Double Glazing Technology: How Insulated Glass Units Work and Why They Matter for Australian Homes

Double glazing has become the baseline expectation for new residential construction across Australia. With the National Construction Code (NCC) 2022 raising the minimum NatHERS star rating from 6 to 7 stars, and BASIX requirements in New South Wales tightening energy performance targets, the insulated glass unit (IGU) sitting inside every window and door frame now carries more compliance weight than ever before. Yet many builders, developers, and homeowners still specify double glazing without fully understanding what determines its performance — the gas fill, the spacer technology, the low-emissivity coating placement, and how these components interact with the frame system to produce the U-value and SHGC numbers that satisfy energy modelling.

This article provides a comprehensive technical breakdown of double glazing technology as it applies to Australian residential and commercial construction, with specific reference to how MEICHEN Windows & Doors engineers its IGU configurations to meet AS2047 compliance, BASIX energy targets, and NatHERS 7-star requirements.

What Is an Insulated Glass Unit (IGU)?

An insulated glass unit is a factory-sealed assembly of two or more glass panes separated by a spacer bar and hermetically sealed at the edges. The cavity between the panes is filled with either dehydrated air or, more commonly, an inert gas such as argon or krypton. This gas-filled cavity is the primary insulating layer — still gas is a far poorer heat conductor than solid glass, and the wider the cavity (up to an optimal point), the lower the heat transfer through the unit.

A standard double-glazed IGU consists of:

• Outer pane (pane 1): The exterior glass, typically 5mm or 6mm tempered or laminated safety glass compliant with AS1288.
• Spacer bar: An aluminium or warm-edge composite bar that maintains the gap between panes. The spacer contains desiccant to absorb residual moisture inside the cavity.
• Gas fill: Argon is the industry standard for Australian residential glazing. Krypton is used for ultra-high-performance applications where cavity width is limited.
• Inner pane (pane 2): The interior glass, which may carry a low-emissivity (Low-E) coating on one surface inside the cavity.
• Primary seal: A butyl sealant applied between the spacer and glass surfaces, providing the primary moisture vapour barrier.
• Secondary seal: A polysulfide or silicone sealant applied over the spacer edge, providing structural integrity and additional moisture protection.

MEICHEN’s standard IGU configurations for the Australian market include 5+20A+5 (5mm glass, 20mm argon-filled air gap, 5mm glass) and 6+12A+6 configurations, both using tempered safety glass compliant with AS/NZS 2208 and AS/NZS 4666 standards for insulating glass units.

The Physics of Heat Transfer Through Glazing

Understanding why double glazing outperforms single glazing requires understanding the three modes of heat transfer that occur through a window:

Conduction is the direct transfer of heat through solid materials. Glass itself is a relatively good conductor — a single 4mm pane of clear glass has a U-value of approximately 5.8 W/m²K, meaning it transfers heat rapidly. The gas-filled cavity in a double-glazed unit dramatically reduces conductive transfer because still argon gas conducts heat approximately 33% less effectively than air, and far less effectively than solid glass.

Convection occurs when the gas inside the cavity circulates, transferring heat from the warm side to the cold side. Narrow cavities (below 9mm) suppress convection effectively but provide insufficient insulation. Very wide cavities (above 20mm for argon) allow convection currents to develop, which partially offsets the insulation benefit. The optimal cavity width for argon-filled units is typically 12-16mm, balancing conduction reduction against convection suppression.

Radiation is the transfer of heat via infrared energy across the cavity. This is where Low-E coatings play a critical role. A Low-E coating is a microscopically thin metallic layer applied to one glass surface inside the cavity. It reflects long-wave infrared radiation (heat) back toward its source while allowing visible light to pass through. In Australian climates, a Low-E coating on surface 2 (the inner face of the outer pane) reflects solar heat back outside in summer, while a coating on surface 3 (the outer face of the inner pane) reflects indoor heat back inside in winter.

Low-E Coating Technology: The Hidden Performance Driver

The Low-E coating is arguably the single most important component in a modern IGU. Without it, a double-glazed unit with argon fill achieves a glass U-value of approximately 2.7 W/m²K. With a single Low-E coating and argon fill, that drops to 1.4-1.8 W/m²K — a 40-50% improvement in thermal performance from a coating that is invisible to the human eye.

There are two primary types of Low-E coatings used in Australian window manufacturing:

Hard-coat (pyrolytic) Low-E is applied during the glass manufacturing process while the glass is still hot. The coating becomes part of the glass surface and is highly durable. Hard-coat Low-E typically achieves emissivity values of 0.15-0.20, providing moderate thermal performance. It is less expensive but also less effective than soft-coat alternatives.

Soft-coat (sputtered) Low-E is applied in a vacuum chamber after the glass is manufactured. Multiple microscopic layers of silver and metal oxides are deposited on the glass surface. Soft-coat Low-E achieves emissivity values as low as 0.02-0.05, providing superior thermal performance. The coating is delicate and must be applied to a surface inside the sealed IGU cavity, where it is protected from oxidation and mechanical damage.

