Mineral Wool Insulation 100mm: Glass Wool vs Stone Wool, Where Each Goes, and How to Install It Without Itching for a Week

Compress mineral wool into a cavity that's too tight and you don't get thinner insulation performing at full capacity. You get a flattened batt with its air pockets crushed, delivering a fraction of the rated thermal resistance shown on the packaging. That mistake, made thousands of times a year in UK lofts and stud walls, turns a cost-effective insulation choice into expensive padding. Mineral wool is the cheapest mainstream insulation you can buy. But cheap only matters if you install it correctly.

What it is and what it's for

Mineral wool is spun fibre insulation made from either molten glass (glass wool) or molten volcanic rock (stone wool, commonly called rock wool). The fibres trap millions of tiny air pockets, and those air pockets are what insulate. Not the fibres themselves. This is why compression destroys performance: squeeze the wool and the air escapes.

It comes in two formats: rolls for large open areas like lofts and between floor joists, and rigid or semi-rigid slabs for stud walls, rafter spaces, and anywhere you need the insulation to stay put vertically. Both formats are Euroclass A1 non-combustible under BS EN 13501-1. That's the highest fire classification available. Stone wool won't melt below 1,000 degrees Celsius; glass wool melts at around 600 degrees. Both are classified identically for building regulations purposes, but stone wool gives you genuinely longer fire resistance in practice.

This fire performance is mineral wool's biggest advantage over PIR boards. PIR carries Euroclass E or F (combustible) and must always be covered by plasterboard. Mineral wool is non-combustible full stop. In party walls, fire-rated partitions, and loft conversions where 60-minute fire protection is needed, mineral wool isn't just an option. It's what building control expects.

The trade-off is thickness. Mineral wool has a lambda (thermal conductivity) of 0.032-0.044 W/mK depending on product. PIR achieves 0.022 W/mK. In practical terms, you need roughly 150-180mm of mineral wool to match the thermal performance of 100mm of PIR. Where space is tight (between rafters with a ventilation gap to maintain, or in thin cavity walls), PIR wins. Where space isn't a constraint and you need fire resistance or acoustic performance, mineral wool is cheaper, easier to work with around pipes and cables, and won't burn.

Glass wool vs stone wool

These are different products with different properties, not interchangeable options. Understanding which to use where saves money and gets the job done properly.

The price difference is substantial. A Knauf Earthwool Loft Roll 44 at 100mm covers 11-14 m2 per roll for around £26-36. A pack of four Rockwool RWA45 slabs at 100mm covers 2.88 m2 for around £27. Per square metre, you're paying roughly four times more for stone wool slabs than glass wool rolls.

That difference is justified when you need it. For acoustic partitions between rooms, stone wool's higher density (45 kg/m3 for RWA45) absorbs significantly more sound than glass wool at 10-20 kg/m3. For loft insulation at ceiling level where you're rolling it out between joists, the cheaper glass wool roll does the same thermal job.

The products you'll see on shelves

Knauf Earthwool Loft Roll 44 - glass wool, lambda 0.044 W/mK, Euroclass A1. The standard budget loft insulation roll. ECOSE Technology means no added formaldehyde and noticeably less itchy than traditional glass wool. Comes in 1140mm width that splits into two 570mm or three 380mm strips (Combi-Cut). Around £26 for 11 m2 at Wickes, £36 for 14 m2 from insulation specialists.

Rockwool RWA45 - stone wool slab, lambda 0.035 W/mK, density 45 kg/m3, Euroclass A1. The acoustic and fire slab. Stiff enough to friction-fit between studs without sagging. 1200 x 600mm slabs, four per pack covering 2.88 m2. Around £27 per pack including VAT from online insulation suppliers.

Rockwool RW3 and RW5 - denser stone wool (60 and 100 kg/m3 respectively), both lambda 0.034 W/mK. For specialist applications where higher compressive strength is needed (warm flat roofs, external renders). Most homeowners won't need these.

Superglass Multi Acoustic Roll - glass wool, lambda 0.044 W/mK, minimum density 10 kg/m3, up to 84% recycled glass. Budget acoustic roll option. Cheaper than Rockwool RWA45 for acoustic floor infill, but lower density means less sound absorption.

Knauf OmniFit Slab 35 - premium glass wool slab, lambda 0.035 W/mK. Knauf's answer to RWA45. Provides 167% more insulation per pallet than comparable low-density rock wool according to Knauf's own data. Lighter than stone wool, which makes a genuine difference when you're lifting slabs above your head into rafter spaces.

How to work with it

PPE first

This is not optional. Mineral wool fibres are mechanically irritating. Stone wool fibres are thicker and coarser (they physically pierce the outer layer of skin), while glass wool fibres are finer but more likely to become airborne. Either way, exposed skin itches. Exposed eyes sting. Inhaled fibres irritate your throat and lungs.

The reaction ranges from mild itching to full-body redness and wheezing that lasts days. Forum posts from homeowners who "just did a quick loft insulation job" without PPE are universally regretful. One described being unable to sleep for two nights from itching. Mineral wool is not classified as a carcinogen (EU REACH confirmed this), but the immediate irritation is real and entirely avoidable.

