Windows are one of the most influential parts of a residential thermal-performance assessment. They provide daylight, views and ventilation, but they can also transfer heat more readily than insulated walls, roofs and floors.
Within BASIX and NatHERS, window performance is determined by more than whether the glass is single or double glazed. The complete window system—including the frame, glass, opening type, size, orientation and shading—affects the modelled heating and cooling demand of the dwelling.
The final assessment may establish maximum U-values, acceptable Solar Heat Gain Coefficient ranges and dwelling-specific window requirements. Those values must then be coordinated with the architectural plans, window schedule, supplier information and installed products.
In Brief
How Do Windows Affect BASIX?
Windows affect BASIX by changing the amount of conducted heat and solar radiation entering or leaving a dwelling. U-value describes heat transfer through the complete glass-and-frame system, while SHGC describes how readily solar energy enters through it. Window area, orientation, opening type, shading and overshadowing also influence the result. The correct specification is therefore project-specific and must match the final BASIX or NatHERS documentation.
Knowledge Navigation
Understanding Window Performance
Window System
Glass and Frame Together
Why glass-only values do not describe the complete installed window.
Performance Values
U-Value and SHGC
How insulation and solar-gain properties influence different loads.
Assessment
BASIX and NatHERS Inputs
How glazing is represented through DIY and Simulation methods.
Delivery
Certification and Substitution
What must be checked when windows are selected or changed.
Why Windows Have Such a Strong Effect on Thermal Performance
Windows perform several architectural and environmental functions at the same time. They provide light, views, ventilation and a connection between the interior and exterior, while also forming part of the thermal envelope.
Heat can move through a window by conduction through the glass and frame. Solar radiation can also pass through the glazing and become heat inside the dwelling.
These two processes can be useful or undesirable depending on the season, climate, orientation and room. Winter solar gain can reduce heating demand in one situation, while the same solar exposure can increase cooling demand during warmer conditions.
Window performance must therefore be considered as a balance between heat transfer, solar access and the wider design intent rather than as a search for one universally superior product.
Complete Product Performance
BASIX Assesses the Whole Window, Not the Glass Alone
A window is a complete system comprising glass, frame, spacers, seals and an opening configuration. The thermal performance of the installed product depends on the interaction of those components.
A centre-of-glass value supplied for an individual pane does not represent the total product performance required for BASIX or NatHERS documentation. The relevant U-value and SHGC must apply to the glass and frame together under the recognised rating conditions.
Glass Only
Incomplete for Compliance
A glass-pane value does not account for the frame, edge conditions, operating arrangement or proportion of glass within the complete product.
Whole Window
Required Performance Basis
The total-product rating incorporates the thermal behaviour of both the glazing and frame as an assembled window or glazed door.
Supplier documentation should therefore identify the whole-window values for the actual product, dimensions and opening configuration rather than only the nominal glass composition.
Understanding U-Value and SHGC
U-value and Solar Heat Gain Coefficient describe different aspects of window performance. They should be read together rather than treated as interchangeable ratings.
| Value | What It Describes | Lower Value | Higher Value |
|---|---|---|---|
| U-value | The rate of non-solar heat transfer through the complete window system. | Better resistance to conducted heat flow. | More heat can transfer through the window. |
| SHGC | The fraction of solar energy admitted through the complete window system. | Less solar heat enters through the window. | More solar heat enters through the window. |
Conducted Heat Transfer
What Does Window U-Value Mean?
U-value measures how readily heat passes through the complete window system because of a temperature difference between indoors and outdoors.
During colder conditions, heat can flow from the warmer interior through the window to outside. During hot conditions, conducted heat can flow inward where the exterior is warmer than the conditioned interior.
A lower total-product U-value indicates that the window provides greater resistance to this heat transfer. Lower U-values can therefore support both heating and cooling performance, although the magnitude of the benefit depends on the climate, window area and wider envelope.
The lowest available U-value is not automatically necessary for every project. The required value should be determined through the applicable BASIX or NatHERS assessment and balanced against the complete design and specification.
What Does Window SHGC Mean?
Solar Heat Gain Coefficient describes how readily radiant solar energy passes through the complete window and contributes heat to the interior.
A lower SHGC allows less solar heat to enter. This can help reduce cooling demand where windows receive strong or unwanted solar exposure.
A higher SHGC permits more solar gain. This may help reduce heating demand where winter sunlight reaches appropriately oriented and shaded glazing.