MEICHEN uses soft-coat Low-E technology in its high-performance glazing systems, achieving glass U-values below 1.6 W/m²K when paired with thermally broken aluminium frames. This configuration satisfies BASIX energy requirements in NSW and contributes to NatHERS 7-star ratings across most Australian climate zones.

Spacer Technology: The Weak Link in Thermal Performance

The spacer bar that separates the two glass panes has historically been the weakest thermal link in a double-glazed unit. Traditional aluminium spacers are excellent conductors of heat, creating a thermal bridge around the perimeter of the IGU. This edge effect can increase the overall U-value of the unit by 0.2-0.4 W/m²K compared to the centre-of-glass value.

Warm-edge spacer technology addresses this problem by replacing the solid aluminium bar with a material that has significantly lower thermal conductivity. Common warm-edge options include:

• Stainless steel spacers: Approximately 10 times less conductive than aluminium, providing measurable thermal improvement at moderate cost.
• Thermoplastic composite spacers: Combining a thermoplastic matrix with a thin stainless steel foil, these spacers achieve thermal conductivity as low as 0.15 W/mK compared to aluminium’s 200 W/mK.
• Silicone foam spacers: The lowest thermal conductivity option, typically used in high-end passive house glazing.

The impact of warm-edge spacers on whole-window performance is significant. A double-glazed unit with a standard aluminium spacer and Low-E argon fill might achieve a whole-window U-value of 2.1 W/m²K. The same configuration with warm-edge spacers can achieve 1.8 W/m²K — a 14% improvement from a component that represents less than 5% of the total window cost.

Gas Fill Options: Argon vs Krypton vs Air

The gas filling the cavity between glass panes directly affects the IGU’s insulating performance. While dehydrated air provides baseline insulation (U-value approximately 2.8 for a 12mm gap with clear glass), inert gases offer substantially better performance:

Argon is the industry standard for Australian residential double glazing. It is non-toxic, non-reactive, abundant, and approximately 33% less thermally conductive than air. An argon-filled 12mm cavity with Low-E coating reduces the glass U-value from approximately 2.7 (air-filled) to 1.8 W/m²K. Argon is cost-effective, adding only 5-10% to the IGU price while delivering a 30%+ improvement in thermal performance.

Krypton provides superior insulation in narrower cavities. It is approximately twice as effective as argon in a 9mm gap, making it ideal for slim-profile windows where cavity width is constrained by frame depth. Krypton is significantly more expensive than argon — typically 5-10 times the cost per unit — and is generally reserved for triple-glazed passive house applications or ultra-slim architectural systems where space is at a premium.

MEICHEN’s standard configurations use argon gas fill across its Australian product range, with krypton available as a custom option for passive house and high-performance projects. All gas-filled units are manufactured to AS/NZS 4666 standards, with gas retention rates exceeding 90% over a 10-year period when tested to the EN 1279 standard for gas leakage.

How Double Glazing Interacts with Frame Systems

A high-performance IGU is only as effective as the frame system that houses it. The whole-window U-value — the number that appears in energy compliance documentation — is a weighted average of the glass U-value and the frame U-value, adjusted for the spacer edge effect.

For a typical residential window with 70% glass area and 30% frame area, the frame contributes disproportionately to overall heat loss. A non-thermally-broken aluminium frame has a U-value of approximately 5.8-6.5 W/m²K — nearly as poor as single glazing. When paired with a high-performance IGU (glass U-value 1.4), the whole-window U-value only reaches approximately 2.5 W/m²K because the frame acts as a thermal highway.

Thermally broken aluminium frames solve this problem. The polyamide (PA66) thermal break strip inserted between the interior and exterior aluminium profiles reduces the frame U-value to 1.2-2.0 W/m²K. When paired with the same IGU, the whole-window U-value drops to 1.4-1.8 W/m²K — meeting or exceeding the most stringent NCC energy requirements.

MEICHEN’s thermally broken systems use PA66 thermal break strips ranging from 20mm to 34mm in width. The wider strips achieve lower frame U-values, with the 34mm configuration delivering whole-window U-values below 1.6 W/m²K when combined with Low-E argon-filled double glazing.

Acoustic Benefits of Double Glazing

While double glazing is primarily specified for thermal performance, it also delivers meaningful acoustic improvements. The weighted sound reduction index (Rw) of a standard single-glazed 4mm window is approximately 24-27 dB. A double-glazed unit with 6mm and 6mm glass and a 12mm argon cavity typically achieves Rw 29-32 dB.

The acoustic performance of a double-glazed unit can be further enhanced through:

• Asymmetric glass thicknesses: Using different pane thicknesses (e.g., 6mm outer, 10mm inner) reduces the coincidence effect, where both panes resonate at the same frequency. This can improve Rw by 2-4 dB.
• Wider air gaps: Increasing the cavity from 12mm to 20mm or wider improves mid-frequency sound insulation. Each 6mm increase in gap width typically adds 1-2 dB to the Rw rating.
• Laminated glass: Adding a polyvinyl butyral (PVB) interlayer to one or both panes significantly improves acoustic performance, particularly at the coincidence frequency. A laminated inner pane can add 3-5 dB to the Rw rating.
• Acoustic interlayers: Specialised acoustic PVB interlayers (e.g., Saflex QuietGlass) outperform standard PVB, adding an additional 2-3 dB.