Cutting

For rolls, the best method is to cut the roll while it's still compressed and bagged. Stand it upright and use a long panel saw (a 22-inch hardpoint saw works well) to cut across the width. This is cleaner and produces far less airborne fibre than unrolling the wool and trying to cut it flat on the floor.

For slabs, use a long serrated bread knife or an insulation knife (a purpose-made tool with a long serrated blade, around £8-15 from builders' merchants). Mark your line, press the slab flat, and cut in a single smooth stroke. Don't saw back and forth repeatedly; it tears the fibres and creates more dust.

Cut batts 10-15mm wider than the cavity they're going into. Mineral wool should friction-fit between studs, joists, or rafters with gentle compression at the edges. Not rammed in. Not loose enough to fall out. The slight oversize creates a push fit that holds the batt in place and ensures no gaps at the edges.

Loft installation (ceiling level)

Click to enlarge

Building regulations require 270mm of mineral wool in lofts at ceiling level to achieve a U-value of 0.16 W/m2K. The standard method is two layers: 100mm between the joists, then 170mm laid perpendicular on top, crossing over the joists. This second layer covers the timber joists themselves, which would otherwise act as thermal bridges (heat conducting through the timber, which insulates poorly).

Don't push insulation tight into the eaves. Leave a minimum 50mm ventilation gap between the insulation and the roof underlay at the eaves. This airflow prevents condensation forming on the underside of the roof structure. Blocking it causes damp timber, mould, and eventually rot. Proprietary eave ventilator combs (plastic strips that maintain the gap) cost pennies and are worth fitting.

Cables must run above the insulation, not beneath it. Cables buried under 270mm of mineral wool overheat because the insulation traps their waste heat. This is a genuine fire risk and a building control fail point.

Stud wall installation (the acoustic air-gap principle)

This is the counterintuitive part that most installation guides get wrong. For acoustic insulation in timber stud partitions, do not full-fill the cavity.

A 95mm deep stud filled with 95mm of mineral wool touching the plasterboard on both sides transmits sound through direct contact. The insulation absorbs airborne sound, but the physical contact transfers vibration straight through to the plasterboard. The plasterboard then acts as a speaker membrane, radiating sound into the next room.

The correct approach: use 50-70mm mineral wool in a 95mm stud cavity, leaving a 25-45mm air gap between the insulation and one face of plasterboard. The insulation absorbs airborne sound. The air gap decouples the insulation from the plasterboard, breaking the vibration path. This combination outperforms a full-fill cavity for sound insulation.

Building Regulations Approved Document E requires a minimum density of 10 kg/m3 for mineral wool in acoustic partitions, with a minimum 25mm thickness in partitions and 100mm in intermediate floors. Rockwool RWA45 at 45 kg/m3 far exceeds this minimum and is stiff enough to stay in place without any additional support.

Click to enlarge

Double plasterboard (two layers of 12.5mm board on each side) often does more for sound reduction than increasing mineral wool thickness. If you're chasing acoustic performance, spend on extra plasterboard before you upgrade to thicker or denser mineral wool.

Floor joists (intermediate floors)

Between floor joists separating two heated rooms, mineral wool is acoustic insulation, not thermal. No vapour control layer is needed when both rooms are at similar temperature. Lay the mineral wool between joists, resting on the ceiling below. 100mm is standard; going thicker than 100mm gives less than 5% acoustic improvement in most domestic floor constructions. Your money is better spent on acoustic resilient bars under the plasterboard ceiling or on decoupling strips on top of the joists.

How much do you need

Loft (ceiling level, 270mm total)

For a loft, you need two layers totalling 270mm. Calculate the loft floor area in square metres.

Worked example: 8m x 5m loft (40 m2)

First layer (100mm between joists): 40 m2 of 100mm rolls. Knauf Loft Roll 44 at 11 m2 per roll = 3.6 rolls, round up to 4 rolls. Add 10% waste = 4.4 rolls, round up to 5 rolls.

Second layer (170mm over joists): 40 m2 of 170mm rolls. Coverage per roll varies by product. At approximately 8 m2 per roll = 5 rolls. Add 10% waste = round up to 6 rolls.

Total: 11 rolls. At £26 per roll (Wickes), that's roughly £286 in materials for a 40 m2 loft. Under £300 to insulate your entire loft to current building regulations.

Stud walls

Measure the total stud wall area (height x length, minus door openings). For Rockwool RWA45 slabs at 2.88 m2 per pack:

Worked example: stud partition 3.5m long x 2.4m high (8.4 m2)

8.4 / 2.88 = 2.9 packs. Add 10% waste = 3.2, round up to 4 packs. At approximately £27 per pack, that's around £108.

For glass wool rolls used in partitions (Knauf Acoustic Roll), coverage is higher per pack and cost per m2 is lower. If acoustic performance isn't critical, glass wool rolls at around £3-4/m2 will do the job.

Cost and where to buy

Mineral wool pricing splits sharply by product type. Glass wool loft rolls are among the cheapest insulation materials you can buy. Stone wool acoustic slabs cost four to five times more per square metre, but you're paying for density and acoustic performance that glass wool rolls can't match.