Neither a low nor high SHGC is universally preferable. The suitable range depends on the climate, orientation, window size, external shading, overshadowing and whether the dwelling is constrained by heating or cooling.
Selecting one low-SHGC specification for every façade can reduce useful winter solar gain. Conversely, using high-SHGC glazing without effective summer control can increase cooling loads.
Performance Balance
Why U-Value and SHGC Must Be Selected Together
A window may have a low U-value but an SHGC that is unsuitable for its orientation. Another may provide useful solar gain but allow more conducted heat transfer than the project can accommodate.
Lower U-value with lower SHGC
Can reduce conducted heat transfer and unwanted solar gain, which may assist cooling-dominated façades or climates.
Lower U-value with higher SHGC
Can reduce conducted heat loss while allowing useful winter solar gain where the orientation and shading support that strategy.
One specification across every orientation
Can simplify procurement but may be less responsive where different façades have substantially different solar exposure.
Orientation-specific specifications
Can produce a more targeted response but require clear window schedules, product control and construction coordination.
Window Performance Depends on More Than the Product Rating
The same window product can have a different effect when its size, orientation or shading changes.
Window Area
Larger windows increase the amount of the façade governed by the window’s heat-transfer and solar-gain properties.
Orientation
Different façades receive different amounts and timing of solar exposure throughout the day and year.
External Shading
Eaves, balconies, awnings and adjustable devices can change the direct solar radiation reaching the glazing.
Overshadowing
Neighbouring buildings, vegetation and parts of the development can reduce solar access to individual windows.
Room and Zone
The room served by the window, its use and relationship to other spaces influence the thermal assessment.
Operability
Fixed, sliding, awning, casement and other opening arrangements may have different whole-window performance data.
Window Construction
How Frames and Opening Types Affect Performance
The frame forms part of the thermal pathway through the complete window. Two products using similar glass can therefore have different U-values and SHGC values where their frame systems or operating arrangements differ.
| Frame or Configuration | Relevant Characteristic | Assessment Implication |
|---|---|---|
| Standard aluminium | Aluminium readily conducts heat through an unbroken frame section. | The whole-window U-value can remain relatively high even where improved glass is used. |
| Thermally broken aluminium | A less conductive separator interrupts the direct thermal path through the frame. | Can improve whole-window performance where the product is appropriately rated and documented. |
| Timber, uPVC or fibreglass | These materials generally conduct less heat than unbroken aluminium. | The final result still depends on the complete frame profile, glass and window construction. |
| Fixed, sliding or hinged | Different operating types use different frame proportions and assembled configurations. | The product data should correspond with the operating type nominated in the assessment and schedule. |
The frame description alone does not prove compliance. The final product should have recognised whole-window values that satisfy the assessed requirements.
Single, Double, Tinted and Low-E Glazing
Glazing descriptions help communicate the general product construction, but they should not replace the complete U-value and SHGC requirements where performance values are nominated.
| Glazing Type | General Characteristic | Project Consideration |
|---|---|---|
| Single glazing | Uses one pane of glass within the window system. | May be suitable in some projects, but its total-product performance must satisfy the assessment. |
| Double glazing | Uses two panes separated by a sealed air- or gas-filled space. | Can improve insulating performance, but the frame, coatings, spacer and complete system remain important. |
| Tinted glazing | Reduces selected portions of solar and visible transmission. | Can lower solar gain but may also affect daylight and useful winter solar access. |
| Low-emissivity glazing | Uses a coating designed to alter radiant heat transfer. | Different low-e products can have different U-values and solar-gain characteristics and are not interchangeable by description alone. |
There is no universal BASIX rule requiring double glazing for every new home. Some projects can meet their requirements with carefully designed single glazing, while others need double glazing, low-e coatings, improved frames or façade-specific combinations.
The correct answer comes from the project-specific thermal assessment rather than the glazing label alone.
Assessment Methods
How Windows Are Entered in BASIX and NatHERS
The way window information is entered depends on whether the project uses the BASIX DIY Method or NatHERS Simulation Method.
| Assessment Method | Window Entry | Resulting Documentation |
|---|---|---|
| BASIX DIY Method | Windows and glazed doors are entered with numbers, dimensions, operating type, frame, glass, orientation, shading and overshadowing information. | The BASIX Certificate records the applicable descriptions or maximum U-values and acceptable SHGC ranges. |
| NatHERS Simulation Method | The assessor enters each relevant window into the thermal model using default or recognised custom whole-window performance data. | The NatHERS Certificate and stamped plans record the assessed window specifications and applicable substitution requirements. |
Window numbers should correspond with the architectural window schedule wherever practical. This allows each assessed opening to be traced into the documentation and checked later during certification.