MEICHEN’s Slim & Silent system achieves Rw ratings up to 45 dB through a combination of asymmetric glass thicknesses, wide air gaps, laminated acoustic interlayers, and multi-cavity frame sealing with EPDM gaskets. This level of acoustic performance is suitable for homes near major roads, airports, rail corridors, and dense urban environments.

Double Glazing and Condensation Control

Condensation forms on window surfaces when warm, moisture-laden indoor air contacts a cold surface. Single-glazed windows in Australian homes commonly develop condensation during winter mornings, particularly in bathrooms, kitchens, and bedrooms with high humidity.

Double glazing dramatically reduces condensation on the inner glass surface because the inner pane stays significantly warmer than a single pane exposed to exterior temperatures. In a typical Melbourne winter morning (interior 20°C, exterior 5°C), the inner surface of a single-glazed window drops to approximately 7-8°C — well below the dew point of indoor air. The inner surface of a double-glazed Low-E argon unit stays at approximately 14-16°C, above the dew point for most indoor humidity levels.

This condensation control benefit extends the service life of window frames, sills, and surrounding finishes by preventing moisture damage, mould growth, and timber rot.

SHGC: Managing Solar Heat Gain in Australian Climates

While U-value measures heat loss (or unwanted heat gain) through conduction, the Solar Heat Gain Coefficient (SHGC) measures the fraction of solar radiation that passes through the glazing. SHGC ranges from 0 to 1, with lower values indicating less solar heat transmission.

Australian climate zones have fundamentally different SHGC requirements:

Low-E coatings can be tuned for different SHGC values. High-solar-gain Low-E coatings (SHGC 0.50-0.65) allow more solar heat through, suitable for heating-dominated climates. Low-solar-gain Low-E coatings (SHGC 0.25-0.40) block more solar heat, appropriate for cooling-dominated or west-facing glazing.

MEICHEN offers Low-E glass configurations tuned for different Australian climate zones and orientations, with SHGC values ranging from 0.25 to 0.62 to optimise energy performance in each specific application.

FAQ: Double Glazing Technology

Q1: How long do double-glazed units last?
Quality double-glazed units manufactured to AS/NZS 4666 standards typically last 15-25 years before the seals degrade and gas leakage compromises performance. MEICHEN provides a 10-year warranty on insulated glass units, covering seal failure and gas leakage. Units with warm-edge spacers and high-quality secondary sealants tend to have longer service lives.

Q2: Can I retrofit double glazing into my existing aluminium frames?
Retrofitting IGUs into existing frames is possible only if the frame has sufficient glazing rebate depth to accommodate the thicker unit (typically 20-28mm versus 4-6mm for single glazing). Most older aluminium frames do not have adequate rebate depth. Full-frame replacement with a thermally broken system is the recommended approach for achieving meaningful energy performance improvements.

Q3: Is triple glazing worth the cost in Australia?
For most Australian climate zones, the marginal improvement from triple glazing over double glazing does not justify the 40-60% cost premium. Triple glazing becomes cost-effective in alpine regions (Zone 8), passive house projects, or buildings with very large glazing areas where the additional thermal performance translates into measurable energy savings. MEICHEN offers triple-glazed configurations as custom options for these specialised applications.

Q4: What is the difference between argon and vacuum-insulated glazing?
Vacuum-insulated glass (VIG) places a vacuum between two panes instead of gas fill. VIG achieves U-values as low as 0.5 W/m²K in a unit only 6-8mm thick. However, VIG technology is expensive, limited in maximum size, and still maturing in the Australian market. Argon-filled double glazing remains the practical and cost-effective choice for Australian residential construction.

Q5: How does double glazing affect the visible light transmission?
Clear double glazing reduces visible light transmission (VLT) from approximately 90% (single glazing) to 80-82%. Adding a Low-E coating further reduces VLT to 60-75%, depending on the coating type. For most residential applications, this reduction is acceptable. MEICHEN offers high-light-transmission Low-E coatings that maintain VLT above 70% while still delivering thermal performance improvements.

Conclusion

Double glazing is no longer a premium upgrade — it is a regulatory baseline for Australian construction. Understanding the technology behind insulated glass units, from gas fills and Low-E coatings to warm-edge spacers and frame thermal breaks, enables builders, architects, and developers to specify glazing systems that not only satisfy compliance but deliver measurable improvements in thermal comfort, acoustic performance, and energy efficiency.

MEICHEN Windows & Doors manufactures AS2047-certified double-glazed window and door systems with Low-E argon-filled IGUs, thermally broken PA66 frames, and configurations tuned for Australian climate zones. With 40+ Australian and New Zealand compliance certifications and a 10-year warranty on insulated glass units, MEICHEN provides glazing solutions that meet the performance requirements of modern Australian construction.