Double Glazing Australia: Complete Guide to Insulated Glass Units, U-Value & SHGC

Double glazing has become the default standard for energy-efficient windows in Australia. Since the National Construction Code (NCC) 2022 raised the minimum NatHERS energy rating for new homes to 7 stars, single-glazed windows — once the norm across most of the country — are no longer sufficient to achieve compliance without compensating insulation upgrades elsewhere in the building envelope. Double glazing, properly specified, is the most cost-effective way to meet the new energy requirements while delivering tangible improvements in thermal comfort, acoustic insulation and condensation control.

This guide explains what double glazing is, how insulated glass units (IGUs) are constructed, how to read U-value and SHGC ratings, how the WERS scheme certifies window energy performance, and how manufacturers such as MC Windows & Doors — the Australian-facing brand of Meicheng International Windows & Doors (美呈国际门窗) — engineer high-performance IGUs for Australian climate zones.

1. What Is Double Glazing?

Double glazing refers to a window that uses two panes of glass separated by a sealed gap, rather than a single pane. The two panes and the gap between them form an insulated glass unit (IGU) — a factory-sealed assembly that is installed as a single component into the window frame.

The insulating principle is simple: the gap between the two glass panes (typically 12 to 16 mm) is filled with still air or an inert gas (argon or krypton) that conducts heat far more slowly than solid glass. This gap, combined with a low-emissivity (Low-E) coating on one of the internal glass surfaces, creates a thermal barrier that reduces heat transfer by 50 to 70% compared to single glazing.

1.1 Single glazing vs double glazing: the performance gap

The U-value difference is the critical metric. A single-glazed window with a Uw of 5.5 W/m²K loses approximately 55 watts of heat per square metre for every degree of temperature difference between inside and outside. A double-glazed window with a Uw of 1.8 W/m²K loses only 18 watts — a 67% reduction. Over a winter heating season, this translates to hundreds of dollars in energy savings per window in cool-climate homes.

2. Insulated Glass Unit (IGU) Construction

A double-glazed IGU is a precision-engineered assembly with five critical components:

2.1 Glass panes

The two glass panes are the primary structural and optical elements of the IGU. Common glass types used in Australian IGUs include:

• Clear float glass (3mm to 10mm): Standard transparent glass. Used on the internal pane where solar control is not required.
• Toughened glass (4mm to 10mm): Heat-treated glass that shatters into small, blunt fragments rather than sharp shards. Required by AS 1288 for safety glazing in doors, low-level windows and bathroom applications. Typically 4 to 6 times stronger than annealed glass of the same thickness.
• Laminated glass (6.38mm to 10.38mm): Two glass plies bonded with a PVB (polyvinyl butyral) or ionoplast interlayer. If broken, the interlayer holds the fragments together. Provides security, acoustic damping and UV filtration. Commonly specified as 6.38mm (6mm + 0.38mm interlayer) for residential applications.
• Low-E glass: Glass with a microscopically thin metallic oxide coating applied to one surface. The coating reflects long-wave infrared heat back toward the warmer side while allowing visible light to pass. Low-E coatings are classified as “hard coat” (pyrolytic, applied during glass manufacture) or “soft coat” (sputtered, applied after manufacture). Soft-coat Low-E offers superior thermal performance but must be sealed inside the IGU cavity to prevent oxidation.
• Tinted glass: Body-tinted glass (grey, bronze, green, blue) that absorbs solar radiation and reduces solar heat gain. Often used on the exterior pane of an IGU in hot climates.
• Acoustic laminated glass: Laminated glass with a specially formulated acoustic PVB interlayer that provides 3 to 5 dB better sound insulation than standard laminated glass of the same thickness.

2.2 Spacer bar

The spacer bar maintains the gap between the two glass panes and houses the desiccant. Traditionally, spacer bars were made from aluminium — an excellent thermal conductor that creates a “thermal bridge” at the glass edge, increasing heat transfer and promoting condensation. Modern IGUs use warm-edge spacers made from:

• Stainless steel: Lower thermal conductivity than aluminium (16 W/mK vs 209 W/mK for aluminium), with excellent structural rigidity and gas retention.
• Thermoplastic spacer (TPS): A butyl-based material applied as a continuous extruded bead that eliminates metal entirely from the edge seal. Offers the best thermal performance at the glass edge.
• Hybrid spacers: A combination of thin stainless steel with a thermal break polymer, balancing structural performance with thermal isolation.

Warm-edge spacers can improve the whole-window U-value by 0.1 to 0.3 W/m²K compared to aluminium spacers — a meaningful improvement that can be the difference between achieving and missing a NatHERS star rating.

