NatHERS Window Performance Guide
Double glazing can improve a NatHERS result, but the effect depends on the complete window system, local climate, orientation, solar exposure, window area and the wider dwelling design.
Double glazing is frequently presented as a direct route to a higher NatHERS star rating. It can be an effective improvement, particularly where heat transfer through the windows is limiting the dwelling’s thermal performance. It is not, however, a guaranteed or identical solution for every project.
NatHERS does not assess the number of glass panes in isolation. The model uses the thermal properties of the complete window system, including the glass, frame, seals and other relevant components. Window size, location, orientation and shading also influence how that system performs within the dwelling.
This guide focuses specifically on when double glazing may improve the model, what the window performance values mean and which information should be coordinated with the assessor and supplier.
For the broader relationship between climate, orientation, shading, insulation and building form, see How to Design a 7 Star NatHERS Home.
In Brief
Double Glazing Can Help, but the Specification Matters
Conductive Performance
Double glazing can reduce non-solar heat transfer through the window where the complete system has an improved U-value.
Solar Performance
The number of panes does not by itself determine how much solar heat enters. The system’s SHGC must also be considered.
Project-Specific Benefit
The improvement depends on climate, orientation, window area, frame type, shading and which rooms are driving the result.
A suitable double-glazed system may provide a meaningful improvement, but the selected product should be tested in the actual NatHERS model rather than treated as a universal requirement.
Window Construction
What Does Double Glazing Actually Change?
A double-glazed unit uses two panes of glass separated by a sealed gap. The gap may contain air or another gas and is intended to reduce heat transfer through the glazed unit compared with a basic single pane.
The two panes do not need to be identical. A double-glazed unit may use clear glass, tinted glass, low-emissivity coatings or another combination selected to provide particular conductive and solar-performance characteristics.
Its performance can be influenced by:
For NatHERS purposes, the relevant question is therefore not simply whether the window is double glazed. It is how the complete selected window system performs.
Two Window Performance Values
What Is the Difference Between U-Value and SHGC?
Window performance is commonly described using two separate values. They represent different forms of heat transfer and should not be treated as interchangeable.
Non-Solar Heat Transfer
U-Value
U-value describes how readily heat is conducted through the window system because of a temperature difference between indoors and outdoors.
Lower generally means better insulation. A lower whole-window U-value indicates greater resistance to conductive heat flow.
Direct Solar Heat
Solar Heat Gain Coefficient
SHGC describes how much incident solar heat is transmitted through and absorbed by the window system before being released indoors.
Lower means less solar heat transmission. Whether a lower or higher value is preferable depends on climate, elevation, season and shading.
A double-glazed window may have a substantially improved U-value while still allowing a comparatively high level of solar heat through. Another product may combine double glazing with coatings or glass selections that also reduce SHGC.
The most suitable combination depends on what the dwelling needs. A heating-dominated room may benefit from retaining useful solar access while reducing heat loss. A cooling-dominated room may require stronger solar control.
Beyond the Glass
Why Does the Window Frame Matter?
The frame forms part of the assessed window system. A high-performing insulated glass unit can be combined with different frame materials and configurations, producing different whole-window U-values and SHGCs.
Frame conductivity, profile size and the proportion of frame to glass all influence the final performance value. A large sliding or stacking system may therefore behave differently from a smaller fixed window using similar glass.
Glass-Only Value
Describes the thermal properties of the glazing component without representing the complete installed window system.
Whole-Window Value
Combines the glazing and frame performance and is the relevant basis for NatHERS window inputs.
Product Configuration
Opening type, frame design, pane configuration and product dimensions can affect the rated system characteristics.
Window quotations and schedules should therefore identify the complete product or provide whole-window performance values rather than relying only on a description such as “double glazed” or “low-e glass”.
Likely Performance Benefit
When Is Double Glazing More Likely to Improve the NatHERS Result?
Double glazing is more likely to provide a meaningful improvement where conductive heat transfer through the windows forms an important part of the dwelling’s heating or cooling load.
This may occur where the design includes:
A stronger window specification may also improve the balance between rooms. For example, an exposed bedroom or highly glazed living area may benefit more than a protected room with limited glazing.
The size of the improvement can only be established by testing the proposed system within the complete dwelling model.
Alternative Window Responses
When Might Double Glazing Not Be the First Measure to Test?
Double glazing may provide limited benefit where the primary problem is not conductive heat transfer through the window system.
Excessive Solar Gain
Review SHGC and Shading
A double-glazed system with a relatively high SHGC may not adequately address strong direct solar exposure on a cooling-constrained elevation.
Oversized Openings
Review Window Area
A better product may reduce heat transfer without fully offsetting the effect of an unusually large or exposed glazed area.
Another Fabric Weakness
Identify the Actual Constraint
The roof, floor, wall construction or one poorly performing room may be affecting the result more strongly than the windows.
Already Strong Windows
Check Diminishing Improvement
Further upgrades may provide only a limited rating benefit where the proposed whole-window values are already comparatively strong.
This does not mean that double glazing is technically unsuitable. It means that the project should identify whether another change addresses the dominant load more directly.
Targeted Specification
Does Every Window Need the Same Glazing System?
