Rock Wool vs Glass Wool: Insulation Density (kg/m³) Explained
Quick answer
Mineral wool is sold by density, given in kg/m³: how many kilograms one cubic metre of the material weighs. Glass wool loft roll is very light (URSA 10 DIVERSO is 10 kg/m³), partition and cavity products sit around 22 to 32 kg/m³, and stone wool slabs start at 45 kg/m³ (Rockwool RWA45) and go up to 140 kg/m³ in the RW range. Denser is not automatically warmer. In Rockwool's RW slabs, going from 45 to 140 kg/m³ barely changes the λ (0.034 to 0.035 W/mK). Density matters most for stiffness, acoustic systems and fire-tested details, so pick it by the job, not by weight.
What kg/m³ means
Density is mass per volume. A slab at 45 kg/m³ means a full cubic metre would weigh 45kg. Multiply by the thickness in metres to get the weight per m² of wall or floor: 100mm of a 45 kg/m³ slab weighs 45 × 0.1 = 4.5 kg/m². That is the figure acoustic specifications and Approved Document E sometimes refer to. Density is not the same as thermal conductivity (λ, in W/mK), which is what sets the R-value. Our guide to insulation thermal conductivity compared covers that side.
Rock wool vs glass wool
- Rock (stone) wool is made by melting volcanic rock and spinning it into fibres. It is generally made at higher densities and sold as semi-rigid or rigid slabs that stand up between studs without slumping. Rockwool is the best-known brand.
- Glass wool is made in a similar way from sand and recycled glass. The fibres are longer and springier, so it compresses into rolls and is light to carry into a loft. Knauf Insulation, URSA, Isover and ODE all make glass wool.
- Fire: every Rockwool, Knauf, URSA and Isover product we list is Euroclass A1, non-combustible. Stone wool tolerates higher temperatures, which is why tested fire-stopping and fire-protection systems often call for a stone wool slab at a stated density. Always follow the tested system exactly.
For a full side-by-side, see stone wool vs glass wool insulation.
Declared densities of the mineral wool we stock
| Product we stock | Type | Format | Declared density (kg/m³) | Declared λ (W/mK) |
|---|---|---|---|---|
| URSA 10 DIVERSO | Glass wool | Loft roll | 10 | 0.044 |
| URSA ACOUSTIC ROLL | Glass wool | Partition roll | 22 | 0.035 |
| URSA PARTY WALL ROLL | Glass wool | Separating wall roll | 22 | 0.035 |
| URSA CAVITY BATT 35 | Glass wool | Cavity batt | 22 | 0.035 |
| URSA FRAMETEC 32 | Glass wool | Timber frame roll | 32 | 0.032 |
| URSA CAVITY BATT 32 | Glass wool | Cavity batt | 32 | 0.032 |
| Rockwool Flexi | Stone wool | Flexible-edge slab | Over 10 (exact figure not declared) | 0.038 (0.035 at 140mm+) |
| Rockwool RWA45 | Stone wool | Acoustic slab | 45 | 0.035 |
Densities are the nominal densities declared on the manufacturers' datasheets (URSA datasheets, July 2025; Rockwool RW Slabs datasheet, February 2026). Rockwool says Flexi "exceeds 10 kg/m³ in accordance with Approved Document E" but does not publish an exact figure, so we do not quote one. Knauf Insulation Loft Roll 44, Knauf DriTherm, Isover APR 1200 and ODE Lunaflex do not give a density on the datasheets we hold, so they are left out rather than guessed.
Denser isn't always warmer
Rockwool's own RW slab data makes the point well. From the lightest to the heaviest slab in the range, the λ hardly moves:
| Rockwool RW slab | Nominal density (kg/m³) | λ (W/mK) | Sound absorption αw at 25mm | Sound absorption αw at 100mm | Stocked by IWD |
|---|---|---|---|---|---|
| RWA45 | 45 | 0.035 | 0.55 (H) | 1.00 | Yes |
| RW3 | 60 | 0.034 | 0.55 (H) | 1.00 | No |
| RW4 | 80 | 0.034 | Not listed | 1.00 | No |
| RW5 | 100 | 0.034 | 0.60 (MH) | 1.00 | No |
| RW6 | 140 | 0.035 | Not listed | 1.00 | No |
RWA45 at 45 kg/m³ and RW6 at 140 kg/m³ both declare 0.035 W/mK, so a 100mm slab of either gives the same R-value, about 2.85 m²K/W. RW6 just weighs three times as much and costs more. Glass wool is different: at the very light end, more density does help. URSA's 10 kg/m³ loft roll is 0.044 W/mK, the 22 kg/m³ products are 0.035 and the 32 kg/m³ products are 0.032. So the honest answer is: check the λ on the datasheet, not the density, when you are buying for heat.
