BASIX, NatHERS, and Aluminium Windows: Meeting Australian Energy Efficiency Requirements

Table of Contents

1. Introduction: The Regulatory Landscape for Window Energy Performance
2. Understanding BASIX: NSW’s Building Sustainability Index
3. Understanding NatHERS: The National House Energy Rating Scheme
4. U-value and SHGC: The Two Numbers That Define Window Energy Performance
5. How Aluminium Windows Achieve Low U-values
6. Glass Selection for Energy Compliance: Beyond Double Glazing
7. How Window Selection Affects BASIX and NatHERS Scores
8. Thermal Break Technology in Aluminium Windows
9. Comparison: Aluminium vs. Timber vs. uPVC for Energy Performance
10. FAQ: Common Questions About Window Energy Ratings
11. References

1. Introduction: The Regulatory Landscape for Window Energy Performance

Energy efficiency has moved from a nice-to-have to a mandatory requirement in Australian residential construction. Every new home and major renovation in Australia must now demonstrate compliance with energy performance standards that address the building envelope — and windows and doors are among the most significant variables in that calculation.

The Australian Building Codes Board (ABCB) has progressively tightened energy provisions in the National Construction Code (NCC). The 2022 edition raised the minimum NatHERS star rating for new homes from 6 to 7 stars in most jurisdictions, and jurisdictions like New South Wales operate additional frameworks — notably BASIX — that impose further targets on water and energy consumption (ABCB, 2022).

For building professionals — architects, designers, builders, and developers — the practical question is: Can aluminium windows meet these standards? The answer is unequivocally yes, but it requires understanding the relationship between frame design, glass selection, and thermal performance, and specifying products that have been engineered for energy compliance from the outset.

This article provides a technical guide to navigating BASIX and NatHERS requirements when specifying aluminium windows and doors for Australian residential projects.

2. Understanding BASIX: NSW’s Building Sustainability Index

2.1 What BASIX Is and Who It Applies To

BASIX (Building Sustainability Index) is a NSW Government planning regulation that applies to all new residential dwellings and alterations and additions valued at $50,000 or more. It sets mandatory targets in three areas:

1. Water: Reduction in mains water consumption compared to a benchmark dwelling.
2. Thermal comfort: Heating and cooling loads, expressed in megajoules per square metre per year (MJ/m²/yr), must not exceed predetermined thresholds.
3. Energy: Overall energy consumption, including fixed appliances, lighting, and hot water.

BASIX compliance is assessed through an online tool (the BASIX Assessment Tool) that models the proposed dwelling’s energy and water performance. The assessment must be submitted with the development application (DA) and construction certificate (CC) application. A BASIX certificate is legally required before a DA can be approved in NSW (NSW Department of Planning, 2024).

2.2 How Windows Affect BASIX Scores

Windows affect both the thermal comfort and energy sections of a BASIX assessment through three primary mechanisms:

• Heat gain in summer: Solar radiation transmitted through windows adds to the cooling load. The Solar Heat Gain Coefficient (SHGC) of the glazing determines how much solar energy enters the building.
• Heat loss in winter: Conduction through the window assembly — frame and glass — represents a significant portion of a building’s total heat loss. The U-value (thermal transmittance) of the window determines the rate of conductive heat transfer.
• Natural ventilation: Operable windows can reduce the need for mechanical cooling when the BASIX assessment accounts for cross-ventilation and night-purging strategies.

A BASIX assessment for a typical Sydney single-storey house generally shows that windows account for 25-40% of the total heating and cooling load. Optimising window selection — particularly U-value and SHGC — is therefore one of the most cost-effective ways to improve a BASIX score.

2.3 BASIX Targets by NSW Climate Zone

NSW spans multiple climate zones under the National Construction Code, and BASIX heating and cooling caps vary accordingly:

In Sydney’s Zone 5, the prevailing strategy is a “balanced” window — U-value below approximately 3.0 W/m²·K for the total unit, with SHGC between 0.4 and 0.6, paired with appropriate external shading for north-facing and west-facing glazing. MEICHEN’s thermally broken systems with double-glazed Low-E units achieve U-values below 1.6 W/m²·K for the glass component and below 2.5 W/m²·K for the total window unit — substantially exceeding the minimum threshold and providing headroom in the BASIX assessment for other design features.

