Double glazing for residential builds: IGU construction, U-values, and selection

TL;DR

Double glazing in residential is an Insulated Glass Unit (IGU): two panes of glass separated by a spacer bar that contains desiccant (silica gel beads) and seals an air or inert gas cavity between the panes. The Australian standard is AS/NZS 4666:2012; window-and-door system compliance sits under AS 2047:2014. The thermal advantage over single glazing: a clear-clear double glazed unit (4-12-4 build-up, air-filled) delivers a U-value of ~3.0 W/m².K vs ~5.7 for single 6 mm float. Add a Low-E coating (typically on the inside face of the inner pane) and an argon gas fill and the U-value drops to ~1.5 to 1.8: competitive with the wall around it. The two specification calls are gas fill (air for budget, argon for volume residential, krypton for premium narrow-cavity) and spacer type (older aluminium with cold-edge thermal bridge, or modern warm-edge spacer in foam, plastic, or thermally-broken stainless steel that reduces edge condensation and improves overall performance). The two job-killers: seal failure (the perimeter butyl + polysulphide seal eventually fails on every IGU; quality units last 15-25 years, cheap ones 5-10; failure shows as fog or moisture inside the cavity), and Low-E coating on the wrong face (the coating should sit on surface #2 in cool climates, surface #3 for solar control in warm humid; wrong orientation reduces performance materially).

What it is

An Insulated Glass Unit is two (or more) panes of glass separated by a structural spacer bar that:

Holds the panes parallel at a fixed gap (typically 8 to 20 mm)
Contains desiccant beads (silica gel) that absorb residual moisture in the cavity
Seals the cavity at the perimeter (primary butyl seal + secondary polysulphide or silicone seal)
May contain a gas fill (argon, krypton, or air) for thermal performance

The two glass panes are typically annealed float glass, but one or both can be toughened or laminated where AS 1288 requires safety glass or where acoustic or security needs it.

The cavity gap drives both thermal and acoustic performance:

8-12 mm gap: standard, optimised for thermal (the convection cell in the gap suppresses heat transfer)
16 mm gap: marginally better thermal; common in premium products
20 mm gap: maximum useful thermal benefit; beyond this, convection in the cavity reduces gains
Larger gaps (acoustic): 50-100 mm asymmetric gaps for acoustic benefit; uncommon in residential

Build-up nomenclature

IGUs are specified as: outer pane thickness + cavity gap + inner pane thickness, all in mm.

Build-up	Description
4-12-4 (~20 mm)	4 mm outer + 12 mm cavity + 4 mm inner; volume residential standard
6-12-6 (~24 mm)	6 mm + 12 mm cavity + 6 mm; larger panes, residential
6-12-4	Asymmetric; outer pane sized for AS 1288 wind load, inner for thermal
4-16-4 (~24 mm)	Wider cavity; slightly better thermal
6-16-6	Premium standard
4-12-6.38 (mixed laminated)	4 mm + 12 mm cavity + 6.38 laminated inner pane; safety glass for AS 1288 zones
Triple-glazed (4-12-4-12-4)	Three panes, two cavities; passive house, cold-climate residential

Window-and-door fabricators typically stock 4-12-4 and 6-12-6 as their volume builds. Custom build-ups require longer lead time.

Gas fills

The cavity can be filled with air (cheapest), argon (volume residential), or krypton (premium, narrow-cavity).

Gas	Thermal conductivity (W/m.K)	Where used	U-value impact
Air (dry)	0.026	Budget IGU	Baseline
Argon	0.018	Volume residential	~10-20% U-value improvement over air
Krypton	0.0095	Premium narrow-cavity (8-12 mm)	~30-40% improvement over air
Xenon	0.0055	Specialist research	Rare in residential

Argon-filled IGUs are now the volume Australian residential default, slightly more expensive than air-filled but materially better thermal. Argon gradually leaks from the IGU through the perimeter seal at approximately 1% per year; quality manufacture should retain 80%+ argon at 20 years.

