Rockwool (stone wool) insulation: fire-rated walls and acoustic applications

TL;DR

Rockwool (also called stone wool or mineral wool) is a different insulation category from glasswool: made from melted basalt rock and slag rather than recycled glass, spun into denser fibres with materially higher fire resistance and acoustic performance. Standard rockwool batts run 40 to 80 kg/m3 density vs glasswool’s 11 to 24 kg/m3, which gives rockwool the edge in fire-rated walls (FRL-rated separations, inter-tenancy boundaries) and acoustic-critical applications (party walls, home theatres, music rooms). The trade-off is cost: rockwool typically runs 40 to 80% more expensive than glasswool at equivalent R-value, plus rockwool’s higher density means more product weight per square metre (less economic on volume residential where R-value is the only driver). The Rockwool Group brand is the global leader in stone-wool insulation, with Australian supply through national merchant networks; Bradford Insulation’s heritage Soundscreen product is the volume Australian acoustic batt (mixed glasswool / mineral-wool product line); Knauf produces a small stone-wool range. Standards are the same AS/NZS 4859.1:2018 (general) and AS/NZS 3999:2015 (install) as glasswool. The two job-killers: substituting rockwool for the same R-value glasswool with no other purpose (you pay 50%+ more for thermal performance that’s identical), and using rockwool in fire-rated walls without the specific tested system (FRL ratings are tested-assembly-specific; generic stone wool is not interchangeable with a tested system).

What it is

Rockwool is a mineral fibre insulation made from natural rock (typically basalt or diabase) plus iron-and-steel-mill slag, melted at high temperature (~1500°C) and spun into fine fibres by a centrifugal spinning process. The fibres are coated with a low-percentage binder, formed into a batt or board, and cured. The trapped air between fibres is the thermal insulator; the dense fibre matrix and the high melting point of the stone give the product its fire resistance.

The Rockwool Group brand (Danish multinational) dominates the global stone-wool market and the term “Rockwool” has become genericised, but the technical product class is mineral wool or stone wool. Glasswool (recycled glass) is a separate chemistry that doesn’t carry the same fire performance.

Key technical differences from glasswool:

Property	Glasswool	Rockwool (stone wool)
Base material	Recycled glass	Basalt + slag
Fibre melt point	~600°C	~1000°C (above building fire test temperatures)
Density range (typical)	11 to 24 kg/m3	40 to 80 kg/m3
Sound absorption coefficient (typical)	0.5 to 0.7	0.7 to 0.95
Fire classification (AS/NZS 1530.3)	Variable; Group 2-4	Group 1 (typically)
Water absorption	Hydrophilic (absorbs water)	Hydrophobic (sheds water)
Compressive strength	Low (compresses easily)	Higher (resists compression)
Cost premium over glasswool	Baseline	40 to 80% over equivalent R-value

Where rockwool is the right call

Fire-rated separation walls (FRL applications): where the NCC requires a Fire Resistance Level (FRL-30, FRL-60, FRL-90, FRL-120), the wall is a tested assembly combining studs, lining, and insulation. Many FRL-rated wall systems use rockwool because the stone fibre survives the fire test temperatures without significant degradation, giving the wall the rated fire performance.

FRL requirement	Where in residential
FRL-60	Inter-tenancy walls (townhouse separation, dual occupancy boundary, apartment party wall)
FRL-90	Higher-density residential, Class 2 mid-rise
FRL-120	Multi-residential Class 2 or higher

Class 1a single dwellings have minimal FRL requirements; the bulk of residential rockwool use is in townhouse and dual-occupancy boundary walls.

Acoustic-critical applications: rockwool’s higher density gives materially better sound absorption per thickness than glasswool. The applications:

Inter-tenancy party walls: NCC Volume Two F5 sets the Rw rating; rockwool helps achieve Rw 50 to Rw 55
Home theatres and music rooms: dedicated acoustic isolation; rockwool in walls + ceilings
Mechanical plant rooms: pump and HVAC noise containment
Around bathrooms / toilets in living areas: privacy walls

High-temperature applications: industrial overlap; rockwool tolerates flue surrounds, hot ducts, and other elevated-temperature environments where glasswool would degrade.

Wet or splash-prone applications: rockwool’s hydrophobic surface (water beads off rather than soaking in) makes it more recoverable from accidental wetting than glasswool.

Where rockwool is wrong

Standard residential where R-value is the only driver: pay 50%+ more for the same R-value with no other benefit. Use glasswool or polyester.
Owner-builder DIY where allergen sensitivity is the driver: stone wool fibres irritate skin and respiratory tract similarly to glasswool. Use polyester for low-irritant install.
Roof spaces over Class 1a residential where the NCC doesn’t require FRL: extra cost without compliance benefit.
Underfloor on lightweight subfloor: rockwool’s higher weight may load lightweight underfloor netting beyond rating. Confirm subfloor support before specifying.