3. Understanding NatHERS: The National House Energy Rating Scheme

3.1 How NatHERS Works

NatHERS (Nationwide House Energy Rating Scheme) is a star-rating system (0-10 stars) that quantifies the thermal performance of a dwelling’s design. It is the primary compliance pathway for NCC energy efficiency provisions in most Australian states and territories. A design achieving 7 stars (the current NCC 2022 minimum in most jurisdictions) is estimated to require significantly less heating and cooling energy than a 6-star design (Nationwide House Energy Rating Scheme, 2024).

NatHERS accredited software tools (such as AccuRate, BERS Pro, and FirstRate5) model the dwelling using detailed inputs including:

• Building geometry and orientation
• Construction materials and insulation levels (R-values for walls, roof, floor)
• Window and door dimensions, orientation, frame type, glass type, and shading
• Climate data for the specific location

The software calculates an annual heating and cooling load (MJ/m²/yr) and converts this to a star rating.

3.2 Window Inputs Required for NatHERS

For each window and door in the model, the NatHERS assessor needs:

• U-value (total unit): The thermal transmittance of the complete window assembly (frame + glass + edge effects), in W/m²·K. This is the single most important number for heating-dominated climates.
• SHGC (total unit): The fraction of incident solar radiation admitted through the window assembly. This matters more in cooling-dominated and mixed climates.
• Air infiltration: The rate of air leakage through the closed window assembly, typically expressed in L/s·m² at a reference pressure. This is generally taken from the window’s AS2047 air infiltration test result or a default value from the NatHERS Technical Manual.
• Dimensions and orientation: Area, aspect ratio, and compass bearing of each window or door.

These inputs are ideally sourced from the window manufacturer’s certified performance data. If certified data is unavailable, the NatHERS assessor must use conservative default values from the NatHERS generic window library, which typically penalise uncertified products with higher U-values and lower SHGC values than the product may actually achieve — potentially costing 0.5-1 star on the final rating.

4. U-value and SHGC: The Two Numbers That Define Window Energy Performance

4.1 Understanding U-value

A window’s U-value (also written as Uw or U-factor) measures how readily heat conducts through the assembly. The units are watts per square metre per degree Kelvin (W/m²·K): a U-value of 2.0 means that for every square metre of window area, 2.0 watts of heat is transferred for every degree of temperature difference between inside and outside.

Lower is better. A window with Uw = 1.5 loses half as much heat as a window with Uw = 3.0 under the same conditions.

Three components contribute to the total window U-value:

• Uf (frame U-value): The thermal performance of the frame material and profile design.
• Ug (glass centre U-value): The thermal performance of the glazing unit at its centre, away from edge effects.
• Psi (ψ, linear thermal transmittance): The additional heat flow at the edge of the glazing unit, where the glass meets the frame and where the spacer bar is located.

The total window U-value (Uw) is calculated as:

Uw = (Af × Uf + Ag × Ug + lg × ψ) / (Af + Ag)

Where Af is frame area, Ag is glass area, and lg is the length of the glazing perimeter.

4.2 Understanding SHGC

The Solar Heat Gain Coefficient (SHGC) measures the fraction of incident solar radiation transmitted through the window assembly. An SHGC of 0.5 means 50% of the sun’s heat energy passes through the window and into the interior.

Unlike U-value, the “ideal” SHGC depends on climate and orientation:

• Heating-dominated climates (southern Australia, alpine regions): Higher SHGC (0.5-0.7) is generally beneficial for north-facing windows, capturing winter solar gain. East and west windows should have lower SHGC to avoid summer overheating.
• Cooling-dominated climates (northern Australia): Lower SHGC (0.3-0.5) is preferred on all orientations to reduce air-conditioning loads.
• Mixed climates (Sydney, Perth, Adelaide): Orientation-specific SHGC is the best strategy — higher SHGC on north, lower on east/west, with external shading on north to block high-angle summer sun while admitting low-angle winter sun.

4.3 Typical U-values and SHGC by Window Type

Note: U-value and SHGC vary with glass supplier, coating specification, and frame design. Certified product-specific data should always be used in BASIX and NatHERS assessments. MEICHEN provides NATA-accredited laboratory test reports or CSI-certified thermal performance data for its complete product range.

