On 14 June 2017, a fire broke out in a fourth-floor kitchen in Grenfell Tower, a 24-storey residential block in the Royal Borough of Kensington and Chelsea, London. Within hours, 72 people had died in what became the deadliest structural fire in the United Kingdom since the Second World War.
The subsequent public inquiry, chaired by Sir Martin Moore-Bick, produced two reports totalling more than 1,600 pages. Among the inquiry's central findings was that the fire cladding refurbishment completed in 2016 had specified materials that were dangerously unsuitable for the purpose: aluminium composite material panels (ACM) with a polyethylene core, classified Euroclass B for reaction to fire, and beneath them, rigid polyisocyanurate (PIR) foam insulation boards with a Euroclass C-d0 classification.
Neither material was classified as Euroclass A2-s1,d0 — the minimum classification that UK Building Regulations Part B implied was required for external wall materials on buildings above 18 metres. The inquiry found that manufacturers had misrepresented test data, building control officers had not exercised appropriate scrutiny, and the regulatory framework had failed to prevent the substitution of cheaper, combustible materials for the non-combustible alternatives that should have been specified.
This article is not about the fire itself, the failures of regulation, or the accountability of the individuals and organisations involved — the inquiry has addressed those questions comprehensively. It is about one specific technical intersection that the Grenfell fire brought into sharp focus: the relationship between acoustic insulation performance and fire safety rating, and what acoustic specifiers need to understand about the materials they specify.
The Core Conflict: Acoustic Performance vs Combustibility
Many materials that perform well acoustically are also combustible. This creates a real design conflict that is not always visible to acoustic specifiers, particularly those who are not also fire engineers.
The conflict arises most acutely in four contexts:
- External wall acoustic insulation: Buildings near roads, rail lines, or airports require external wall systems with high acoustic performance. The thermal-acoustic insulation layer behind cladding is a critical acoustic element.
- Internal partition wall insulation: Acoustic partitions in residential and healthcare buildings require cavity insulation to achieve STC 50–60 targets.
- Floor/ceiling acoustic insulation: Impact noise and airborne sound insulation in multi-storey buildings often requires acoustic quilt materials in floor assemblies.
- Roof/ceiling acoustic treatment: Industrial and commercial buildings requiring acoustic absorption in roof voids or ceiling spaces.
Material-by-Material Analysis
Mineral Wool (Rock Wool and Glass Wool)
Mineral wool is the workhorse of both acoustic and fire-rated construction. It exists in two primary forms: rock wool (manufactured from basalt or slag) and glass wool (manufactured from recycled glass). Both have similar acoustic and fire performance.
Acoustic performance:
- Absorption coefficient: NRC 0.75–0.95 for 75–100 mm boards (frequency-dependent; excellent above 500 Hz, moderate at 125 Hz)
- In cavity partitions: 100 mm full-fill mineral wool adds 14–16 dB to STC of a 92 mm metal stud assembly
- Impact sound insulation: 25–35 mm dense mineral wool quilt (80–160 kg/m³) in floating floor systems contributes approximately 17–22 dB improvement to Lw (weighted impact sound pressure level)
- Typical products: Rockwool Safe'n'Sound (North America), Rockwool Flexi (Europe), Knauf Earthwool, Saint-Gobain ISOVER
- Euroclass: A1 or A2-s1,d0 (non-combustible)
- Melting point: > 1,000°C for rock wool, > 700°C for glass wool
- Behaviour in fire: does not ignite, does not produce flaming droplets, contributes no heat to the fire
- Fire resistance contribution: mineral wool batts within a fire-rated partition maintain integrity for the rated period; without mineral wool, many partition systems lose fire resistance prematurely
Melamine Foam (Open-Cell)
Melamine foam — familiar under trade names such as Basotect (BASF) — is a lightweight, open-cell foam with outstanding broadband acoustic absorption at mid and high frequencies.
