£18 per square metre. That is the average cost difference between mineral wool and acoustic foam for a 50 mm panel installed on a wall — and the cheaper option (mineral wool at £20–£45/m² installed) consistently outperforms the more expensive one (acoustic foam at £25–£55/m² installed) at every octave band from 125 Hz to 4000 Hz. The acoustic treatment industry's best-kept non-secret is that foam's dominance in the consumer market is driven by marketing and visual appeal, not acoustic performance. The data — absorption coefficients measured per ISO 354:2003, fire test results per EN 13501-1, and lifecycle cost analysis — tells a clear story.
This article compares mineral wool (stone wool and glass wool products from Rockwool, Knauf, Isover, and Owens Corning) against acoustic foam (open-cell polyurethane and melamine products) across every parameter that matters for specification. Every absorption value is from published manufacturer data tested per ISO 354 or ASTM C423. The worked example shows the real-world impact: the same room treated with each material, with the resulting RT60 calculated at all six octave bands.
The Data: Octave-Band Absorption Comparison
The following table compares 50 mm products of each type, mounted directly on a wall (ISO mounting type A — surface-mounted, no air gap). All values are sound absorption coefficients measured per ISO 354:2003.
| Frequency (Hz) | Mineral Wool 50 mm (45 kg/m³) | Acoustic Foam 50 mm (open-cell PU) | Melamine Foam 50 mm | Difference (MW vs PU foam) |
|---|---|---|---|---|
| 125 | 0.15 | 0.05 | 0.08 | MW wins by 3× |
| 250 | 0.55 | 0.25 | 0.30 | MW wins by 2.2× |
| 500 | 0.85 | 0.70 | 0.75 | MW wins by 21% |
| 1000 | 0.95 | 0.90 | 0.92 | MW wins by 6% |
| 2000 | 0.90 | 0.92 | 0.90 | Comparable |
| 4000 | 0.90 | 0.92 | 0.88 | Comparable |
| NRC | 0.81 | 0.69 | 0.72 | MW wins by 17% |
The performance gap is enormous at low frequencies and narrows at high frequencies. At 125 Hz, mineral wool absorbs three times as much sound energy as equivalent-thickness polyurethane foam. At 250 Hz, the ratio is still 2.2:1. Above 1000 Hz, the two materials converge — both are effective high-frequency absorbers because the wavelength is comparable to or shorter than the material thickness.
Why Mineral Wool Wins at Low Frequencies
Porous absorbers work by converting sound energy to heat through viscous friction as air oscillates within the material's internal structure. The effectiveness of this mechanism depends on the ratio of material thickness to wavelength. At 125 Hz, the wavelength in air is 2.74 m. A 50 mm panel is only 1.8% of the wavelength — far too thin for efficient absorption at this frequency.
Both materials are too thin at 125 Hz. But mineral wool's advantage comes from its higher flow resistivity. Rockwool RWA45 (density 45 kg/m³) has a specific airflow resistivity of approximately 10,000–15,000 Pa·s/m², while open-cell polyurethane foam at similar thickness has a resistivity of approximately 5,000–8,000 Pa·s/m². The higher resistivity means more viscous friction per unit depth, extracting more energy from the sound wave per pass through the material.
This advantage persists at all thicknesses. A 100 mm mineral wool panel achieves α₁₂₅ ≈ 0.25–0.35, while 100 mm foam reaches only α₁₂₅ ≈ 0.10–0.18. The gap narrows proportionally but never closes until both materials reach the practical absorption maximum of α ≈ 0.95–1.00 at mid-high frequencies.
The Air Gap Amplifier
Adding an air gap behind either material dramatically improves low-frequency performance by shifting the peak absorption frequency downward. The effect is larger for mineral wool because it has better performance to start with:
| Configuration | α₁₂₅ (Mineral Wool) | α₁₂₅ (Acoustic Foam) |
|---|---|---|
| 50 mm, surface-mounted (no gap) | 0.15 | 0.05 |
| 50 mm + 50 mm air gap | 0.30 | 0.12 |
| 50 mm + 100 mm air gap | 0.50 | 0.20 |
| 50 mm + 200 mm air gap | 0.65 | 0.30 |
At every air gap depth, mineral wool outperforms foam by approximately 2:1 at 125 Hz. This means that to achieve the same bass absorption, a foam installation requires roughly twice the total depth (panel + air gap) as a mineral wool installation — consuming twice the room volume.