2.3 Desiccant

The air or gas inside the IGU cavity must be dry. Any moisture trapped during manufacture will condense on the internal glass surfaces when the temperature drops — appearing as fog or condensation that cannot be cleaned. To prevent this, the spacer bar is filled with desiccant (typically molecular sieve or silica gel) that absorbs residual moisture from the cavity air during the sealing process and continues to absorb any moisture that penetrates the seal over the IGU’s service life.

2.4 Primary seal (butyl)

The primary seal is a continuous butyl (polyisobutylene) bead applied between the spacer bar and both glass surfaces. Butyl is an excellent moisture vapour barrier and gas retainer — it prevents the argon gas from escaping and moisture from entering the cavity. The primary seal is the critical barrier for IGU longevity.

2.5 Secondary seal (polysulphide or silicone)

The secondary seal is applied over the back of the spacer bar, filling the gap between the two glass edges. It provides structural integrity, holding the two panes together against wind pressure, barometric changes and thermal cycling. The secondary seal is typically:

• Polysulphide: The standard secondary seal for residential IGUs. Excellent gas retention and moisture resistance, but not UV-stable — must be protected from direct sunlight by the frame rebate.
• Silicone: Used for structural glazing applications where the edge seal may be exposed to UV. More expensive than polysulphide but UV-stable and compatible with structural silicone glazing systems.

3. Low-E Coatings: The Key to High-Performance Double Glazing

The Low-E (low-emissivity) coating is the single most important component in a high-performance IGU. Without it, a double-glazed unit achieves a Uw of approximately 2.8 to 3.2 W/m²K. With a soft-coat Low-E on the cavity-facing surface of the exterior pane, the Uw drops to 1.5 to 2.0 W/m²K — a 40 to 50% improvement.

3.1 How Low-E coatings work

Low-E coatings consist of multiple layers of silver and metal oxide deposited onto the glass surface in a vacuum sputtering process. The coating is approximately 100 nanometres thick — thinner than a human hair — but it fundamentally changes the glass’s interaction with electromagnetic radiation:

• Visible light (380 to 780 nm): Transmitted largely unimpeded, maintaining glass transparency.
• Near-infrared solar heat (780 to 2,500 nm): Reflected back toward the source (exterior), reducing solar heat gain.
• Long-wave infrared (4,000 to 100,000 nm): Reflected back toward the warmer side. In winter, interior heat is reflected back into the room. In summer, exterior heat is reflected back outside.

The emissivity of standard glass is approximately 0.89 (it emits 89% of the thermal radiation it absorbs). A high-quality soft-coat Low-E reduces the emissivity to 0.02 to 0.05 — a 95% reduction in radiant heat transfer.

3.2 Low-E coating position

In a double-glazed IGU, the Low-E coating is applied to one of the four glass surfaces:

• Surface 1: Exterior face of the outer pane. Not used for soft-coat Low-E (would degrade in UV and weather).
• Surface 2 (cavity face of outer pane): The most common position for soft-coat Low-E. The coating faces the cavity, protecting it from oxidation while reflecting solar heat back to the exterior and retaining interior heat. Best for hot and mixed climates.
• Surface 3 (cavity face of inner pane): The coating faces the cavity from the interior side. Better for cold climates where maximum solar heat gain is desired in winter — the coating allows solar radiation to pass through the outer pane and enter the room, then reflects interior heat back inward.
• Surface 4: Interior face of the inner pane. Not typically used (coating would be exposed to handling and cleaning damage).

In the Australian market, Surface 2 is the most common Low-E position, as it provides the best balance of summer heat rejection and winter heat retention across most climate zones.

3.3 High-transmission vs low-transmission Low-E

Low-E coatings are classified by their solar heat gain characteristics:

• High-transmission Low-E (high SHGC, low U-value): Allows most solar radiation to pass through while retaining interior heat. Best for south-facing windows in cool climates where winter solar gain is beneficial. Typical SHGC: 0.50 to 0.65.
• Low-transmission Low-E (low SHGC, low U-value): Reflects a significant portion of solar radiation while retaining interior heat. Best for north-, east- and west-facing windows in hot and mixed climates where summer heat gain must be controlled. Typical SHGC: 0.25 to 0.40.
• Spectrally selective Low-E: Advanced coatings that transmit most visible light while blocking near-infrared solar heat. These deliver high visible light transmittance (Tvis > 0.60) with low SHGC (< 0.30), making them ideal for bright, hot-climate applications where daylighting is desired without heat gain.

MC Windows & Doors offers IGU configurations with SHGC values adjustable between 0.2 and 0.6, allowing precise solar control matching to the project’s climate zone, orientation and shading strategy.