Default and Custom Windows in NatHERS
NatHERS software can represent windows through standard default selections or recognised custom products, depending on the software, assessment stage and available product information.
Generic Selection
Default Window
Represents a recognised generic combination of operating type, frame and glazing where the final manufacturer or product has not been fixed.
Product Selection
Custom Window
Represents a specific recognised window product with whole-window performance data available to the assessment software.
Default windows can preserve product flexibility during design and procurement, provided the eventual product complies with the allowable values and other requirements shown in the final documentation.
Custom windows can provide greater product certainty but may tie the assessment more closely to a particular system. Any later substitution should be checked against the Certificate before it is accepted.
The assessor should be told whether the project team wants a performance-based specification or intends to nominate a specific product range.
Compliance Documentation
How Are BASIX Window Requirements Certified?
The required evidence depends on the thermal-performance method and the window type shown on the Certificate.
For certain common clear-glass and frame combinations entered through the BASIX DIY Method, compliance can be based on the installed window matching the description recorded on the BASIX Certificate.
These combinations include:
- standard aluminium frames with single clear glass
- standard aluminium frames with clear air-filled double glazing
- timber, uPVC or fibreglass frames with single clear glass
- timber, uPVC or fibreglass frames with clear air-filled double glazing
For other DIY window selections, the BASIX Certificate generally states a maximum allowable whole-window U-value and an acceptable SHGC range. Product documentation must demonstrate that the installed window falls within those limits.
In those cases, the descriptive glass or frame wording may be indicative rather than exclusive. A different product can potentially be used where its recognised whole-window performance satisfies the Certificate requirements.
For NatHERS Simulation projects, the installed windows should satisfy the window schedule, allowable values and substitution conditions recorded in the NatHERS documentation. The operating type, window dimensions and other modelled characteristics should also remain consistent.
Can a Different Window Be Installed After Assessment?
A substitution may be possible where the replacement remains within every applicable performance and documentation requirement.
The review should consider whether the replacement changes:
- the whole-window U-value
- the whole-window SHGC
- the opening or operating type
- the frame construction
- the glass configuration or coating
- the window dimensions
- the operable area or ventilation assumption
- the external shading arrangement
- the window number or schedule reference
A product with a lower U-value is not automatically equivalent if its SHGC falls outside the allowable range. Likewise, a window with a suitable SHGC may not comply where its U-value is too high.
Where the new product cannot satisfy the Certificate requirements, the assessor should review the design. The thermal model, NatHERS Certificate or BASIX Certificate may need to be revised before the substitution proceeds.
Project Documentation
What Should Appear in the Window Schedule?
The window schedule should allow the assessed requirements to be traced from the BASIX or NatHERS documentation to the plans, supplier quotation and installed product.
| Schedule Information | Purpose | Coordination Check |
|---|---|---|
| Window number | Identifies each window or repeated group. | Use the same reference on plans, elevations, schedules and assessment documents. |
| Dimensions and area | Defines the size represented in the model. | Confirm that later architectural revisions have been reflected in the assessment. |
| Opening type | Distinguishes fixed, sliding, awning, casement and other configurations. | The installed operating type should align with the assessed product data. |
| Maximum U-value | Sets the allowable conducted heat-transfer performance. | The installed whole-window U-value must not exceed the nominated maximum. |
| Acceptable SHGC | Sets the permitted solar-gain performance. | The installed whole-window SHGC must remain within the allowable range. |
| Frame and glass notes | Communicates the intended or indicative construction. | Clarify whether the description itself is mandatory or whether compliance is performance based. |
Where several window specifications are used, the schedule should clearly identify which requirement applies to each opening. Generic notes that cannot be traced to individual windows create avoidable procurement and certification risk.
Window Performance in Townhouse and Apartment Projects
Multi-dwelling projects can contain many repeated windows, but apparently identical dwellings do not always require the same thermal response.
Window requirements may vary because of:
- different dwelling orientations
- end, middle or corner positions
- ground, intermediate and top-floor exposure
- different balcony or shading arrangements
- overshadowing from other buildings
- different window-to-floor-area ratios
- different room layouts or façade treatments
The project may adopt one conservative window specification that satisfies every dwelling, or use controlled variations for different façades, buildings or dwelling groups.
