Acoustic batt (sound insulation)

TL;DR

An acoustic batt is a higher-density variant of glasswool or polyester insulation (typically 24-32 kg/m³ vs ~11-14 kg/m³ for thermal-only batts) used in inter-tenancy separating walls between Class 1b dwellings (e.g. terrace houses) and Class 2 dwellings (e.g. apartments). The batt is rated by Rw acoustic value (weighted sound reduction index) in addition to its thermal R-value. The NCC Volume Two Part F7 (and equivalent in Volume One) sets the Rw + Ctr target for inter-tenancy walls. A thermal batt at the same nominal R does NOT deliver the same acoustic performance; substitution is a common builder error.

What it is

Property	Thermal batt	Acoustic batt
Density	11-14 kg/m³	24-32 kg/m³ (or higher for polyester variants)
Material	Glasswool, polyester, rockwool	Same materials, higher density
Thickness	Sized to wall cavity	Sized to wall cavity
R-value	1.5-3.5 m²·K/W typical	1.5-3.5 m²·K/W (similar)
Rw acoustic rating	Not rated	Rated as part of wall system (e.g. Rw 50, Rw 55, Rw 60)
Cost	$5-$12 per m²	$15-$28 per m²

The Rw value is a system rating (wall assembly), not a single-product rating. The acoustic batt contributes to the system; the system includes the wall framing, lining, and seal details.

Where it’s required

NCC Volume Two Part F7 sets minimum acoustic performance for inter-tenancy separating walls in Class 1b dwellings:

Wall element	Minimum performance
Inter-tenancy wall (between dwellings)	Rw + Ctr ≥ 50
Inter-tenancy wall to a wet area	Rw + Ctr ≥ 50 + discontinuous construction

For Class 2 (apartments), NCC Volume One Part F7 sets typically higher targets:

Wall element	Minimum performance
Inter-tenancy wall (between sole-occupancy units)	Rw + Ctr ≥ 50 (some jurisdictions higher)
Wall to common corridor	Rw ≥ 50
Wall between dwelling and adjacent garage	Rw + Ctr ≥ 50

The wall ASSEMBLY (framing + lining + insulation) is what’s rated; the acoustic batt is one component.

Common wall systems achieving Rw + Ctr 50

Wall system	Construction	Typical Rw + Ctr
Single 13 mm plasterboard + 90 mm timber frame + standard thermal batt + 13 mm plasterboard	Light construction	~30-35 (FAILS NCC)
2x 13 mm plasterboard each side + 90 mm timber frame + acoustic batt	Double-lining each side	~45-50 (marginal)
2x 13 mm plasterboard each side + 90 mm staggered stud + acoustic batt	Staggered stud	~50-55
2x 13 mm plasterboard each side + 90 mm + 90 mm separated walls (discontinuous) + acoustic batt	Full discontinuous construction	~55-65

A simple thermal-batt-in-frame wall does NOT meet NCC. Inter-tenancy walls need engineered assemblies.

Where a thermal batt won’t substitute

Application	Thermal batt	Acoustic batt
Inter-tenancy separating wall	NO	Required
Class 1b / Class 2 floor-ceiling separating system	NO	Required (high-density variant)
External wall (thermal performance)	Yes	Overspec but works
Internal wall (sound privacy bonus)	Marginal benefit	Better; 13 mm + acoustic batt feels noticeably quieter
Roof / ceiling thermal	Yes	Overspec

Cost band (2026 AUD ex-GST)

Item	Indicative cost
Standard thermal batt R2.0	$5-$10 per m²
Acoustic batt R2.0 (high density)	$15-$28 per m²
Premium for inter-tenancy wall (50 m² wall)	+$500-$900 over thermal

Brand and product examples

Knauf Earthwool Acoustic (high-density glasswool).
Bradford Black (high-density polyester acoustic).
Fletcher Insulation Permastop (high-density glasswool).
Pinkfit Acoustic (Pink Batts variant for acoustic).
Rockwool RWA (high-density rockwool, also fire-rated).

Manufacturer datasheets give the wall-system Rw + Ctr ratings for typical assemblies. Always reference the datasheet, not just “an acoustic batt”.

Where NOT to use

In place of thermal-only batt in a single-occupancy dwelling exterior wall: overspec, wasted cost.
As the sole acoustic measure (without considering the rest of the wall assembly): system performance is what’s rated.
In a non-Code-required application without consultation: acoustic engineer can advise on selection.

Installation

Friction-fit between studs: same as thermal batts, but the higher density makes them stiffer.
No compression: don’t compress the batt below its stated thickness.
No gaps: gaps at edges destroy acoustic performance.
Seal penetrations: services penetrations must be acoustically sealed (mastic, intumescent).
Wet-area lining: where wet area abuts an inter-tenancy wall, discontinuous construction may be required (separate wall framing on each side).

Common builder errors

Error	Cost
Thermal batt installed instead of acoustic	NCC F7 fail at certifier inspection
Acoustic batt in single-stud wall expecting Rw 50	Wall system doesn’t achieve target
Penetrations not sealed	Sound bypass through services
Wet-area on inter-tenancy wall without discontinuous construction	Pipe noise transmits

For builders

Spec the wall system, not just the batt: refer to the manufacturer’s tested wall assembly that achieves the Rw + Ctr target.
Brief the chippy and plasterer on discontinuous construction requirements for inter-tenancy walls.
Seal every penetration acoustically: data, electrical, water, gas.
Have the acoustic engineer review for any Class 2 build; Class 1b can often be standard-system, Class 2 sometimes needs custom.
Don’t substitute cheaper thermal batts mid-build to save money; the rework cost on a failed inter-tenancy wall is enormous.

Suppliers

Bunnings (Knauf, Bradford retail).
Fletcher Insulation (trade direct).
Pink Batts (trade direct).
CSR Bradford trade.

References

AS/NZS 4859.1 Materials for thermal insulation (verified 2026-05-16).
AS/NZS 1276.1 Acoustics rating of sound insulation (verified 2026-05-16).
NCC 2022 Volume Two Part F7 Sound Insulation (verified 2026-05-16).
Standards Australia