Glasswool batts in Australian construction: R-values, sizing, and install

TL;DR

Glasswool batts are the volume bulk-insulation choice across Australian construction (Class 1a houses, Class 2 low-rise apartments, and Class 3-9 commercial fitouts): recycled glass fibre formed into resilient mats sized to fit between standard wall studs (typical 600 mm or 450 mm centres) and ceiling joists. Class 2 separating-wall and inter-tenancy applications add acoustic-rated and higher-density product variants that the volume Class 1a stock doesn’t carry. The two specification dimensions are R-value (thermal resistance, ranging from R1.5 for thin walls to R6.0+ for cathedral ceilings) and product fit (wall batt vs ceiling batt vs underfloor; each has different density, facing, and thickness to suit its application). The volume manufacturers are Bradford Insulation (CSR), Knauf Insulation (Earthwool brand), and Pink Batts (Fletcher Building / Tasman Insulation), all producing equivalent products to AS/NZS 4859.1:2018. The compliance driver is NCC 2022 Volume Two Part H6 (Energy efficiency), which sets the Total R-value the wall, ceiling, and floor must achieve per climate zone (climate zone 5, Sydney metro: Total R-value 2.8 in wall, 5.1 in ceiling, 2.25 in floor). The two job-killers: compressing the batt during install (a batt squeezed to 70% thickness loses 30% of its R-value), and leaving gaps at junctions (a 5% gap area drops effective R by roughly 50% via thermal short-circuit). Installation must comply with AS/NZS 3999:2015 (verified 2026-05-13).

What it is

Glasswool batts are insulation mats made from recycled glass (typically 50 to 80% recycled content) melted and spun into long fine fibres, formed into resilient mats with a binder, and cut to size. The fibres trap still air; the still air is the thermal insulator. The batt material itself contributes about 4% of the insulation; the trapped air does the rest.

Alternative bulk insulation materials cover slightly different markets:

Polyester batts: recycled-plastic-fibre based; allergen-free; preferred for allergy-sensitive households; slightly higher cost
Sheep wool batts: niche premium; natural fibre; specialty applications
Mineral wool / rockwool batts: stone-fibre based; higher fire rating; specialty fire-rated walls
Rigid foam boards (EPS, XPS, PIR): different product class for continuous external insulation; not bulk batts

Glasswool is the volume default because it’s cheapest per R-value-unit and ubiquitous in Australian merchant supply chains. Polyester is the volume alternative on residential builds where allergen sensitivity or fibre-handling concerns drive selection away from glasswool.

The Australian standard for insulation materials is AS/NZS 4859.1:2018 (general criteria) and 4859.2:2018 (design). Installation sits under AS/NZS 3999:2015.

R-values and what they mean

The R-value is the material’s resistance to heat flow, in m2.K/W. Higher R = more insulation. R-values are additive: a wall with R2.5 batt insulation plus R0.5 from the surrounding structure delivers a Total R-value (the actual wall’s resistance) close to R3.0.

But the additive math only works if the batt is installed correctly: full thickness, no compression, no gaps, no thermal bridge bypass. A compressed or gappy batt delivers materially less R-value than the nominal product spec.

R-value (product label)	Typical thickness	Typical application
R1.5	75 mm	Thin wall studs, internal partition
R2.0	90 mm	Standard 90 mm external wall stud
R2.5	90-110 mm	Higher-performance wall, climate zone 4-5
R2.7	90-130 mm	High-spec wall, climate zone 6-7
R3.0	145 mm	Wider stud wall (deeper cavity), high-performance
R3.5	175 mm	Standard ceiling, climate zone 5
R4.0	200 mm	Cool-climate ceiling, climate zone 6-7
R5.0	240 mm	Cold-climate ceiling, climate zone 7-8
R6.0	290 mm	Premium cold-climate ceiling

For a given product family, R-value scales with thickness. Knauf Earthwool R3.5 ceiling batts are about 175 mm thick; R5.0 is about 240 mm.

Product variants

Product type	Where used	Key spec
Wall batt (snug-fit)	Between wall studs at 600 mm or 450 mm centres	Standard width 580 mm (for 600 mm centres) or 430 mm (for 450 mm centres); standard length 1160 mm or 1200 mm
Ceiling batt	Above ceiling joists, in roof space	Wider product (1160 mm or 1200 mm wide rolls); lower density
Underfloor batt	Below subfloor in suspended floor systems	Higher density to resist sagging; usually held by netting or wire
Mid-floor / inter-tenancy	Within suspended floor cavities (acoustic + thermal)	Acoustic-rated; meets NCC Volume Two F-rating requirements for separating walls
Sound batt (acoustic)	Inter-tenancy walls (Class 1b, 2 separation walls)	Higher density (24+ kg/m3); rated by acoustic Rw value not just thermal R

The same R3.5 R-value can come in different product variants (a ceiling batt and an acoustic batt at the same nominal R3.5 are differently dense and faced).

Australian manufacturers

Brand	Product range	Where supplied
Bradford Insulation (CSR)	Wall batts, ceiling batts, sound batts, Optimo (premium high-density)	National
Knauf Insulation (Earthwool)	Wall, ceiling, underfloor, acoustic; lower-irritant formula	National
Pink Batts (Fletcher Building / Tasman Insulation)	Wall, ceiling, fluffy underfloor; the legacy household brand	National
Fletcher Insulation (Permastop, Permaflux brands)	Wall, ceiling; volume residential	National
Bondor / Kingspan	Rigid foam alternatives (not glasswool batts)	Specifier-led

All four glasswool brands produce equivalent product to AS/NZS 4859. Brand selection is mostly a function of merchant stock and habit; product specs are interchangeable at the same R-value rating.