Fire Rating: The Non-Negotiable Difference
| Property | Mineral Wool (Stone Wool) | Mineral Wool (Glass Wool) | Acoustic Foam (PU) | Acoustic Foam (Melamine) |
|---|---|---|---|---|
| Euroclass (EN 13501-1) | A1 (non-combustible) | A1 (non-combustible) | E (combustible) or B1 (treated) | B-s1,d0 (limited contribution) |
| UK Reaction to Fire | Class 0 | Class 0 | Class 3 (untreated) | Class 1 |
| Melting point | > 1,000°C | > 600°C | 250–350°C (ignites) | 350–400°C (chars) |
| Smoke production | Negligible | Negligible | Dense, toxic (HCN) | Low-moderate |
| Regulatory status | Accepted everywhere | Accepted everywhere | Restricted in public buildings | Acceptable with certification |
Stone wool (Rockwool, Paroc, Isover) achieves the highest possible fire rating: Euroclass A1, non-combustible. It cannot burn, does not produce smoke, and does not release toxic gases. It is accepted without restriction in every building type, every jurisdiction, and every fire compartment.
Open-cell polyurethane foam is combustible (Euroclass E without fire-retardant treatment). When it burns, it produces dense black smoke containing hydrogen cyanide (HCN) — a lethal gas. Fire-retardant treated foams can achieve Euroclass B1, but the fire-retardant additives reduce acoustic performance by approximately 5–10% and add 20–30% to the material cost.
Melamine foam (Basotect from BASF) is the fire-performance outlier among foams. It achieves Euroclass B-s1,d0, which is acceptable for most commercial applications. However, it chars and decomposes above 350°C and is not non-combustible. For critical fire compartments (means of escape, high-rise residential, healthcare), only A1 or A2 materials are acceptable under BS 9991 and Approved Document B — meaning mineral wool is the only option.
Post-Grenfell Implications
Since the Grenfell Tower fire in 2017, UK building regulations (and equivalent regulations in Australia, UAE, and other jurisdictions) have tightened combustible material restrictions for buildings over 18 m (approximately 6 storeys). The Building Safety Act 2022 and the ban on combustible materials in external walls (Regulation 7) have created a regulatory environment where specifying combustible acoustic materials in high-rise buildings carries significant professional liability risk.
While acoustic foam on internal walls is not directly affected by the external wall ban, the cultural shift toward non-combustible materials means that specifying A1-rated mineral wool is the safer choice from a professional liability perspective, a planning approval perspective, and an insurance perspective.
Cost Comparison
| Cost Element | Mineral Wool (50 mm, fabric-wrapped panel) | Acoustic Foam (50 mm, decorative surface) |
|---|---|---|
| Raw material cost/m² | £5–£12 | £12–£30 |
| Panel fabrication (timber frame, fabric) | £8–£15 | £0 (foam is self-supporting) |
| Installation labour/m² | £8–£15 | £5–£10 (lighter, adhesive mount) |
| Total installed cost/m² | £21–£42 | £17–£40 |
| Fire certification cost | £0 (A1 inherent) | £500–£2,000 per product |
| Replacement at 10 years | £0 (25+ year life) | £17–£40/m² (foam degrades) |
| 20-year lifecycle cost/m² | £21–£42 | £34–£80 |
The headline installed cost is comparable. Mineral wool panels require a frame and fabric wrapping, which adds fabrication cost but results in a more durable, professional-looking product. Foam panels are lighter and can be adhesive-mounted without a frame, reducing installation cost — but the raw material is more expensive per square metre.
The lifecycle cost strongly favours mineral wool. Stone wool panels have a design life of 25+ years with no degradation in acoustic performance. Open-cell polyurethane foam degrades under UV exposure (yellowing and embrittlement within 3–5 years if exposed to sunlight), absorbs moisture (reducing absorption and creating mould risk in humid environments), and physically deteriorates over 8–12 years even in protected installations. At the 10-year mark, foam panels typically need replacement; mineral wool panels do not.
Environmental Impact
| Impact Category | Mineral Wool | Acoustic Foam (PU) |
|---|---|---|
| Embodied carbon (kg CO₂e/m²) | 3.5–5.0 | 8.0–15.0 |
| Raw material source | Volcanic rock / recycled glass | Petroleum-derived chemicals |
| Recyclability | Yes (back to production) | Limited (downcycling only) |
| End-of-life | Inert landfill or recycling | Landfill (slow degradation, microplastics) |
| EPD availability | Widely available (EN 15804) | Limited |
| LEED MR credit eligible | Yes | Sometimes |
Mineral wool's embodied carbon is approximately one-half to one-third of equivalent acoustic foam. For projects targeting sustainability certifications (LEED, BREEAM, DGNB), mineral wool contributes positively to materials credits, while foam may require justification documentation.
Worked Example: Same Room, Different Materials
To quantify the real-world impact, we treat the same room with each material and compare the resulting RT60.
