“Mineral wool” covers two quite different products that are often confused: stone wool (sold under brand names like Rockwool and Rocksilk) and glass wool (sold as Knauf Earthwool, Isover, and others). Both are made from inorganic fibres bonded into batts or rolls, and both provide thermal and acoustic insulation — but their performance profiles, fire behaviour, and best applications differ meaningfully.
Understanding the distinction helps you specify the right product rather than defaulting to whatever the merchant has in stock.
What Is Mineral Wool?
All mineral wool insulation is made by melting raw material and spinning it into fine fibres, which are then bonded with a binder (usually a thermosetting resin) and compressed into rolls, batts, or rigid slabs. The fibre is naturally non-combustible — the raw material either melts at very high temperatures (stone wool) or softens and fuses without propagating flame (glass wool).
The key distinctions:
| Property | Stone wool (Rockwool) | Glass wool |
|---|---|---|
| Raw material | Basalt rock / blast furnace slag | Recycled glass (70%+ typically) |
| Melting point | >1,000°C | ~700°C |
| Density range | 30–180 kg/m³ | 10–48 kg/m³ |
| Typical λ (thermal conductivity) | 0.033–0.040 W/mK | 0.032–0.044 W/mK |
| Fire classification | A1 (non-combustible) | A1 (non-combustible) |
| Acoustic performance | Higher (denser, better sound absorption) | Moderate |
| Relative cost | Higher | Lower |
| Water resistance | Good (hydrophobic treatment standard) | Moderate |
Thermal Performance Compared
Both materials achieve similar thermal conductivity values when comparing like-for-like densities. The λ value (W/mK) is the number to compare: lower is better.
| Product type | Typical λ (W/mK) | Density (kg/m³) |
|---|---|---|
| Glass wool loft roll (standard) | 0.040–0.044 | 10–14 |
| Glass wool loft roll (premium) | 0.032–0.036 | 16–20 |
| Stone wool roll (general purpose) | 0.036–0.040 | 23–30 |
| Stone wool rigid slab (Rockwool RWA45) | 0.035–0.037 | 45 |
| Stone wool rigid slab (high-density, Rockwool RS60) | 0.033–0.035 | 60 |
In practice, the difference in λ between comparable products is small enough that thickness matters more than brand. A 200 mm layer of good glass wool roll (λ = 0.038) achieves U = 0.16 W/m²K in a standard ventilated loft — well below the Building Regulations Part L target of 0.16 W/m²K for new dwellings. The same thickness of stone wool roll would be marginally better but not meaningfully so for loft use.
Where stone wool’s density advantage shows up is in rigid-slab applications: a 50 mm Rockwool slab at 45 kg/m³ is stiffer and more dimensionally stable than glass wool at the same thickness, which matters in cavity walls and rafter applications where the product must stay put without mechanical restraint.
Fire Resistance: Where Stone Wool Wins Clearly
Both stone wool and glass wool carry an A1 Euroclass fire classification under BS EN 13501-1, meaning neither contributes to fire growth. However, there is an important distinction in their behaviour at extreme temperatures:
- Stone wool (basalt): Retains its structure up to 1,000°C+ because the raw material melts at that temperature. This makes it suitable as a fire-protection product — not just insulation. It is used in fire-rated duct wraps, structural steel protection, and fire barriers in compartment walls.
- Glass wool: The glass softens at around 700°C. It will not burn or propagate flame, but it loses structural integrity at temperatures that stone wool withstands. It is not suitable for applications where the insulation must maintain integrity during a fire.
For residential renovation purposes this matters in:
- Party wall fire barriers (Building Regs Part E/B)
- Roof void fire stops in HMOs and flats
- Penetration seals around pipes and cables in fire-rated compartments
- Fire protection of structural steelwork (in conversions and extensions)
In all of these applications, specify stone wool. Glass wool is not an appropriate substitute even though both are technically “mineral wool” and both carry A1 classification.
Acoustic Performance
Stone wool’s higher density makes it the better acoustic insulator. The key metric for partitions is the weighted sound reduction index (Rw) — and for mid-frequency sound (voices, TV), denser insulation in a timber-frame partition can add 3–6 dB of additional attenuation compared to lighter glass wool.
For separating floors in flats or conversions subject to Part E of the Building Regulations (Rw + Ctr ≥ 45 dB for walls, ΔRw ≥ 13 dB for floors), stone wool is the standard specified product. Rockwool’s RWA45 and RS60 slabs appear in pre-tested robust details published by Robust Details Ltd.
For simple draught-proofing or thermal loft insulation where sound is not a concern, glass wool is perfectly adequate.
Cost Comparison (2026 UK Supply Prices)
| Product | Format | Cost (supply only) |
|---|---|---|
| Glass wool loft roll, 100 mm | Roll (~8 m²) | £3.50 – £5.50/m² |
| Glass wool loft roll, 200 mm | Roll (~5 m²) | £5.50 – £8.00/m² |
| Glass wool rafter/wall batt | 100 mm, per m² | £4.00 – £7.00/m² |
| Stone wool roll (general) | 100 mm, per m² | £5.00 – £8.00/m² |
| Rockwool RWA45 slab, 50 mm | Per m² | £6.00 – £9.00/m² |
| Rockwool RWA45 slab, 100 mm | Per m² | £10.00 – £14.00/m² |
| Rockwool RS60/RS80 slab | 50 mm, per m² | £8.00 – £12.00/m² |
| Rockwool acoustic partition slab | 50 mm, per m² | £7.00 – £11.00/m² |
Prices are for merchant supply; DIY-store prices for rolls are typically 10–20% higher. Installation costs vary enormously by application — loft rolls are a standard DIY task; fitting batts between rafters or in party walls is typically contractor work at £10–£20/m² labour.
Which to Choose for Common Applications
Loft insulation (accessible, between joists): Glass wool roll is the cost-effective choice. It’s lighter, easier to manoeuvre in a low loft, and performs identically to stone wool thermally at this application. Install 100 mm between joists and a second 270 mm layer across the joists for a total 370 mm to achieve approximately U = 0.11 W/m²K.
Rafter insulation (warm roof or between-and-below): Stone wool batts cut more cleanly and hold their shape between rafters without sagging. For a warm loft conversion, you’re typically fitting batts between rafters and a second layer below — stone wool’s dimensional stability earns its price premium here.
Timber-frame wall (structural): Either works thermally; use stone wool if Part E acoustic performance is required, which it is in new dwellings (Approved Document E) and most conversions that introduce a new habitable room next to a party element.
Party wall or separating floor: Stone wool only. Specify to the Robust Detail tested system to guarantee compliance without site testing.
Fire-stopping or fire-rated duct wrap: Stone wool only. Do not substitute glass wool.
Cavity wall (retrofit, professional injection): Neither roll product — cavity wall installers use purpose-made blown mineral wool, typically a stone-wool-based product to PAS 2030 and BS 5617.
Handling and Health
Both products can cause temporary skin and throat irritation from loose fibres during cutting. Modern formulations are certified as non-carcinogenic under IARC Group 3 classification, but standard precautions apply: gloves, dust mask (FFP2), and eye protection when cutting. Stone wool’s higher density means it generates fewer airborne fibres than lighter glass wool during installation.