4. Gas Fills: Argon and Krypton

The cavity between the two glass panes can be filled with still air, or with an inert gas that has lower thermal conductivity than air.

4.1 Argon

Argon is the most common gas fill for Australian IGUs. It is:
– Non-toxic, non-reactive and abundant (approximately 1% of the Earth’s atmosphere)
– 36% less thermally conductive than air (0.016 W/mK vs 0.025 W/mK)
– Cost-effective — adds approximately $15 to $30 per square metre to the IGU cost
– Effective at the standard 12 mm cavity width

Filling the cavity with argon instead of air improves the IGU U-value (Ug) by approximately 0.3 W/m²K — a meaningful improvement that contributes to the overall whole-window U-value (Uw).

4.2 Krypton

Krypton is a denser, less conductive gas than argon, offering superior thermal performance:
– 52% less thermally conductive than air (0.0094 W/mK)
– Effective at narrower cavity widths (6 to 9 mm optimal, compared to 12 to 16 mm for argon)
– Significantly more expensive than argon — typically 10 to 20 times the cost
– Used in triple glazing and ultra-high-performance applications where cavity width must be minimised to reduce IGU weight

For most Australian applications, argon provides the best performance-to-cost ratio. Krypton is reserved for passive house projects and architectural applications where every decimal of U-value matters.

5. Understanding U-Value and SHGC

5.1 U-value (thermal transmittance)

The U-value measures the rate of heat transfer through the complete window assembly — glass, frame, spacer and seals — per degree of temperature difference. Lower is better.

Three U-value terms are commonly used:

• Ug (centre of glass): The U-value at the centre of the glass pane, ignoring the frame and edge effects. This is the number most commonly quoted in marketing materials and can be misleadingly low.
• Uf (frame): The U-value of the frame profile alone.
• Uw (whole window): The U-value of the complete window assembly, including glass, frame and edge-of-glass effects. This is the number that matters for energy compliance and should be used in NatHERS modelling.

For a typical thermally broken aluminium window with Low-E double glazing and argon:
– Ug = 1.1 to 1.4 W/m²K
– Uf = 1.8 to 3.0 W/m²K (depends on thermal break width)
– Uw = 1.5 to 2.2 W/m²K

The difference between Ug and Uw (approximately 0.4 to 0.8 W/m²K) is the “penalty” for the frame and edge-of-glass heat loss. This penalty is minimised by using warm-edge spacers and thermally broken frames.

5.2 SHGC (solar heat gain coefficient)

SHGC measures the fraction of incident solar radiation that passes through the complete window assembly, expressed as a number between 0 and 1. Lower SHGC means less solar heat enters the building.

The optimal SHGC depends on:
– Climate zone: Hot climates need low SHGC (0.20 to 0.35) to minimise cooling load. Cool climates benefit from higher SHGC (0.50 to 0.65) on north-facing windows to capture winter solar gain.
– Orientation: North-facing windows in southern Australia (Melbourne, Hobart, Canberra) can use higher SHGC because the high winter sun angle provides useful heating. West-facing windows in all climates need low SHGC to control summer afternoon heat.
– Shading: If the window is permanently shaded (eaves, pergola, adjacent buildings), a higher SHGC may be acceptable because direct solar gain is already controlled.

5.3 Visible light transmittance (Tvis / VLT)

Tvis measures the percentage of visible light that passes through the window. High Tvis (0.60 to 0.80) is desirable for daylighting, reducing the need for artificial lighting. However, very high Tvis can cause glare and excessive solar heat gain. Low-E coatings with low SHGC typically reduce Tvis to 0.50 to 0.70 — a trade-off that should be evaluated for each room and orientation.

6. WERS: The Window Energy Rating Scheme

The Window Energy Rating Scheme (WERS), administered by the Australian Glass and Window Association (AGWA), provides a standardised 0 to 10 star rating for window energy performance. WERS rates the complete window assembly, not just the glass.

6.1 How WERS ratings work

Each WERS-rated window product receives:
– Heating star rating (0 to 10): How well the window retains heat in winter. Higher is better for cool climates.
– Cooling star rating (0 to 10): How well the window rejects solar heat in summer. Higher is better for hot climates.
– Whole-window U-value (Uw): Measured under AFRC (Australian Fenestration Rating Council) protocols.
– Whole-window SHGC: Measured under AFRC protocols.
– Visible light transmittance (Tvis): Percentage of visible light transmitted.
– Air infiltration rate: Measured at 25 Pa pressure differential.