NCC compliance and Total R-value

The NCC sets the Total R-value the wall, ceiling, and floor must achieve, not just the batt rating. Total R = batt R + structural R (timber, plasterboard, sarking) + air gap R.

NCC 2022 Volume Two Part H6 Total R-value targets for residential walls, by NatHERS climate zone:

Climate zone (NatHERS)	Wall Total R-value	Ceiling Total R-value	Floor Total R-value
1 (Darwin, FNQ)	2.8	5.1	1.5 (suspended)
2 (Brisbane, Cairns coastal)	2.8	5.1	1.5
4 (Mildura, Inland NSW)	2.8	6.0	2.25
5 (Sydney, Perth metro)	2.8	5.1	2.25
6 (Melbourne, Adelaide hills)	2.8	6.0	2.75
7 (Canberra, Hobart, Ballarat)	2.8	6.5	3.25
8 (Alpine)	3.8	7.0	3.75

The product R-value to achieve these Totals: typically R2.5 wall batt (delivering ~R2.8 Total) and R3.5 to R5.0 ceiling batt (delivering R5.1-R6.0 Total). Verify against the specific NatHERS energy assessment for the project.

For a Sydney metro single-storey house, the volume residential specification is R2.5 in walls, R5.0 in ceiling, no underfloor (slab-on-ground). This is the entry-level NCC-compliant package.

Install requirements

AS/NZS 3999:2015 sets installation requirements. The critical install rules:

Rule	Why
Full thickness, no compression	Compression reduces R-value proportionally; 30% compression loses ~30% R
Full coverage, no gaps	A 5% gap in coverage drops effective R by approximately 50% (thermal short-circuit)
Continuous behind plumbing, electrical, junctions	Insulation must continue behind every penetration; cutting around services creates thermal bridges
Clearance from downlights (recessed luminaires)	Old halogen downlights: 50 mm clearance per AS 5110; LED IC-F rated downlights: 0 mm clearance acceptable
No contact with flue or hot duct	50 mm clearance from any flue, B-vent, or hot duct; combustion risk and fire hazard
Vapour control in cool climates	Climate zones 6-8: vapour-permeable membrane or vapour control layer specified per condensation analysis
Sarking compatibility	Wall sarking sits between batt and external cladding; the batt must not press against sarking

The installer typically uses a hand-pull-and-place technique: pull the batt to its full length, push into the cavity, fold around penetrations. A scoring blade is used to cut to length and around services. Friction-fit is the typical retention; staples or strips are used in occasional cases.

Health and safety

Glasswool fibres are skin and respiratory irritants. The 1990s-era CCI fibre was classified as a possible carcinogen by IARC; modern glasswool products use bio-soluble fibres that are no longer classified as carcinogenic (IARC reclassification 2001). But all glasswool products still require basic PPE:

Safety glasses: fibre to eye is unpleasant and a notifiable incident
Respirator (P2 dust mask): for any extended install work
Disposable coveralls: fibres embed in clothing and irritate skin for days
Cotton or nitrile gloves: skin contact is irritating, not seriously hazardous, but prolonged contact builds up

The Knauf Earthwool product range is marketed as “ECOSE technology” with a low-irritant formula; some installers find it materially easier to handle than legacy glasswool products.

Common defects and on-site issues

Compressed batt under low ceiling: a R5.0 batt installed in an attic with only 180 mm clearance is compressed to 75% thickness. Loses ~25% of R-value. The fix is to spec a thinner higher-R batt that fits the actual depth (R3.5 at 175 mm rather than R5.0 at 240 mm).
Missing batts at wall-floor and wall-ceiling junctions: thermal bridge at corner perimeter. Installer should continue insulation into the corner geometry; common oversight on quick-install jobs.
Wrong density for application: a wall batt installed in a ceiling (sags within a year); a ceiling batt installed in a wall (slumps from gravity). Match the product variant to the application.
Service penetrations bypassing batt: every plumbing run, electrical cable, and HVAC duct that punches through the wall must have the batt installed behind it. Common omission.
Vapour barrier missing in cool climate: condensation forms on the cold-side face of the batt in cool climates; the wall cavity becomes wet. AS/NZS 4859 design analysis required.
Downlight burning the insulation: legacy halogen downlight with batt installed flush against the housing. Replace downlight with LED IC-F or maintain 50 mm clearance.
Batts not covering top plate: insulation stops at the top of the wall plate; thermal bridge across the plate. Insulation must continue across the top plate up to the ceiling line.
Tape on the wrong face of foil-faced batt: foil-faced wall batt has a reflective face; if installed with reflective face buried, the R-value drops materially. Confirm install orientation.

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

Product	Per square metre
R2.0 wall batt (90 mm, standard)	$7-11
R2.5 wall batt (90 mm, premium)	$9-14
R3.5 ceiling batt	$12-18
R5.0 ceiling batt	$18-26
R6.0 ceiling batt	$25-35
Acoustic / sound batt (high-density, R2.5)	$14-22
Underfloor batt (R2.5, dense)	$14-22
Premium low-irritant (Knauf Earthwool)	+$2-4/m2 over standard

Installed cost (supply + install) typically adds $5-8 per m2 for ceiling and $8-14 per m2 for wall, varying by accessibility and complexity.

Standards and references

Standards Australia, AS/NZS 4859.1:2018 Thermal insulation materials for buildings, Part 1: General criteria and technical provisions. 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).
Australian Building Codes Board, NCC 2022 Volume Two Part H6 Energy efficiency. https://ncc.abcb.gov.au/editions/ncc-2022/adopted/volume-two/h-class-1-and-10-buildings/part-h6-energy-efficiency (verified 2026-05-13).