance. Specify semi-rigid or rigid boards for acoustic panels.
Mistake 3: Treating NRC as the sole comparison metric. NRC (250–2,000 Hz average) hides low-frequency performance. For bass-critical applications, check the 125 Hz and 250 Hz coefficients separately.
Mistake 4: Ignoring the facing material. The absorption coefficient of a panel depends on the core material and the facing. Open-weave acoustic fabric transmits sound to the core; tight-weave fabric blocks it. Always test the system absorption, not just the core.
Summary Verdict
| Criterion | Fiberglass | Mineral Wool | Winner |
|---|---|---|---|
| Mid/high frequency absorption (500–4000 Hz) | Excellent | Excellent | Tie |
| Low frequency absorption (125–250 Hz) | Good | Better | Mineral wool |
| Fire rating | A2 | A1 | Mineral wool |
| Moisture resistance | Moderate | High | Mineral wool |
| Weight | Lower | Higher | Fiberglass |
| Cost (at similar performance) | Comparable | Comparable | Tie |
| HVAC duct lining | Preferred | Less common | Fiberglass |
| Health hazard | Low (modern) | Low (modern) | Tie |
For most acoustic panel and cavity-fill applications, mineral wool is the marginally superior choice — better fire rating, better moisture resistance, and better low-frequency performance at high density. The performance advantage of fiberglass is limited to weight-sensitive applications (hanging baffles) and HVAC duct lining where the factory-applied facing provides important practical benefits.
Use AcousPlan's material library to model either product at your specified density and thickness, calculate the resulting RT60, and verify compliance against your target standard before finalising the specification.