Thermal Break Aluminium Windows: How Multi-Chamber Technology Meets Australia’s 7-Star NatHERS Requirements

On 1 May 2024, the National Construction Code (NCC) 2022 adoption took full effect across most Australian states and territories, raising the minimum energy efficiency requirement for new residential dwellings from 6 stars to 7 stars under the Nationwide House Energy Rating Scheme (NatHERS). This single regulatory change has reshaped the Australian window market. Windows and doors, which typically account for 40–60% of a home’s total heat gain and loss, are now subject to performance benchmarks that standard single-glazed, non-thermal-break aluminium frames simply cannot meet.

This article explains the engineering behind thermal break aluminium windows, the physics of heat transfer through window assemblies, and how MC Windows & Doors (Meichen International Windows & Doors) has engineered its thermal break systems to achieve whole-window U-values as low as 1.2 W/m²·K—performance that comfortably satisfies 7-star NatHERS requirements across all Australian climate zones.

The Problem: Aluminium Without a Thermal Break

Aluminium is an excellent conductor of heat. Its thermal conductivity is approximately 200 W/m·K, which is roughly 1000 times greater than that of polyamide (0.25 W/m·K) and 500 times greater than timber (0.13 W/m·K). In a standard aluminium window frame without a thermal break, the interior and exterior surfaces of the profile are connected by a continuous metal path, allowing heat to flow freely between inside and outside.

In practical terms, this means:

• In summer, the aluminium frame on the exterior of the building heats up to near-ambient outdoor temperature (40°C+ in many Australian cities) and conducts that heat directly to the interior frame surface, which may reach 35°C or higher, radiating heat into the room
• In winter, the interior frame surface drops close to the outdoor temperature, creating cold spots that cause condensation, mould growth, and thermal discomfort
• The whole-frame U-value of a non-thermal-break aluminium frame is approximately 5.8 W/m²·K, which is far too high for 7-star NatHERS compliance in most Australian climate zones

The thermal break was invented to solve this problem. By physically interrupting the metal-to-metal path, it dramatically reduces the rate of conductive heat transfer through the frame.

What Is a Thermal Break?

A thermal break is a low-conductivity material inserted between the interior and exterior aluminium profiles of a window frame to reduce thermal bridging. The most common thermal break material is polyamide 66 (PA66) reinforced with 25% glass fibre, which offers:

• Thermal conductivity of approximately 0.25 W/m·K (vs. 200 W/m·K for aluminium)
• Mechanical strength sufficient to carry structural loads between the two aluminium profiles
• Thermal stability up to 120°C (important for powder coating cure temperatures)
• Dimensional stability across temperature cycling
• Compatibility with aluminium extrusion and assembly processes

The Polyamide Strip Manufacturing Process

Thermal break polyamide strips are manufactured as I-beams, U-sections, or multi-chamber profiles, typically 14mm to 34mm wide. They are mechanically interlocked with the aluminium profiles using one of two methods:

1. Knurled-and-rolled (debossing): The aluminium profile is knurled (patterned with small teeth) along the edges where the polyamide strip will be inserted, and the strip is pressed in under high pressure, creating a mechanical interlock
2. Extruded through: The polyamide strip is co-extruded through a die that shapes it directly into the aluminium profile’s groove, then cured

Both methods create a bond that is structurally sound and thermally effective, with the polyamide strip carrying the shear load between the two aluminium sections while preventing conductive heat transfer.

Multi-Chamber Technology: Beyond the Basic Thermal Break

MC Windows & Doors’ thermal break window systems go beyond a single polyamide strip. They incorporate multi-chamber frame design, which uses the trapped air in internal cavities as additional thermal insulation.

How Multi-Chamber Design Works

A multi-chamber profile has internal cavities created by the geometry of the extrusion and the polyamide strips. Each cavity contains a volume of still air, which is a poor conductor of heat (thermal conductivity 0.024 W/m·K at 20°C). The cavities work in series: heat must pass through aluminium, then polyamide, then air cavity 1, then polyamide, then air cavity 2, then aluminium interior. Each transition creates a thermal resistance (R-value), and the total thermal resistance is the sum of the individual resistances.

The relationship is:

R_total = R_aluminium_ext + R_polyamide_1 + R_air_1 + R_polyamide_2 + R_air_2 + R_polyamide_3 + R_aluminium_int

Since each R = thickness / conductivity, and aluminium is highly conductive (thin R), the polyamide and air chambers dominate the total resistance. A 5-chamber profile with 24mm polyamide strips and 3 air cavities can achieve a frame U-value of 1.5–2.0 W/m²·K, compared to 5.8 W/m²·K for a non-thermal-break frame.

