Rockwool insulation review: stone wool for fire and sound

Quick answer: Rockwool is stone wool: denser, heavier and dearer than glass wool, Euroclass A1 non-combustible, and the best mainstream choice for sound. RWA45 slab at 100mm is £24.67 ex VAT for 2.88m², about £8.57 per square metre, against £2.32 for a Knauf loft roll. Prices checked 27 July 2026.

What stone wool is

Rockwool spins molten volcanic rock into fibre. Glass wool, which is what Knauf and Superglass make, spins recycled glass instead. The chemistry differs, but the difference that shows up on site is density: Rockwool’s RW Slabs data sheet puts RWA45 at a nominal 45 kg/m³, and a loft roll runs at around a quarter of that.

Density buys three things. Slabs hold their own shape, so they friction-fit into a stud bay and stay there rather than slumping after five years. Mass absorbs sound, which is why RWA45 turns up in every acoustic partition specification in Britain. And the material is Euroclass A1 to BS EN 13501-1, the top of the reaction to fire scale, with no combustible binder to worry about.

We haven’t tested these products. This review works from published data sheets, certification and merchant pricing, checked 27 July 2026.

The range and what it costs

The Twin Roll line is the one to argue about. It’s 0.044 W/mK, exactly the same declared conductivity as a Knauf or Superglass glass wool roll, and it costs £10.36 per square metre against £2.32. You’re paying more than four times over for the same thermal number. What you get back is A1 non-combustibility and a roll that won’t compress under a stored suitcase. In an ordinary cold loft, that isn’t worth four times the money.

Fire and sound, which is the real case for Rockwool

Euroclass A1 means the material makes no contribution to a fire at all. PIR boards are Euroclass F. Glass wool is A1 too, so Rockwool has no monopoly on fire performance. Where it pulls ahead is temperature. Stone wool holds together far hotter than glass wool, which is why it’s specified for fire stopping around service penetrations and in cavity barriers.

On sound the density does the work. A stud partition with 100mm RWA45 in the void performs measurably better than the same partition with a lightweight roll stuffed in, because the fibres absorb the air-borne energy instead of letting it pass. If you’re splitting a room, converting a garage or sharing a party wall with a noisy neighbour, this is the material. Our fire safety guide explains where A1 is a regulatory requirement rather than a preference.

Durability

Rockwool says its stone wool lasts at least 50 years with no degradation in quality or performance, based on testing products recovered during the renovation of an airport hangar in Copenhagen. Nothing in the material ages the way a foam blowing agent does. The declared conductivity and the reaction to fire of a factory-made mineral wool don’t drift with time.

Treat that as a floor rather than a promise. Insulation fails through water ingress, compression and rodents, none of which are the material’s fault and all of which are common in British lofts. Nothing degrades stone wool itself, but a leaking roof will still wreck the layer under it.

Cavity walls and external systems

Rockwool also makes full-fill cavity batts, and it supplies the insulation layer inside a lot of external wall insulation systems. That’s where the A1 classification stops being a nice-to-have. Render systems on stone wool can go up at heights where an expanded polystyrene system can’t, because the material isn’t restricted by regulation 7(2) in England.

For a two-storey house that restriction isn’t binding. So the decision comes back to money and moisture. Stone wool batts cost more than glass wool batts and rather more than blown bead. They tolerate a driven-rain exposure better than most fills, and they let the wall dry, which matters on older solid-ish construction. If a surveyor has flagged your property as severe or very severe exposure, this is the conversation to have.

Where it’s the wrong choice

A plain loft top-up. If you have joists, headroom and a budget, glass wool at a quarter the price hits the same 0.044 W/mK. Buy Knauf or Superglass rolls and put the difference towards more depth, which is what actually moves the U-value. Our guide to loft insulation thickness sets out the target.

Anywhere depth is constrained. At 0.035 W/mK, RWA45 needs roughly 60% more thickness than a PIR board for the same resistance. In a loft conversion with 150mm rafters, board wins on geometry regardless of what it costs. Compare the materials in mineral wool, PIR and EPS and the arithmetic in U-values explained.

For a modelled cost of the whole job, the calculator and insulation product guide use a £2,950 mid estimate for loft and cavity wall insulation, based on Energy Saving Trust installed prices and adjusted by region, with the method in how we calculate costs.

FAQs

Is Rockwool better than glass wool?

For fire resilience, sound and shape retention, yes. For plain thermal performance per pound in a loft, no. Rockwool Twin Roll and Knauf Loft Roll 44 both declare 0.044 W/mK, and the Rockwool costs over four times as much per square metre.

Does Rockwool itch like old fibreglass?

Stone wool fibres are coarser than old-style glass fibre, so it sheds less fine airborne dust. It’s still a mineral fibre, though. Long sleeves, gloves, goggles and a dust mask are the standard precautions for either material.

Is Rockwool waterproof?

It’s water repellent rather than waterproof. Stone wool doesn’t absorb moisture readily and dries out without losing performance, but don’t put it anywhere that stays wet. See phenolic foam boards for the closed-cell alternative in damp conditions.