5. How Aluminium Windows Achieve Low U-values

5.1 The Thermal Break

Aluminium is an excellent conductor of heat — approximately 1,000 times more conductive than timber or uPVC. Without intervention, an aluminium frame is a thermal bridge, efficiently conducting heat between the outside and inside environments.

The solution is the thermal break: a strip of low-conductivity material — typically glass-fibre-reinforced polyamide (PA66 nylon) — inserted between the interior and exterior aluminium extrusions. The thermal break physically separates the two sides of the frame, dramatically reducing the conductive heat transfer path.

A well-designed thermal break can reduce the frame U-value (Uf) from approximately 7-9 W/m²·K (non-thermally broken) to 2.5-4.0 W/m²·K (single thermal break) or even 1.8-2.5 W/m²·K (multi-cavity thermal break).

5.2 MEICHEN’s Multi-Cavity Thermal Break System

MEICHEN employs a multi-cavity PA66 thermal break design that goes further than a simple single-strip separation:

1. Multiple insulating chambers within the PA66 profile create dead-air spaces that further reduce thermal conduction.
2. Warm-edge spacer bars in the insulating glass unit (stainless steel with a thermal break, or non-metallic “super spacer”) reduce the linear thermal transmittance (ψ) at the glass edge — the zone where condensation risk is highest.
3. Optimised glazing rebate depth ensures the warm-edge spacer is positioned in the warmest zone of the frame, minimising edge-of-glass heat loss.

The result is a total window U-value below 2.5 W/m²·K for standard configurations and below 1.6 W/m²·K (glass centre) for high-performance configurations — values that satisfy BASIX thermal comfort targets in all NSW climate zones and support NatHERS ratings of 7 stars and above.

6. Glass Selection for Energy Compliance: Beyond Double Glazing

6.1 Low-E Coatings

The single most impactful glass upgrade for energy performance is a Low-E (low-emissivity) coating. This is a microscopically thin, transparent metallic oxide layer applied to one surface of the glass during manufacture. It works by reflecting long-wave infrared radiation (heat) while transmitting short-wave solar radiation (visible light and near-infrared).

The position of the Low-E coating within the double-glazed unit is critical:

• Surface #2 (the interior surface of the outer pane): The most common position for mixed and cooling-dominated climates. It reflects solar heat outward (reducing SHGC) while still providing good insulation (low U-value).
• Surface #3 (the interior surface of the inner pane): Used in heating-dominated climates. It reflects interior heat back into the room, maximising winter performance but admitting more solar gain.
• Double Low-E (surfaces #2 and #3): The highest-performance option for cold climates, achieving the lowest U-value but with a lower SHGC and slightly reduced visible light transmission.

6.2 Argon and Krypton Gas Fills

Replacing the air in the cavity between glass panes with argon or krypton gas improves the Ug value because these gases are denser than air and conduct heat less efficiently:

• Air fill, 12 mm cavity: Ug ≈ 2.7 W/m²·K (clear glass)
• Argon fill, 12 mm cavity: Ug ≈ 2.5 W/m²·K (clear glass, approximately 7% improvement)
• Argon fill, 16 mm cavity: Ug ≈ 2.3 W/m²·K with Low-E coating

The improvement is modest for clear glass but more significant when combined with Low-E coatings, because the coating already reduces the radiative component of heat transfer, leaving conduction and convection through the gas as the dominant mechanisms — and these are what argon and krypton address.

6.3 Warm-Edge Spacer Technology

The spacer bar that separates the two glass panes around the perimeter is traditionally aluminium — a highly conductive material that creates a cold line around the glass edge. Warm-edge spacers — made from stainless steel with a thermal break, silicone foam, or thermoplastic — reduce this edge-of-glass heat loss by 30-50%. For a typical residential window, this can improve the total window U-value by 0.1-0.2 W/m²·K — a small but meaningful improvement that can be the difference between meeting and missing a BASIX heating cap in a marginal design.

7. How Window Selection Affects BASIX and NatHERS Scores

7.1 Sensitivity Analysis

Research by the CSIRO and the Australian Glass and Window Association has shown that window performance is the most sensitive variable in a NatHERS rating after ceiling insulation and roof colour. A typical sensitivity analysis for a single-storey, 200 m² house in western Sydney shows:

Improving window performance alone can contribute more than a full star to a NatHERS rating, making it one of the most cost-effective interventions available to a designer struggling to meet the 7-star target.