Practical Impact on Whole-Window U-Value

The whole-window U-value is a weighted average of the frame U-value and the glass U-value, weighted by the respective areas. For a typical residential window with 70% glass area and 30% frame area:

• Non-thermal-break frame (Uf 5.8) + standard double glazing (Ug 2.7): Uw = 0.7 × 2.7 + 0.3 × 5.8 = 3.63 W/m²·K
• Thermal break frame (Uf 2.0) + Low-E double glazing (Ug 1.7): Uw = 0.7 × 1.7 + 0.3 × 2.0 = 1.79 W/m²·K
• Multi-chamber thermal break frame (Uf 1.5) + triple Low-E glazing (Ug 0.8): Uw = 0.7 × 0.8 + 0.3 × 1.5 = 1.01 W/m²·K

The progression from 3.63 to 1.01 W/m²·K represents a 72% reduction in heat transfer through the window. This is the difference between a home that requires constant air-conditioning and one that maintains comfortable internal temperatures passively.

Glass Technology: The Other Half of the Equation

While the frame accounts for 20–40% of the window’s thermal performance, the glass is the dominant element. MC Windows & Doors’ thermal break systems are paired with advanced glazing to achieve the lowest possible whole-window U-values.

Low-Emissivity (Low-E) Coatings

Low-E coatings are microscopically thin layers of silver (typically 10–20 nanometres) deposited on the glass surface within the double-glazed unit. These coatings have high reflectance in the far-infrared (wavelengths 3–50 micrometres, which is the wavelength of heat radiated by warm objects) and high transmittance in the visible spectrum (wavelengths 0.38–0.78 micrometres).

In practical terms, this means:
– In winter: Heat from the interior heating system is reflected back into the room rather than passing through the glass to the exterior
– In summer: Heat radiated from hot outdoor surfaces (paving, walls, roads) is reflected away from the interior

A double-glazed unit with a Low-E coating on the #3 surface (the inner face of the inner pane) can achieve a centre-of-glass U-value of approximately 1.7 W/m²·K, compared to 2.7 W/m²·K for uncoated double glazing.

Argon Gas Fill

The sealed air gap in a double-glazed unit can be filled with argon gas instead of air. Argon has a thermal conductivity of 0.016 W/m·K (compared to 0.024 W/m·K for air), which reduces the convective and conductive heat transfer across the gap. The improvement is modest (approximately 0.3 W/m²·K in the glass U-value) but meaningful when combined with Low-E coatings.

Warm Edge Spacer Technology

The edge of a double-glazed unit is sealed with a spacer bar, traditionally made of aluminium. However, aluminium spacers create a thermal bridge at the glass edge that can increase heat loss by 10–15%. MC Windows & Doors uses warm edge spacer bars made of thermoplastics with a thin stainless steel barrier. These reduce the thermal bridge effect at the edge, improving the overall U-value of the glazing by approximately 0.1–0.2 W/m²·K and reducing condensation at the glass edge in winter.

Triple Glazing for Maximum Performance

For projects targeting passive house standards or the most demanding climate zones (e.g., alpine areas, Tasmania’s southern regions), MC Windows & Doors offers triple-glazed units with two Low-E coatings, dual argon-filled cavities, and warm edge spacers. These configurations achieve a centre-of-glass U-value as low as 0.7 W/m²·K and a whole-window U-value (with multi-chamber thermal break frames) as low as 1.0–1.2 W/m²·K.

Solar Heat Gain Coefficient (SHGC): Managing Summer Heat

While U-value addresses conductive and convective heat transfer, SHGC addresses radiative heat transfer—the fraction of incident solar radiation that passes through the window. In most Australian climate zones, managing SHGC is as important as managing U-value.

Orientation-Specific SHGC

MC Windows & Doors specifies different glass configurations based on the window’s orientation:

• North-facing windows: Moderate SHGC (0.35–0.45) to allow beneficial winter solar gain while blocking excessive summer gain. A hard Low-E coating on the #2 surface (outer face of inner pane) is typical
• East and west-facing windows: Low SHGC (0.20–0.30) to block the intense low-angle morning and afternoon sun. A soft Low-E coating combined with a tinted or reflective outer pane is recommended
• South-facing windows: Higher SHGC acceptable (0.45–0.60) as solar gain is limited. Emphasis on U-value and visible light transmittance

Tinted and Coated Options

MC Windows & Doors offers a range of solar control glass options:

• Body-tinted glass (grey, bronze, green): Absorbs solar radiation, reducing SHGC by 20–40%. Can cause thermal stress in the glass; heat-strengthened glass is recommended for tinted outer panes
• Reflective coatings: Mirror-like coatings that reject solar radiation. Effective for SHGC reduction but reduce visible light transmittance and can cause aesthetic issues at night (interior becomes visible from outside)
• Spectrally selective Low-E: The most advanced option. These coatings selectively transmit visible light while rejecting near-infrared radiation (which carries approximately 50% of solar energy). They achieve SHGC values of 0.25–0.30 with visible light transmittance of 60–70%, making them the preferred choice for high-performance projects

NatHERS and BASIX Compliance: How It All Comes Together

NatHERS 7-Star

The NatHERS rating is a simulation-based assessment of the whole-home thermal performance, calculated using AccuRate software. It considers the building envelope (walls, roof, floor, windows, doors), insulation levels, orientation, shading, and climate zone. Each element contributes to the overall star rating.