6.2 Typical WERS ratings for double-glazed aluminium windows

6.3 Using WERS in NatHERS modelling

WERS ratings are designed to integrate directly with NatHERS-accredited energy modelling software (FirstRate5, AccuRate, BERS Pro). When the energy assessor inputs the actual WERS data for the specified windows — rather than using conservative default values — the model accurately reflects the window’s contribution to the home’s thermal performance. This often unlocks additional glazing area or reduces the insulation burden elsewhere in the building envelope, making the difference between a 6-star and a 7-star NatHERS result.

7. NCC 2022 and 7-Star NatHERS Compliance

The NCC 2022 energy efficiency provisions require new residential dwellings to achieve a minimum 7-star NatHERS rating (up from the previous 6-star minimum). This change has made double glazing effectively mandatory for most new homes in southern and alpine Australia, and strongly recommended elsewhere.

7.1 How double glazing contributes to 7-star compliance

In a typical Australian home, windows account for 25 to 40% of the total heating and cooling energy load. Upgrading from single to double glazing with Low-E and argon can reduce the window-related energy load by 50 to 70%, which translates to approximately 0.5 to 1.5 NatHERS stars depending on the climate zone and glazing area.

In practice, most 7-star compliant designs in cool-temperate climates (Melbourne, Canberra, Hobart) specify:
– Double-glazed, thermally broken aluminium frames on all external openings
– Low-E coatings on surfaces 2 or 3 (climate-dependent)
– Argon gas fill in 12 to 16 mm cavities
– SHGC tuned to orientation (low on west, moderate on north, high on south)
– Uw target of 1.5 to 2.0 W/m²K

7.2 BASIX (NSW) compliance

In New South Wales, the Building Sustainability Index (BASIX) sets energy and water targets for new dwellings. BASIX uses a different methodology from NatHERS but achieves a similar outcome: double glazing with Low-E is typically required for most window orientations in the Sydney and regional NSW climate zones.

MC Windows & Doors’ IGU specifications are designed to meet both NCC/NatHERS and BASIX requirements, with U-values as low as 1.2 W/m²K and SHGC adjustable from 0.2 to 0.6 to suit any Australian climate zone.

8. Condensation Control

One of the most immediately noticeable benefits of double glazing is condensation reduction. In cool-climate homes with single glazing, interior condensation is a persistent problem — moisture from cooking, bathing and breathing condenses on the cold inner surface of single-glazed windows, leading to mould growth, timber rot and health issues.

Double glazing raises the interior glass surface temperature to near room temperature, dramatically reducing or eliminating condensation. The internal pane of a Low-E, argon-filled IGU in a 20-degree room with 0-degree exterior temperature will typically measure 15 to 18 degrees — well above the dew point for normal indoor humidity levels (below 60% RH).

For extremely humid environments or passive house projects, triple glazing may be required to eliminate condensation entirely under all conditions.

9. Acoustic Performance of Double Glazing

While double glazing is primarily specified for thermal performance, it also delivers significant acoustic improvements over single glazing. The improvement comes from two mechanisms:

1. Mass doubling: Two panes of glass have approximately twice the mass of one, increasing sound transmission loss by 3 to 5 dB across most frequencies.
2. Cavity resonance decoupling: The air (or argon) gap between the panes decouples them acoustically, breaking the direct vibration transmission path. The cavity also introduces a mass-air-mass resonance that, when correctly sized, improves mid-frequency performance.

9.1 Typical acoustic performance

For homes near busy roads, flight paths or rail corridors, specifying an IGU with acoustic laminated glass on one pane and an asymmetric configuration (different glass thicknesses on each pane, e.g., 8mm/12mm/6mm) can achieve Rw 38 to 42 — sufficient to reduce road traffic noise from “intrusive” to “barely noticeable.”

10. Triple Glazing: When Is It Worth It?

Triple glazing uses three glass panes with two sealed cavities, further reducing the U-value to 0.9 to 1.5 W/m²K. It is worth considering for:

• Alpine and cool-temperate regions (ACT, alpine Victoria, Tasmania) where winter temperatures regularly drop below zero
• Passive house certified projects targeting Uw below 1.0 W/m²K
• Projects requiring Rw 38+ acoustic performance where the second cavity provides additional sound isolation
• Net-zero energy homes where every watt of heating and cooling load matters

The trade-offs are weight (a triple-glazed IGU is 40 to 60% heavier than double-glazed), cost (60 to 100% premium over double glazing), and frame thickness (the wider IGU requires deeper frame profiles). For most of temperate Australia (Sydney, Perth, Adelaide, Brisbane), double glazing with Low-E and argon provides sufficient performance at a more economical price point.