7.2 Practical Compliance Strategies

For a project targeting 7-star NatHERS compliance or a challenging BASIX heating/cooling cap, the following window strategies are typically effective:

1. Specify thermally broken aluminium frames as a minimum — non-thermally broken frames will almost certainly preclude 7-star compliance in climate zones 5, 6, and 7 (the most populous zones in NSW, Victoria, and SA).
2. Use double glazing with Low-E coating and argon fill as the standard specification for all windows and doors. The cost uplift over clear double glazing is approximately $40-60/m² and typically pays back the BASIX heating load reduction needed.
3. Differentiate SHGC by orientation: Higher SHGC glazing (0.55-0.65) on north-facing windows, lower SHGC (0.35-0.45) on east and west. This is achievable by specifying different glass types for different facades.
4. Limit west-facing glazing area or specify deep external shading (eaves, awnings, or vertical screens) for any large west-facing windows.
5. Include the window certification documentation in the BASIX/NatHERS submission. Using certified rather than default window performance values can improve the rating by 0.3-0.8 stars (CSIRO, 2023).

7.3 Case Study: 2 Murray Rose Avenue, Sydney Olympic Park

The 2 Murray Rose Avenue apartment project at Sydney Olympic Park specified MEICHEN’s 150 mm commercial sliding door and awning window systems with non-thermally broken Low-E double-glazed units. The project was required to meet BASIX energy targets for a Class 2 (multi-residential) building in NSW Climate Zone 5. By using Low-E coated glass with optimised SHGC and the thermal performance data certified through AS2047 testing, the project satisfied its BASIX thermal comfort targets without resorting to more expensive triple-glazed or thermally broken frame options on all facades — demonstrating that glass specification, not just frame design, is a critical lever for BASIX compliance.

8. Thermal Break Technology in Aluminium Windows

8.1 How Thermal Breaks Are Manufactured

There are two primary methods for creating a thermally broken aluminium profile:

1. Pour-and-debridge (P&D): A liquid polyurethane is poured into a channel in the aluminium extrusion, allowed to cure, and then the aluminium bridge at the base of the channel is machined away (debridged), leaving the cured polyurethane as the structural connection between the interior and exterior halves. This method is common in North America but less prevalent in Australia.
2. Polyamide strip (PA66) insertion: Pre-formed glass-fibre-reinforced polyamide strips are mechanically rolled into grooves in the interior and exterior aluminium extrusions, physically locking the two halves together. This is the dominant method in the Australian and European markets and is the system used by MEICHEN.

The PA66 method offers higher shear strength (important for structural window frames), better long-term dimensional stability, and the ability to create multi-chamber thermal breaks by using shaped PA66 profiles with internal cavities.

8.2 Thermal Break Performance Verification

A thermal break should not be taken on trust. Key verification points include:

• Positioning: The thermal break should be positioned to intercept all conductive heat paths — including the glazing rebate, where the warm-edge spacer of the glass unit sits. A thermal break that does not extend into the rebate zone leaves a significant thermal bridge at the glass edge.
• Depth: The PA66 strip should be a minimum of 18-24 mm deep for effective thermal separation. Thinner strips (12-16 mm) provide measurably less thermal resistance.
• Certified Uf value: The frame U-value should be certified through testing in accordance with AS/NZS 4859.1 or NFRC 100, not estimated from generic material properties. MEICHEN provides frame thermal performance data from NATA-accredited laboratory testing for its thermally broken ranges.

9. Comparison: Aluminium vs. Timber vs. uPVC for Energy Performance

9.1 Whole-of-Life Comparison

Builders and architects often face a material choice between aluminium, timber, and uPVC for window frames. While this article focuses on aluminium — the dominant material in Australian multi-residential and commercial construction — a fair comparison helps inform specification decisions:

For Australian conditions — particularly the combination of high structural demands (wind loads, large spans), coastal durability requirements, and bushfire safety regulations (BAL ratings) — thermally broken aluminium represents the most balanced solution for the majority of residential and commercial projects. The BASIX and NatHERS compliance pathway is well established, and the durability and low-maintenance characteristics align with Australian consumer preferences.

10. FAQ: Common Questions About Window Energy Ratings

Q1: Can I get a 7-star NatHERS rating with aluminium windows?