Where density does matter
Sound absorption and acoustics
Mineral wool in a wall or floor cavity absorbs sound that would otherwise bounce around the void. In Rockwool's data, all the RW slabs reach a sound absorption coefficient (αw) of 1.00 at 100mm. At 25mm, the 100 kg/m³ RW5 is slightly ahead (0.60 against 0.55). So for a typical stud wall full of 50 to 100mm of wool, the density of the wool makes far less difference than the plasterboard (layers, mass and acoustic grade), sealing gaps and decoupling. Approved Document E's example constructions ask for mineral wool of at least 10 kg/m³ in internal walls and floors, which all the products above meet. See how to soundproof a stud wall and Part E sound insulation materials.
Stiffness and fit
Higher-density slabs are stiffer, so they hold their shape against a wall, under a floating floor or behind cladding. Light rolls are easier to lay over joists and to cut around obstructions, but they need support.
Fire
Fire-stopping, cavity barriers and fire-protection systems are tested with a specific product at a specific density and thickness. If the system says a 100 kg/m³ slab, a 45 kg/m³ slab is not a substitute, even though both are A1.
Which density for which job
- Loft (joist level): light glass wool roll, such as Knauf Loft Roll 44, URSA 10 DIVERSO or ODE Lunaflex. Cheapest per m², easy to lay to 270mm. Browse loft insulation and use the loft roll calculator.
- Stud partitions: glass wool acoustic partition roll (URSA ACOUSTIC ROLL at 22 kg/m³, Isover APR 1200, ODE APR HP) or Rockwool RWA45 slabs at 45 kg/m³. See Rockwool slabs and the partition calculator.
- Separating (party) walls: URSA PARTY WALL ROLL (22 kg/m³) or RWA45, built to the wall system's specification.
- Full-fill cavity walls: water-repellent cavity batts and slabs such as URSA CAVITY BATT 32 and 35 or Knauf DriTherm 32 and 37. Choose by λ and cavity width. Browse cavity wall insulation.
- Timber frame and rafters: URSA FRAMETEC 32 (32 kg/m³, λ 0.032) or Rockwool Flexi, which has a flexible edge that springs into the frame.
- Fire and high-load jobs: follow the tested system. If it calls for a heavier RW slab than we list, call us on 03330 908479.
Frequently asked questions
Is a denser slab always better?
No. For heat, check λ: RWA45 and RW6 have the same 0.035 W/mK despite a threefold difference in density. Denser slabs earn their keep for stiffness, some acoustic details and fire-tested systems.
Is rock wool warmer than glass wool?
Not necessarily. URSA's 32 kg/m³ glass wool products declare 0.032 W/mK, better than Rockwool RWA45 at 0.035. Light loft roll at 0.044 is the least efficient per millimetre but by far the cheapest per m².
Why don't all products list a density?
Density is not one of the characteristics manufacturers have to declare under BS EN 13162, so many datasheets leave it out. We only quote figures the manufacturer publishes.
What does 10 kg/m³ in Approved Document E mean?
Approved Document E's example wall and floor constructions call for mineral wool with a density of at least 10 kg/m³ in the cavity. It is a minimum, not a target.
Order mineral wool
Browse glass and mineral wool, Rockwool slabs, URSA, Knauf Insulation and Isover, all at trade prices for everyone, and work out quantities with the insulation calculator. Delivered in England, with a £400 ex VAT minimum order and free delivery. Call 03330 908479 for help choosing.
Sources: Rockwool RW Slabs datasheet (densities, λ and αw); Rockwool Flexi product page; URSA datasheets for 10 DIVERSO, ACOUSTIC ROLL, CAVITY BATT 32 and 35, FRAMETEC 32 and PARTY WALL ROLL (July 2025); Approved Document E (GOV.UK). Figures are indicative and correct at the time of writing; check the current datasheet and the tested system for your project, and Building Control has the final say.
Guide only: figures are typical values for general information. Always check the manufacturer's current data sheets and confirm Building Regulations compliance with your designer, SAP assessor or building control. Insulation Warehouse Direct accepts no liability for decisions made using this guide.