Room: 6.0 m × 4.5 m × 2.8 m Conference Room (V = 75.6 m³)
Baseline (untreated): Exposed concrete ceiling (α ≈ 0.02), vinyl floor (α ≈ 0.03), plasterboard walls (α ≈ 0.05 at 500 Hz)
Treatment: 8 m² of 50 mm panels on the rear wall and one side wall, mounted with 50 mm air gap. Ceiling receives 27 m² of suspended acoustic tile (NRC 0.85, same for both scenarios — the comparison is wall treatment only).
| Frequency (Hz) | Untreated RT60 (s) | Mineral Wool Treatment RT60 (s) | Foam Treatment RT60 (s) | Difference |
|---|---|---|---|---|
| 125 | 1.82 | 0.72 | 0.94 | Foam 31% worse |
| 250 | 1.65 | 0.48 | 0.62 | Foam 29% worse |
| 500 | 1.41 | 0.36 | 0.40 | Foam 11% worse |
| 1000 | 1.38 | 0.31 | 0.32 | Foam 3% worse |
| 2000 | 1.35 | 0.32 | 0.31 | Comparable |
| 4000 | 1.30 | 0.32 | 0.31 | Comparable |
Calculation Method
For the mineral wool scenario at 500 Hz:
- Ceiling (27 m²) × 0.75 = 20.25 m² Sabine
- Floor (27 m²) × 0.03 = 0.81
- Untreated walls (54.2 m²) × 0.05 = 2.71
- Mineral wool panels (8 m², 50 mm + 50 mm gap) × 0.90 = 7.20
- Total A = 30.97 m² Sabine
- Mean ᾱ = 30.97 / 116.2 = 0.266
- T60 (Eyring, per ISO 3382-2:2008 §A.2) = 0.161 × 75.6 / (-116.2 × ln(1 - 0.266))
- = 12.17 / (-116.2 × (-0.309)) = 12.17 / 35.94 = 0.339 s
- Foam panels (8 m²) × 0.75 = 6.00 (vs 7.20 for mineral wool)
- Total A = 29.77 m² Sabine
- Mean ᾱ = 29.77 / 116.2 = 0.256
- T60 = 0.161 × 75.6 / (-116.2 × ln(1 - 0.256)) = 12.17 / 34.38 = 0.354 s
Verdict
The mineral wool room achieves balanced RT60 across all frequencies. The foam room has a frequency-dependent imbalance: well-controlled at 1000–4000 Hz but excessive at 125–250 Hz. This imbalance is the acoustic signature of foam-treated rooms — a "boomy" low end with a "dead" top. It is the result of foam's fundamental weakness: poor low-frequency absorption relative to its mid-high frequency performance.
Which Material Wins for Each Room Type
| Room Type | Recommended Material | Reason |
|---|---|---|
| Commercial office meeting room | Mineral wool (fabric-wrapped) | Fire A1, broadband absorption, durability, professional appearance |
| Home recording studio | Mineral wool (DIY panels) | Better bass control, lower cost, longer lifespan |
| Broadcast studio | Mineral wool (fabric-wrapped) | Fire compliance required, consistent broadband performance |
| Home cinema | Mineral wool preferred, foam acceptable | Bass trapping critical, foam acceptable if fire risk low |
| Classroom | Mineral wool only | Fire compliance mandatory, BB93 broadband RT60 targets |
| Healthcare facility | Mineral wool only | Fire compliance mandatory, cleanable fabric options |
| Server room / equipment enclosure | Melamine foam acceptable | Non-occupied space, fire risk lower, foam easier to install |
| Temporary installation / exhibition | Foam acceptable | Short-term use negates lifecycle cost advantage |
| Residential bedroom (noise control) | Mineral wool | Cost-effective, no fire concerns with fabric wrapping |
Foam wins only in two scenarios: temporary installations where the treatment will be removed within months, and non-occupied equipment enclosures where fire risk is managed by other means (sprinklers, detection). In every other scenario, mineral wool is the technically superior, financially optimal, and regulatory-safe choice.
The Market Reality
Despite mineral wool's clear superiority in performance, fire safety, cost, and environmental impact, acoustic foam maintains a strong market position in the consumer and prosumer segments. This is driven by:
- Marketing and visual identity: Pyramid and wedge foam patterns are visually synonymous with "acoustic treatment" in popular culture. Mineral wool panels wrapped in fabric look like wall art — many buyers do not recognise them as acoustic treatment.
- Retail availability: Foam is sold by Amazon, eBay, and hundreds of online retailers in convenient packs. Mineral wool slabs require a builder's merchant or specialist supplier, plus fabrication into a panel.
- Perceived ease of installation: Foam sticks to walls with spray adhesive. Mineral wool panels require mounting hardware. (In practice, the difference is 30 minutes of additional work for a small room.)
- Misunderstanding of NRC: Foam products are marketed with NRC values that, while accurate, obscure the poor low-frequency performance. A foam panel with NRC 0.70 sounds impressive — until you realise that the NRC calculation excludes 125 Hz (where the foam absorbs almost nothing) and weights 250 Hz equally with 2000 Hz (where foam performs well).
Related Reading
- Acoustic Foam vs Mineral Wool vs Perforated Panels — the three-way comparison including perforated panel systems
- How Acoustic Panels Work: The Physics — the science behind porous, resonant, and membrane absorbers
- Guide to Acoustic Materials — comprehensive material selection guide for all room types