The price gap between loft rolls and acoustic slabs reflects a genuine difference in material. Don't pay stone wool slab prices for a loft at ceiling level where a glass wool roll performs identically for thermal purposes. Equally, don't use a glass wool loft roll in an acoustic partition where you need the density and rigidity of RWA45.

£1,000–2,000 is a typical budget for roof insulation materials and installation on a single-storey extension, covering both PIR and mineral wool options depending on specification.

VAT: the hidden saving

Installation of insulation in residential buildings carries 0% VAT when a contractor supplies and installs in a single contract. This relief runs until March 2027 in Great Britain. If you buy the materials yourself from Wickes or Screwfix, you pay 20% VAT on the materials. If a contractor supplies the same materials as part of a supply-and-install contract, the entire job (labour and materials) is zero-rated.

On a £500 insulation materials bill, that's a £100 saving. Worth structuring the job as supply-and-install rather than buying materials yourself, even if the contractor's material markup eats some of the saving.

Where to buy

Wickes and Toolstation carry Knauf loft rolls and basic acoustic rolls. Stock is reliable and pricing is competitive for glass wool products. Neither stocks a wide range of stone wool slabs.

Insulation specialists (Online Insulation Sales, Insulation Superstore, Insulation Hub) carry the full range of both Knauf and Rockwool products at competitive prices, often with free delivery above a modest threshold. For stone wool slabs, these are usually cheaper than builders' merchants.

Travis Perkins and Jewson stock Rockwool products on their trade counters but pricing varies by branch and trade account status. Worth checking if your builder has a trade account, but compare against online specialists before assuming the trade account price is best.

Alternatives

PIR boards (Celotex, Kingspan) - dramatically better thermal performance per millimetre (lambda 0.022 W/mK vs 0.035-0.044 for mineral wool). 50mm of PIR matches roughly 100mm of mineral wool thermally. Use PIR where space is tight: cavity walls, under rafters where every millimetre of headroom matters. But PIR is combustible (Euroclass E/F), costs more, and has zero acoustic benefit. For party walls and fire-rated partitions, PIR is not acceptable.

EPS (expanded polystyrene) - lambda 0.032-0.038 W/mK, roughly similar to premium mineral wool slabs. Lighter and easier to handle. Used primarily in floor insulation under screed and in external wall insulation systems. Not suitable for acoustic applications.

Cavity wall insulation batts - a specific mineral wool product (typically 50-75mm glass wool) designed to friction-fit in masonry cavity walls during construction. A different form factor from general-purpose mineral wool for a different application. If your walls are masonry cavity construction, your builder will use cavity batts, not loft rolls or acoustic slabs.

Insulated plasterboard - plasterboard with a layer of insulation (PIR or phenolic foam) pre-bonded to the back. Used to add insulation to solid walls in a single fixing operation. More expensive per m2 than buying insulation and plasterboard separately, but saves labour.

Where you'll need this

• Insulation - wall, floor, and roof insulation specification and installation

Mineral wool appears across any extension, loft conversion, or renovation project that involves thermal or acoustic insulation. It's not specific to any one build type.

Common mistakes

Compressing batts to fit. The single most common installation error. If your 100mm batt goes into a 75mm cavity, you haven't saved 25mm of space. You've crushed the air pockets that provide insulation and reduced thermal performance by up to 50%. Cut a thinner batt or buy the right thickness. National Physical Laboratory research confirmed this: compression halves R-value.

Boarding a loft directly on top of insulation. Chipboard or OSB laid straight onto mineral wool compresses it flat. The insulation under the boards delivers a fraction of its rated performance. Use raised loft legs (stilts that lift the boarding above the insulation) to maintain the full 270mm depth. Loft legs cost around £2-3 each and save the entire cost of the insulation from being wasted.

Blocking eave ventilation. Pushing insulation tight into the eaves blocks the airflow that prevents condensation on roof timbers. The 50mm gap at the eaves is not optional. Fit eave ventilator combs to maintain the gap.

Burying electrical cables. Cables under 270mm of mineral wool overheat. All cables in an insulated loft must run above the insulation or be derated (specified for higher temperature operation). This is a building control fail point and a fire risk.

Using loft rolls in vertical stud walls. Glass wool loft rolls at 10-15 kg/m3 density are too light and floppy to stay in place between vertical studs. They sag over time, leaving the top of the cavity uninsulated. Use semi-rigid slabs (RWA45, Knauf OmniFit) or acoustic rolls designed for vertical installation.

Ordering dense stone wool for a loft. Rockwool RWA45 at £9-12/m2 is four times the price of a glass wool loft roll at £2.35-2.60/m2. For loft insulation at ceiling level, the cheaper glass wool roll delivers identical thermal performance. Stone wool's acoustic and fire advantages don't matter in an unoccupied loft void. Save the money.

Skipping PPE. Covered above, but the number of forum posts from people who "just did a quick job" without gloves and a mask, then spent two days itching, suggests this warning cannot be repeated too often. Wear the PPE. Every time.