Spacer types

The spacer bar is the perimeter element holding the panes apart. Two generations:

Spacer	Construction	Pros	Cons
Aluminium spacer (cold-edge)	Aluminium tube with desiccant	Cheap; standard	High thermal conductivity creates a thermal bridge at the perimeter; condensation forms on the inner-pane edge in cool conditions
Warm-edge spacer	Plastic, foam, stainless steel thermally broken, or composite	Reduces edge condensation; improves overall U-value	~10-20% premium over aluminium

Common warm-edge brands: Super Spacer (Edgetech), TGI (Technoform), Swisspacer Ultimate, Chromatech (Saint-Gobain). The visible spacer colour and profile differ; performance is roughly equivalent across brands.

For Australian climate zones 6-8 (cool-climate residential), warm-edge spacer is the volume choice; it avoids the edge condensation pattern that aluminium spacer creates around January in unheated rooms.

Low-E coating

The thermal performance jump from single glazing to double glazing is large; from clear double glazing to Low-E double glazing is similarly large. Low-E is the key technology.

Low-E (low-emissivity) is a thin metallic coating on one face of the IGU that reflects long-wavelength infrared radiation (heat) while allowing visible light through.

Coating position: which surface?

IGU surfaces are numbered from outside to inside:

Surface	Position
Surface 1	Outside face of outer pane (weather-exposed)
Surface 2	Inside face of outer pane (cavity-facing, outer side)
Surface 3	Outside face of inner pane (cavity-facing, inner side)
Surface 4	Inside face of inner pane (room-facing)

Climate	Low-E location	Why
Cool climate (zones 5-8)	Surface 2 or 3	Keeps internal heat in; reflects internal radiation back into the room
Warm humid (zones 1-3)	Surface 2 (solar control coating)	Reflects external solar radiation back outside
Mixed climate (zones 4-5)	Selection by orientation; consider west-facing solar load	Each window needs analysis

Wrong-surface Low-E (e.g. coating on surface 4 in a cool climate) materially reduces performance. The fabricator confirms surface orientation when ordering; verify against the manufacturer’s data sheet.

Pyrolytic vs sputter coating

Coating	Process	Where used
Pyrolytic (hard coat)	Applied to float glass during manufacture; pyrolytic deposition at high temperature	Single-pane Low-E (less common); inner surface of IGU
Sputter (soft coat)	Applied after float; sputtered metallic layers in vacuum chamber	Inside cavity of IGU; the volume IGU choice

Sputter Low-E delivers better thermal performance (lower emissivity) but cannot be exposed to air long-term: it must sit inside the sealed IGU cavity.

U-values: what they mean

The U-value (W/m².K) is the rate of heat flow through a square metre of the unit per degree of temperature difference. Lower U = better insulation.

Glazing system	Approximate U-value
Single 4 mm clear float	5.7
Single 6 mm clear float	5.6
Clear-clear 4-12-4 IGU (air fill)	3.0
Clear-clear 4-12-4 IGU (argon fill)	2.8
4-12-4 IGU, Low-E inner, argon fill	1.7-1.9
4-12-4 IGU, double Low-E, argon, warm-edge spacer	1.4-1.6
Triple-glazed 4-12-4-12-4, double Low-E, argon	0.8-1.1

The NCC NatHERS energy assessment requires window U-values that match the climate-zone target. In Melbourne (zone 6) a typical residential build needs window U-values of 2.5-3.0; in Hobart (zone 7) ~2.0-2.5; in alpine zone 8 below 2.0. Single glazing cannot achieve these without performance window frames; double glazing with Low-E + argon is the volume residential path.