R-values and product variants

Rockwool products span the same R-value range as glasswool, at the same approximate thickness. The product variants:

Product type	Density (kg/m3)	Typical R-value	Where used
Standard residential wall batt	40-50	R2.0 to R2.5	External walls (residential)
High-density acoustic batt	60-80	R2.5 to R3.0	Inter-tenancy walls, home theatre, music room
Fire-rated batt (lower-density, tested system)	32-45	R1.5 to R2.7	FRL-rated tested assemblies (specific brand and product per the tested system documentation)
Ceiling batt	32-40	R3.5 to R5.0	Multi-residential, Class 2 ceiling cavities
Rigid board (board format)	80-150	R0.5 to R1.5 (per board)	Continuous external insulation, fire-rated ducting

The Rockwool brand uses product designators like RW3, RW45, etc. that map to density; verify the specific product against the tested system documentation when FRL-rating is required.

Australian manufacturers and supply

Brand	Range	Where supplied
Rockwool Group (RWA Insulation)	Full stone-wool range; the global market leader brand	National, specifier-led; through major merchants
Knauf Insulation	Limited stone-wool range alongside their main glasswool products	National
Bradford Soundscreen (CSR)	Acoustic batt; mixed glasswool / mineral-wool product line; the volume Australian acoustic product	National
Fletcher Insulation	Niche stone-wool range	National

For FRL-rated walls, the specific tested system is what matters; the system documentation names the brand, product, and assembly. Substituting a different brand or product within the same chemistry category invalidates the FRL rating.

NCC compliance

For thermal insulation, the same NCC Part H6 Total R-value targets apply regardless of insulation type (rockwool, glasswool, polyester all deliver the same thermal performance at equivalent product R-value).

For fire-rated walls, NCC 2022 Volume Two Specification 4 sets the test methods. The tested wall system is the compliance basis; the insulation is one component of that system. Common tested systems use:

Specific brand and product code of rockwool (e.g. Rockwool Flexi or Rockwool RWA)
Specific stud size and centres (typically 600 mm or 450 mm)
Specific plasterboard type and number of layers (e.g. 2 x 13 mm fire-rated)
Specific edge sealing detail

Generic rockwool of the same density is not interchangeable with the tested product unless specifically certified. This is the compliance trap: substituting an “equivalent” rockwool product that breaks the tested-system rating.

Install requirements

Install rules are the same as glasswool under AS/NZS 3999:2015. The main practical differences:

Heavier batt: 50 to 100% more per square metre. Manual handling considerations on long install runs.
Easier to cut cleanly: rockwool cuts with a serrated insulation knife cleanly; less fibre release than glasswool during cutting.
PPE same as glasswool: respirator, safety glasses, gloves, coveralls.
Cavity-fit may need adjustment: higher density retains shape better; tighter friction-fit between studs.
Compression rule applies: compressed rockwool still loses R-value proportionally to compression.

For fire-rated walls, the install detail must match the tested system exactly: stud spacing, batt thickness, gap-free coverage, plasterboard fix pattern, edge sealant.

Common defects and on-site issues

Substituted brand or product in FRL system: the tested system specifies the exact rockwool product. Substituting another brand at the same density looks identical but invalidates the FRL. The certifier should check the system documentation against what’s on site.
Gaps at top plate or wall corners in fire-rated wall: any gap in the insulation envelope in a fire-rated wall reduces the FRL because fire can travel through the gap. Insulation must be gap-free around the perimeter.
Compressed rockwool in over-thick cavity: a 90 mm thick batt installed in a 75 mm cavity is compressed 17%; loses ~17% R-value and may not fit the tested system thickness requirement.
Wrong density for acoustic application: standard-density rockwool used where high-density is specified. Verify product label and density on each pack.
Rockwool around halogen downlight: same 50 mm clearance rule as glasswool. LED IC-F rated luminaires allow direct contact.
Wet rockwool during install: stone wool sheds water, but extended wet exposure can still impact the binder and reduce R-value. Cover and protect during install in wet conditions.

Pricing (2026 indicative, ex-GST, supply only)

Product	Per square metre
Standard wall batt R2.5 (Rockwool)	$18-26
High-density acoustic batt R2.5	$26-38
Fire-rated tested-system batt	$26-44 (product + system documentation)
Ceiling batt R3.5	$24-34
Ceiling batt R5.0	$36-50
Bradford Soundscreen (mixed glass/mineral wool)	$20-30
Rigid stone-wool board (50 mm)	$35-55

Compare to glasswool: rockwool is roughly 40 to 80% more expensive at equivalent product R-value. For acoustic and fire-rated applications, the premium is justified; for general thermal use, glasswool is the volume choice.

Standards and references

Standards Australia, AS/NZS 4859.1:2018 Thermal insulation materials for buildings. https://store.standards.org.au (verified 2026-05-13).
Standards Australia, AS/NZS 3999:2015 Bulk thermal insulation, Installation. https://store.standards.org.au (verified 2026-05-13).
Standards Australia, AS/NZS 1530.3:1999 Methods for fire tests on building materials. https://store.standards.org.au (verified 2026-05-13).
Australian Building Codes Board, NCC 2022 Volume Two and Specification 4 (FRL test methods). https://ncc.abcb.gov.au/editions/ncc-2022/adopted/housing-provisions (verified 2026-05-13).