Rockwool vs Other Mineral Wool: Which to Use

“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:

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.

Rockwool’s own datasheet puts RWA45 slabs at 0.035 W/mK and its denser RW3 slab at 0.034 W/mK.¹ Knauf’s standard glass wool loft rolls, for comparison, are 0.040–0.044 W/mK.² Rockwool’s own loft rolls (Roll, Twin Roll and Rollbatt) are 0.044 W/mK, so a stone wool roll is no better thermally than a glass wool roll.³

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) gives roughly U = 0.18 W/m²K in a standard ventilated loft, short of the 0.16 W/m²K that Building Regulations Part L sets when an existing roof is renovated; a total of around 250–270 mm gets there.⁴ The same thickness of stone wool roll performs much the same.

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.²¹ However, there is an important distinction in their behaviour at extreme temperatures:

• Stone wool (basalt): Resists temperatures of over 1,000°C, so it keeps its structure in a fire.¹ 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 and melts at a much lower temperature than stone wool.⁵ Rockwool, which makes stone wool, puts glass wool’s melting point at around 600°C.⁶ 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)⁷
• 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 gives it an edge as an 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 a few decibels of attenuation compared with lighter glass wool, although in a lightweight stud partition the difference is often small.

For separating floors in flats or conversions subject to Part E of the Building Regulations, stone wool is a common choice. The figures are worth getting right because three things get mixed up. The measure is DnT,w + Ctr, tested in the finished building, not Rw, which is a laboratory value. A purpose built flat needs 45 dB or better on separating walls and floors; one formed by a material change of use, which is what most conversions are, needs 43 dB.⁸ And a separating floor carries a second figure that walls do not: impact sound, at a maximum of 62 dB L’nT,w purpose built or 64 dB on a conversion.⁸ Rockwool’s Flexi slab appears in registered Robust Details for separating walls, such as E-WT-1 (twin timber frame).⁹

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)

RS60 and RS80 are Knauf’s Rocksilk stone wool slabs, not Rockwool products; a 50 mm pack of RS60 covering 6.48 m² sold for £53.92 including VAT in September 2026, about £8.30/m².¹⁰

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 at least as well as a stone wool roll thermally in 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 for separating walls and floors in new homes and in homes formed by a material change of use, such as a house converted into flats (Approved Document E).⁸

Party wall or separating floor: Use the insulation the tested construction specifies, which is usually a stone wool product. On a new house or flat, building to a registered Robust Detail can replace pre-completion sound testing; a conversion formed by a material change of use still has to be tested.⁸

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, either glass wool (such as Knauf Supafil) or stone wool, installed to PAS 2035/2030:2023 under the system’s own BBA certificate or ETA.¹¹ Note that BS 5617, still quoted in places for cavity fill, was specific to urea-formaldehyde foam and was withdrawn on 2 January 2024.¹²

Handling and Health

Both products can cause temporary skin and throat irritation from loose fibres during cutting. Modern mineral wool fibres are made to be bio-soluble, and Rockwool states that its stone wool is not suspected of being carcinogenic under the CLP Regulation.¹³ Standard precautions still apply: gloves, eye protection and a disposable dust mask when cutting. Rockwool’s safe use sheet specifies at least an FFP1 mask to EN 149 in confined spaces or dusty work; an FFP2 or FFP3 mask gives a wider margin in a cramped loft.¹³

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.