Acoustic performance:
- Absorption coefficient: NRC 0.95–1.00 for 50 mm thickness (among the highest achievable)
- Excellent absorption above 500 Hz; limited below 250 Hz without additional thickness
- Used primarily as surface absorber (hung baffles, wall panels, ceiling tiles) rather than cavity insulation
- Weight: 9–11 kg/m³ (very light, relevant for suspended ceiling installations)
- Euroclass: B-s1,d0 to C-s1,d0 depending on density and thickness
- Melamine foam does not have the non-combustible classification of mineral wool
- Behaviour in fire: chars and self-extinguishes; does not produce flaming droplets in standard test conditions; produces limited smoke (s1 classification)
- Not classified A2 — cannot be used in external wall applications above 18 m in the UK under current Building Regulations
Polyurethane Foam and Acoustic Foams
Open-cell polyurethane foam is widely sold as "acoustic foam" — wedge-shaped panels, pyramid foam tiles, and egg-crate foam are all PU foam products marketed primarily to home studios and domestic acoustic treatment.
Acoustic performance:
- Absorption coefficient: NRC 0.70–0.90 for 50 mm thickness at mid-high frequencies
- Significantly worse than mineral wool or melamine below 500 Hz
- Primarily absorbs above 1 kHz; poorly controls bass frequencies
- Euroclass: E or F (combustible, high contribution to fire)
- Behaviour in fire: ignites readily, burns with visible flame, produces dense toxic smoke (hydrogen cyanide and carbon monoxide from PU combustion), produces flaming droplets
- Explicitly prohibited in all publicly accessible buildings under European regulations, and in residential buildings above ground floor level in most jurisdictions
- The foam panelling in the Station Nightclub fire (Rhode Island, 2003, 100 deaths) was non-fire-retardant PU foam used as acoustic treatment. The fire spread from the foam panels within 90 seconds of ignition.
Extruded Polystyrene and Expanded Polystyrene
EPS and XPS are used extensively as thermal insulation, and appear in some acoustic specifications as impact sound attenuation layers in floating floor systems.
Acoustic performance:
- EPS: Dynamic stiffness 10–50 MN/m³; used in floor underlays for impact sound insulation
- A 30 mm EPS underlay achieves approximately ΔLw = 19–24 dB (impact sound improvement)
- Negligible airborne sound insulation contribution
- Euroclass: E (EPS) or E-F (XPS without fire retardant)
- Fire-retardant XPS achieves Euroclass B-s1,d0 in some products
- Non-FR EPS burns rapidly, producing significant quantities of styrene vapour and dense black smoke
- The Grenfell Tower cladding fire was significantly worsened by the presence of PIR foam (polyisocyanurate, a relative of PU foam) which ignited behind the ACM panels and contributed continuous fuel to the fire spread
The UK Regulatory Framework Post-Grenfell
The Building Safety Act 2022 and the associated updates to Building Regulations Part B (Fire Safety) and Part E (Resistance to Sound) have created a more stringent regulatory environment for acoustic specification in buildings above 18 m.
Key changes affecting acoustic specifiers:
External walls above 18 m: All materials in the external wall system must meet Euroclass A2-s1,d0 minimum. This explicitly prohibits PIR foam, EPS, and standard PU foam as the insulation layer behind cladding. Acoustic-grade mineral wool (such as Rockwool Flexi or Knauf FKD) is the only widely available insulation that meets both the acoustic and fire requirements simultaneously.
Cavity barriers: Acoustic cavities in external wall systems (between outer cladding and inner lining) must now be interrupted by fire-stopping cavity barriers at each floor level and at horizontal intervals not exceeding 2 m. Acoustic specifiers must coordinate with fire engineers to ensure cavity barrier positions do not create acoustic bridges that undermine the sound insulation performance.
Desktop studies: The Building Safety Act has severely restricted the use of "desktop studies" — assessments of fire performance based on similarity to previously tested systems rather than direct test data. For acoustic assemblies that are also part of the fire-rated envelope, full test data (EN 1365 or equivalent) must now be available. This affects composite wall and floor systems where the acoustic treatment is integral to the fire-rated assembly.