Acoustic performance

Double glazing provides 3-7 dB Rw improvement over single glazing of equivalent thickness. The acoustic benefit increases with:

Asymmetric panes (4-12-6 outperforms 4-12-4 for sound)
Wider cavity (acoustic gaps 50-100 mm)
Laminated glass in one pane (PVB damps sound)
Acoustic PVB (Saflex Q in a laminated pane within the IGU)

For traffic-noise-affected residential (within 50 m of busy road, near rail, under flight path), a specific acoustic IGU specification is the difference between “OK” and “noticeable”. A standard 4-12-4 IGU rates ~Rw 32-35; an acoustic 6.76-12-6 laminated IGU rates ~Rw 38-42.

Australian manufacturers

Most Australian double glazing is assembled locally from imported components (glass from Viridian or G.James, spacers and seals from European or Asian suppliers, Low-E coatings on coated float). The main window-and-door fabricators handle the IGU assembly:

Fabricator	Range
Viridian Glass (BlueScope)	Float glass + IGU assembly; full range
G.James Glass & Aluminium	Float + IGU + window fabrication integration
Stegbar / A&L	Window-and-door + IGU
Capral Aluminium	Window framing + IGU
Hanlon Windows	Premium thermally-broken aluminium + IGU
Paarhammer (Vic)	Premium European-style timber + IGU
Architectural Window Systems (AWS)	Specifier-led, full range
uPVC window manufacturers (Pazen, Logikhaus, Eurochoice)	uPVC frame + IGU

For volume residential, the IGU is part of the window-and-door system, not specified separately. The fabricator integrates the IGU specification with the frame.

Common defects and on-site issues

Seal failure (fog or moisture in the cavity): the perimeter butyl + polysulphide seal degrades over time. Quality IGUs last 15-25 years; cheap IGUs 5-10. Visible as condensation or milky fog between the panes that cannot be cleaned. Replace the IGU.
Argon loss: argon leaks through perimeter seal at ~1% per year. Acceptable up to ~20-30% loss before performance drops noticeably. Cannot be re-charged.
Wrong-surface Low-E: coating on surface 4 instead of surface 3 in cool climate; reduces performance materially. Verify on data sheet.
Aluminium spacer condensation pattern: noticeable condensation strip around the edge of the IGU on cold mornings. Specify warm-edge spacer in cool-climate residential.
Cavity gap too narrow: gap below 8 mm doesn’t get the convection-suppression benefit; thermal performance degrades. Specify 12 mm minimum.
Cavity gap too wide: gap above 20 mm allows convection in the cavity; thermal performance degrades. 16 mm is the practical maximum for argon.
Glass-to-frame thermal bridge: aluminium frames without thermal breaks bypass the IGU’s performance. Thermally-broken aluminium, uPVC, or timber frames complete the system.
AS 1288 zone non-compliance: IGU with annealed glass in a safety-required position (low-level window adjacent to door). At least one pane must be safety glass (toughened or laminated).
Field damage during transit and install: IGU edges chip easily; chip can propagate over time. Inspect on receipt; reject any chipped unit.

Pricing (2026 indicative, ex-GST, supply only, sized for a standard residential window)

IGU build-up	Per square metre (supply)
4-12-4 clear + clear, air fill, aluminium spacer	$140-200
4-12-4 clear + clear, argon fill, warm-edge spacer	$170-260
4-12-4 Low-E inner, argon, warm-edge	$220-340
4-12-6.38 laminated, Low-E, argon	$300-440
6-12-6 double Low-E, argon, warm-edge	$320-470
Triple-glazed 4-12-4-12-4, double Low-E, argon	$480-720
Acoustic 6.76-12-6 laminated, Low-E	$450-680

IGU supply is part of the window-and-door fabrication cost; the full installed window cost in a typical residential build runs roughly 3-5× the IGU supply cost (frame, hardware, install, certification).

Standards and references

Standards Australia, AS/NZS 4666:2012 Insulating glass units. https://store.standards.org.au (verified 2026-05-13).
Standards Australia, AS 1288:2021 Glass in buildings: selection and installation. https://store.standards.org.au (verified 2026-05-13).
Standards Australia, AS 2047:2014 Windows and external glazed doors in buildings. 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 (verified 2026-05-13).