One common specification simplifies procurement but can require unnecessary upgrades. Multiple specifications can be more efficient but require stronger control across apartment numbers, elevations, window schedules and supplier packages.
Related Design Function
Window Performance Is Not the Same as Ventilation Performance
U-value and SHGC describe thermal properties of the window system. They do not describe how effectively air can move through the dwelling when windows are open.
Natural ventilation depends on the operable area, opening type, inlet and outlet positions, internal airflow paths, wind conditions and how occupants operate the building.
A fixed high-performance window can reduce heat transfer but provide no natural ventilation. A large operable window can support airflow but must still satisfy the required whole-window thermal specification.
Why Window Performance Should Be Reviewed Early
Window decisions are easier to optimise while the façade, openings and shading remain under design.
1. Review the glazing distribution
Identify large glazed areas, exposed orientations and rooms likely to influence heating or cooling demand.
2. Model the current design
Use the applicable BASIX or NatHERS method to establish whether heating, cooling or both are constrained.
3. Test design responses before product upgrades
Review window area, shading and distribution before assuming every opening needs the highest-performing product.
4. Establish the performance specification
Determine the U-value, SHGC, operating type and dwelling-specific variations needed for compliance.
5. Confirm product availability
Check that suppliers can provide compliant products in the intended dimensions, configurations and frame systems.
6. Coordinate the final schedule
Transfer the assessed requirements into the architectural, procurement and certification documentation.
Common Window Performance and Documentation Issues
Glass-only values are used
The quoted performance excludes the frame and does not represent the whole-window values required by the assessment.
A lower U-value is assumed to make any product compliant
The SHGC must also remain within the required range, and the other assessed characteristics must still be satisfied.
Double glazing is specified without performance values
Different double-glazed products can produce materially different whole-window results.
One SHGC is applied without reviewing orientation
Solar-control needs can differ between strongly exposed and seasonally useful façades.
Window sizes change after modelling
Revised elevations or schedules are issued without updating the thermal assessment.
The operating type is substituted
A sliding, fixed or hinged product is substituted without checking whether its whole-window data and modelled assumptions remain compliant.
Window numbers do not match
The architectural schedule, elevations, BASIX Certificate and NatHERS documentation use incompatible references.
Supplier data cannot be verified
The quotation provides marketing descriptions but no recognised whole-window U-value and SHGC information for the proposed products.
Frequently Asked Questions
Window Performance and BASIX FAQs
Why are windows important in BASIX?
Windows influence conducted heat transfer, solar heat gain, heating and cooling loads and the wider thermal response of the dwelling.
What is a window U-value?
U-value measures the rate of non-solar heat transfer through the complete window, including both glass and frame. A lower U-value indicates greater resistance to heat flow.
What is window SHGC?
Solar Heat Gain Coefficient measures the fraction of solar energy admitted through the complete window system. A lower SHGC allows less solar heat to enter.
Is a lower SHGC always better?
No. A lower SHGC can reduce unwanted summer heat, but a higher SHGC may provide useful winter solar gain. The suitable range depends on climate, orientation, shading and the project’s heating and cooling results.
Does BASIX require double glazing?
There is no universal requirement for every BASIX project to use double glazing. The required whole-window performance depends on the design, climate, assessment method and applicable heating and cooling limits.
Are glass-only U-values acceptable?
No. BASIX and NatHERS window performance is based on the whole product, including the glass and frame together.
Can different façades use different window specifications?
Yes. Orientation-specific window requirements can be used where they are clearly identified in the assessment, window schedule and construction documentation.
Can a window product be substituted after the BASIX Certificate is issued?
Potentially. The replacement must satisfy the applicable whole-window U-value, SHGC, operating type and other requirements. If it does not, the assessment and Certificate may need revision.
Does changing a fixed window to a sliding window matter?
It can. Different operating types may have different whole-window ratings and ventilation characteristics. The replacement should be checked against the assessment and Certificate.
What documentation should a window supplier provide?
Where performance verification is required, the supplier should provide recognised whole-window U-value and SHGC information for the proposed frame, glazing and operating configuration so it can be checked against the BASIX or NatHERS requirements.
Related Knowledge
Continue Exploring Glazing and Thermal Performance
Assessment note: Window-performance requirements depend on the project location, thermal-assessment method, window dimensions, orientation, shading and complete building design. Product values should be checked against the project-specific BASIX and NatHERS documentation before procurement or substitution.
For regulatory information, refer to the NSW Planning Portal guidance on BASIX windows and glazed doors.
Last reviewed: July 2026.
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