Golden Thread: The Act introduces a requirement for a "Golden Thread" of building safety information to be maintained throughout a building's life. For occupied buildings, this includes the specification of all materials in fire-rated and acoustically rated assemblies. Acoustic consultants designing systems for higher-risk buildings (above 18 m, or certain occupancy types) should be aware that their specifications will form part of this record.
The European Context: EN 13501 and CE Marking
For acoustic materials sold in the European market (and, post-Brexit, still widely used in the UK as reference standards), the European fire classification system (EN 13501-1) is the primary framework.
The Euroclass system uses the following designations relevant to acoustic materials:
| Euroclass | Description | Example acoustic materials |
|---|---|---|
| A1 | Non-combustible, no contribution to fire | Rock wool, glass wool |
| A2-s1,d0 | Non-combustible, limited smoke, no droplets | Some mineral wool composites |
| B-s1,d0 | Limited combustibility | Melamine foam (some grades) |
| C-s1,d0 | Limited combustibility | Melamine foam (standard), some FRT-PU |
| D-s2,d0 | Combustible, medium smoke | PU foam (FRT) |
| E | Combustible | Standard PU foam, EPS |
| F | No performance determined | Unknown or non-tested materials |
The suffix notation: s (smoke) = s1 (limited), s2 (medium), s3 (high); d (droplets) = d0 (none), d1 (limited), d2 (significant).
For any acoustic material installed in a building, the specifier should request:
- CE marking documentation (mandatory for structural products, but also available for acoustic products)
- The Declaration of Performance (DoP) — the manufacturer's documented evidence for the stated Euroclass
- A current EN 13501-1 test report from an accredited laboratory
Specification Guidance: Combining Acoustic and Fire Performance
For the most common acoustic applications in buildings, the following material choices satisfy both acoustic and fire requirements:
External wall acoustic insulation (buildings > 18 m):
- Specify: Rockwool Fixrock 035 or equivalent rock wool facade slab, minimum 80 mm
- Euroclass: A1
- Acoustic benefit: Rw improvement ~6–8 dB in rainscreen systems; thermal bridging reduction also reduces low-frequency flanking
- Specify: 100 mm full-fill mineral wool batt (Rockwool Safe'n'Sound or Knauf Earthwool 35)
- Euroclass: A1
- Acoustic benefit: STC improvement ~14–16 dB over empty cavity
- Specify: 25–30 mm dense mineral wool floating floor quilt (80–140 kg/m³), e.g., Rockwool Steprock or Knauf FKD-S
- Euroclass: A1
- Acoustic benefit: ΔLw = 20–27 dB depending on density and system construction
- Specify: Mineral wool acoustic ceiling tiles with factory-applied decorative face (Armstrong Ultima+, Ecophon Master E, Saint-Gobain Ecophon Connect)
- Euroclass: A2-s1,d0
- Acoustic benefit: NRC 0.85–0.95
- Specify: Melamine foam (Basotect) panels with intumescent paint if required
- Euroclass: B-s1,d0
- Acoustic benefit: NRC 0.95–1.00; better than mineral wool at mid-high frequencies
Conclusion: Two Specifications, One Material
The lesson from building envelope failures — Grenfell most prominently, but also the Lacrosse Building in Melbourne (2014, ACM cladding fire), the Torch Tower in Dubai (2015, EPS cladding fire), and numerous less-publicised incidents — is not that acoustic insulation is dangerous. It is that the acoustic specification and the fire specification are the same specification, and must be treated as such.
Mineral wool is the default material for acoustic cavity insulation in buildings because it satisfies both requirements simultaneously. It absorbs sound effectively. It does not burn. It is the material that decades of experience, thousands of test reports, and now two major building safety inquiries confirm is appropriate for the purpose.
The acoustic specifier who substitutes PIR foam for mineral wool because the manufacturer offers a slightly higher R-value, or specifies PU foam panels because they are cheaper per unit NRC, is making a decision that has fire safety consequences. Those consequences are no longer abstract. They are documented, investigated, and the subject of criminal prosecutions.
The specification should state: mineral wool, Euroclass A1 or A2-s1,d0 minimum, full test data provided. Everything else requires a fire engineering justification.