11. IGU Longevity and Warranty

11.1 Seal failure

The most common failure mode for an IGU is seal failure — the primary butyl seal or secondary seal degrades, allowing argon gas to escape and moisture to enter the cavity. The visible symptom is permanent condensation or fogging between the glass panes that cannot be cleaned.

Seal failure is caused by:
– Poor manufacturing quality (inadequate butyl application, contaminated surfaces, incomplete curing)
– Thermal cycling stress (repeated expansion and contraction degrades the seal over time)
– Chemical attack (silicone sealants or cleaning agents incompatible with the secondary seal)
– Physical damage (frame deflection under wind load stressing the edge seal)

11.2 Expected lifespan and warranty

A quality IGU manufactured to AS 4666 (Insulating glass units) should achieve a service life of 20 to 30 years before seal failure. The AS 4666 standard requires IGUs to pass:
– High-humidity test: 4 weeks at 50 to 60 degrees Celsius and 95%+ relative humidity
– Weather cycling test: 320 cycles between -20 and +50 degrees with UV exposure
– Gas retention test: Maximum 1% argon loss per year

Premium IGU manufacturers, including MC Windows & Doors’ supply chain, provide 10 to 15-year warranties against seal failure, covering replacement of the failed IGU (labour and materials).

Frequently Asked Questions

Q1: Is double glazing mandatory in Australia?

Since NCC 2022, double glazing is effectively mandatory for new homes in most Australian climate zones to achieve the 7-star NatHERS minimum. While the NCC does not explicitly mandate double glazing, the 7-star requirement is practically impossible to achieve with single glazing in cool-temperate and alpine climates without excessive and uneconomical insulation upgrades elsewhere. In hot-humid climates (Darwin, Cairns), single glazing with appropriate solar control may still be feasible in some designs.

Q2: Can I retrofit double glazing to existing single-glazed windows?

Retrofitting can be done by replacing the existing glass with a double-glazed IGU in the existing frame (if the frame rebate can accommodate the thicker IGU) or by installing secondary glazing (a second pane mounted inside the existing window). Full IGU replacement is more effective but requires the frame to be structurally suitable. Secondary glazing is less thermally efficient but significantly cheaper and suitable for heritage buildings where the original frame must be retained.

Q3: What is the ideal gap between the two panes of glass?

For argon-filled IGUs, the optimal cavity width is 12 to 16 mm. Below 12 mm, the gas layer is too thin to provide effective insulation. Above 16 mm, convection currents within the cavity begin to transfer heat between the panes, reducing the insulating benefit. For air-filled IGUs (no argon), the optimal gap is slightly wider at 16 to 20 mm. Krypton-filled IGUs achieve optimal performance at 6 to 9 mm.

Q4: Does argon gas leak out of the IGU over time?

All IGUs lose a small amount of gas over time — typically less than 1% per year for a quality unit manufactured to AS 4666. Over a 20-year service life, this means the IGU retains at least 80% of its original argon fill, with a corresponding U-value increase of less than 0.1 W/m²K. This is a negligible performance degradation that does not materially affect the window’s energy performance.

Q5: How do I know if my IGU has failed?

The primary symptom of IGU seal failure is permanent condensation or fogging between the two glass panes that does not clear when the temperature changes. Other indicators include a white powdery deposit on the internal glass surface (desiccant dust) or visible cracking of the Low-E coating. If you observe these symptoms, contact the window supplier for warranty assessment.

Q6: What is the difference between Low-E and tinted glass?

Low-E glass uses a microscopically thin metallic coating to selectively reflect infrared heat while transmitting visible light. Tinted glass uses body colour (metal oxides in the glass melt) to absorb solar radiation across all wavelengths. Low-E is far more effective at reducing heat transfer (both U-value and SHGC) while maintaining visible light transmittance. Tinted glass reduces SHGC but does not improve U-value and reduces visible light more aggressively than Low-E. For best performance, specify Low-E glass with a light tint if additional solar control is needed.

Conclusion

Double glazing with Low-E coatings and argon gas fill is the foundation of energy-efficient window performance in Australia. Understanding IGU construction, U-value vs SHGC, WERS ratings and NCC 2022 compliance requirements empowers homeowners and builders to make specification decisions that will deliver thermal comfort, energy savings and condensation-free performance for decades.

MC Windows & Doors engineers IGU systems with Low-E double glazing, argon fill, warm-edge spacers and thermally broken aluminium frames, achieving whole-window U-values as low as 1.2 W/m²K with SHGC adjustable from 0.2 to 0.6. With 19 energy-rated systems certified by AGWA and compliance with AS 4666 IGU manufacturing standards, these systems are designed to meet and exceed the 7-star NatHERS requirements across all Australian climate zones.