Windows are a major contributor because:
– They have significantly higher U-values than insulated walls (a 7-star wall has an R-value of 2.8+, or U-value of 0.36, versus a window at U-value 1.2–2.0)
– They allow solar heat gain (positive in winter, negative in summer)

For a typical 200m² home in Sydney (Climate Zone 17, mixed temperate), upgrading from non-thermal-break single-glazed windows (Uw 5.8, SHGC 0.7) to MC Windows & Doors’ thermal break double-glazed windows (Uw 1.8, SHGC 0.35) can improve the NatHERS rating by 1.5–2.0 stars—often the difference between 5 and 7 stars.

BASIX (New South Wales)

The Building Sustainability Index (BASIX) is NSW’s separate sustainability assessment. It sets maximum U-values and minimum/maximum SHGC values for windows based on climate zone and glazing area. MC Windows & Doors’ thermal break systems comfortably meet the most stringent BASIX requirements, including:

• Maximum U-value of 2.0 W/m²·K for glazing in Sydney metro (Zone 17)
• Maximum U-value of 1.8 W/m²·K for alpine regions
• Orientation-specific SHGC limits

MC Windows & Doors’ Thermal Break Product Range

MC100 Tilt and Turn Window

The flagship thermal break window, featuring:
– 5-chamber polyamide thermal break profile
– Triple-seal EPDM weather seals
– Double or triple Low-E glazing
– Tilt-and-turn operating mechanism (tilt for ventilation, turn for cleaning)
– U-value: as low as 1.2 W/m²·K
– Rw: 36 (acoustic laminated option)

MC100 Awning Window

A thermal break awning window designed for Australian conditions:
– Outward-opening sash, operable in rain
– Multi-chamber thermal break profile
– Chain winder or friction stay opener options
– U-value: as low as 1.4 W/m²·K

MC100 Sliding Window

Thermal break sliding window for larger openings:
– Multi-chamber thermal break profile
– High-load stainless steel rollers
– Interlocking weather seals
– U-value: as low as 1.6 W/m²·K

MC140 Sliding Door

Premium thermal break sliding door:
– 140mm profile depth for large panel sizes
– Double or triple Low-E glazing
– Multi-point locking
– U-value: as low as 1.6 W/m²·K
– Panel capacity: up to 400kg per leaf

Condensation Control: An Often-Overlooked Benefit

Beyond energy efficiency, thermal break windows provide a critical health benefit: condensation control. In non-thermal-break windows, the interior frame surface temperature drops below the dew point of the interior air during cold weather, causing condensation to form on the frame. This moisture can lead to mould growth, timber rot in adjacent joinery, and degradation of paint and sealants.

Thermal break windows maintain interior frame surface temperatures 5–8°C warmer than non-thermal-break frames under the same conditions, which in most Australian climates is sufficient to prevent condensation formation. This is particularly important in:
– Alpine and cool-temperate regions (Tasmania, ACT, alpine Victoria and NSW)
– High-humidity coastal environments where interior moisture loads are elevated
– Bedrooms where moisture from breathing accumulates overnight

Conclusion

The move to 7-star NatHERS is not a marginal change—it is a fundamental shift in what the Australian building industry must deliver. Windows that were “good enough” under 6-star are now inadequate, and builders and architects who continue to specify non-thermal-break, single-glazed windows are not merely failing to comply; they are delivering homes that will cost their occupants thousands of dollars in unnecessary heating and cooling costs over the building’s life.

MC Windows & Doors’ thermal break systems represent the engineering response to this regulatory shift. By combining multi-chamber polyamide thermal break technology with Low-E double and triple glazing, warm edge spacers, and argon fills, these systems achieve whole-window U-values as low as 1.2 W/m²·K—performance that exceeds 7-star NatHERS requirements in every Australian climate zone, while also delivering superior acoustic performance, condensation control, and structural compliance with AS 2047.

For builders, developers, and homeowners who are serious about energy efficiency, comfort, and regulatory compliance, thermal break aluminium windows are no longer an upgrade—they are the new standard. MC Windows & Doors is ready to supply that standard, from its Zhaoqing manufacturing base to project sites across Australia.

For thermal break window specifications, NatHERS and BASIX compliance documentation, and project-specific U-value calculations, contact MC Windows & Doors at Annly@mcwindow.com.au or +61 490 141 931.