Yes — and thousands of homes across Australia have achieved 7 stars and above with thermally broken aluminium windows. The key is specifying thermally broken frames (not non-thermally broken standard aluminium) and selecting double-glazed units with Low-E coatings, argon fill, and warm-edge spacers. With these specifications, aluminium window U-values of 2.0-2.8 W/m²·K are achievable, which is entirely compatible with 7-star and even 8-star NatHERS designs when combined with good insulation, orientation, and shading.

Q2: What is the difference between glass U-value and total window U-value?

The glass U-value (Ug) measures heat transfer through the centre of the glass pane only. The total window U-value (Uw) incorporates the frame (Uf), the glass edges (ψ), and the glass-centre (Ug) into an area-weighted average for the entire assembly. Because aluminium frames — even thermally broken ones — are more conductive than the glass unit, Uw is always higher than Ug. A window with Ug = 1.2 might have Uw = 2.2. Always use Uw for BASIX and NatHERS inputs, not Ug.

Q3: Does a larger window area make it harder to meet BASIX targets?

Not necessarily — it depends on orientation, shading, and glass specification. A large north-facing window with appropriately high SHGC and effective external shading can be a net energy benefit in winter (reducing the heating load). A large west-facing window with high SHGC and no shading will almost certainly cause a cooling load problem. The BASIX tool accounts for orientation, so the strategy is to manage glazing area and specification by orientation rather than simply minimising total glazing.

Q4: Are double-glazed windows mandatory under NCC 2022?

The NCC 2022 requires compliance with the energy efficiency provisions (Part J0 for commercial, Part 13 for residential), but does not specifically mandate double glazing. In practice, however, achieving the required thermal performance (7 stars NatHERS or equivalent elemental Deemed-to-Satisfy provisions) in climate zones 5, 6, 7, and 8 is extremely difficult with single glazing alone. Double glazing — and in many cases, thermally broken frames — becomes a practical necessity, even if not a literal legal requirement.

Q5: Does MEICHEN provide U-value and SHGC data for BASIX and NatHERS assessments?

Yes. MEICHEN provides certified thermal performance data — including total-unit U-values, SHGC, and visible light transmittance — for its complete product range, supported by NATA-accredited laboratory testing and CSI certification documentation. This data is formatted for direct input into BASIX and NatHERS assessment tools, avoiding the performance penalty associated with generic default values. Project-specific thermal performance schedules can be prepared on request.

Q6: How long do thermal breaks last? Do they degrade over time?

Polyamide (PA66) thermal breaks are highly durable. The glass-fibre reinforcement provides mechanical strength, and the nylon base material is resistant to moisture, UV radiation (the break is protected within the extrusion), and temperature cycling. Independent accelerated-ageing studies indicate a service life exceeding 50 years for PA66 thermal breaks in building applications. MEICHEN’s 10-year frame warranty extends to the thermal break as an integral part of the extrusion assembly.

11. References

External References

1. Australian Building Codes Board (ABCB). (2022). National Construction Code 2022, Volume 2 — Energy Efficiency (Part 13). Available from: ncc.abcb.gov.au
2. NSW Department of Planning. (2024). BASIX — Building Sustainability Index. Available from: www.basix.nsw.gov.au
3. Nationwide House Energy Rating Scheme (NatHERS). (2024). NatHERS Technical Manual and Software Protocols. Available from: www.nathers.gov.au
4. CSIRO. (2023). Window Performance and NatHERS Ratings — Sensitivity Study. Available from: www.csiro.au
5. Australian Glass and Window Association (AGWA). (2024). AGWA Thermal Performance Resources for Window Specifiers. Available from: www.agwa.com.au
6. Window & Glass Association New Zealand. NZBC Clause H1 — Energy Efficiency Compliance for Windows. Available from: www.wganz.org.nz

Internal MEICHEN References

• MEICHEN Windows & Doors — Thermal Performance Data. Available from: mcwindow.com.au
• MEICHEN Windows & Doors — Project Portfolio: 2 Murray Rose Avenue, Sydney Olympic Park. Available from: mcwindow.com.au

Prepared by the MEICHEN Windows & Doors technical team. For project-specific BASIX and NatHERS thermal performance data, contact Annly Zhang at Annly@mcwindow.com.au or +61 490 141 931.