Not necessarily. Different elevations and rooms can experience different thermal conditions, so the most useful window improvement may be concentrated in particular parts of the dwelling.
The assessor may test whether selected exposed windows benefit from a lower U-value, different SHGC or improved frame while the remaining windows retain another specification.
Targeted Upgrade
Selected Windows or Elevations
May address the rooms or façades where window performance is materially affecting the model while limiting changes elsewhere.
Consistent Upgrade
One System Across the Home
May simplify procurement, detailing, appearance and documentation, even where some windows contribute less to the thermal improvement.
A mixed specification should be clearly coordinated so that the plans, window schedule, quotation and NatHERS model all identify which system applies to each opening.
For guidance on comparing the cost and value of targeted versus broader upgrades, see How to Achieve 7 Star NatHERS Without Unnecessary Cost.
Project-Specific Selection
How Do Climate and Orientation Change the Best Window Specification?
The same window product can influence the NatHERS result differently when used in another climate, on another elevation or behind different shading.
Heating-Dominated Conditions
Reduce Heat Loss
A lower U-value may provide meaningful benefit, while the SHGC should be selected with useful winter solar access and unwanted heat gain in mind.
Cooling-Dominated Conditions
Control Solar Gain
The SHGC and external shading may be particularly important where direct sun is increasing cooling demand through exposed glazing.
Mixed Conditions
Balance Seasonal Loads
The selected combination may need to reduce conductive transfer while still managing seasonal solar gain in both heating and cooling periods.
Orientation affects the timing and angle of solar exposure. West-facing glazing can experience strong afternoon sun, while other elevations may receive different seasonal exposure and shading conditions.
The selected U-value and SHGC should therefore be considered against the location and role of each window rather than chosen from one generic specification rule.
Solar Exposure
Does Better Glass Remove the Need for External Shading?
Not necessarily. Glazing selection and external shading change solar heat transfer in different ways and should be assessed together.
A lower-SHGC window can reduce the proportion of incident solar heat transmitted through the system. External shading can prevent or reduce direct solar radiation from reaching the glass during particular periods.
Glazing Selection
Changes Window Transmission
The window’s U-value and SHGC influence conductive heat flow and the proportion of solar heat admitted through the complete system.
External Shading
Changes Solar Exposure
Eaves, awnings, balconies, screens and other external elements can alter when direct sunlight reaches the window.
A cooling problem may sometimes be addressed by a lower-SHGC system, targeted shading or a combination of the two. Excessive shading can also restrict useful winter solar access, so the response should be tested seasonally.
Double glazing should therefore not be treated as a direct substitute for reviewing solar exposure and external shading.
NatHERS Option Testing
How Can an Assessor Compare Window Options?
An assessor can compare window-system options within the proposed dwelling model. This allows the project team to see whether a change produces a meaningful improvement in the overall rating, heating load, cooling load or performance of a particular room.
Test One
Improve U-Value
Compare a system with lower conductive heat transfer while retaining an appropriate solar-performance value.
Test Two
Adjust SHGC
Review whether a different solar-transmission value better addresses the seasonal loads on a particular elevation.
Test Three
Target Selected Windows
Apply the stronger system only to the rooms or elevations where the window specification is materially influencing performance.
Test Four
Coordinate Shading
Compare the window system with the documented eaves, awnings or screens that affect its direct solar exposure.
Testing should be controlled so that the project team can understand which change produced the improvement. The preferred option can then be reviewed against cost, product availability, appearance and construction requirements.
Window Documentation
What Window Information Does the Assessor Need?
The assessor needs enough information to identify the size, location and thermal characteristics of the proposed window systems. Preliminary modelling may begin with provisional assumptions, but the final assessment should align with the selected products and project documentation.
Useful information may include:
Descriptions such as “double glazing”, “tinted glass” or “energy-efficient windows” are not sufficiently precise to establish the final NatHERS inputs on their own.
Where the final supplier or product has not yet been selected, the assessor can clarify the performance values that the eventual window system will need to satisfy.
Common Specification Issues
What Window Mistakes Can Affect the NatHERS Result?
Specifying only “double glazed”. This does not identify the whole-window U-value, SHGC, frame or complete product performance.
Using glass-only values. The NatHERS inputs should reflect the complete window system rather than the glazing component alone.
Assuming a lower SHGC is always better. Reducing solar heat may help cooling while also restricting useful winter solar access.
Ignoring the frame. Different frame materials and profiles can materially change the whole-window performance.
Upgrading every window without testing selected areas. The main benefit may be concentrated in particular rooms, openings or elevations.
Treating better glass as a substitute for shading. The glazing system and external solar exposure should be reviewed together.
Changing products after certification. A substitute window may have different U-value and SHGC characteristics and should be reviewed before installation.
Allowing the schedule and quotation to conflict. Product references and performance values should remain consistent across the project documentation.
The Practical Answer
So, Does Double Glazing Help a Home Achieve 7 Star NatHERS?
It can. A double-glazed system with suitable whole-window performance may reduce heating or cooling demand and improve the dwelling’s NatHERS result.
The improvement is not created by the number of panes alone. The result depends on the U-value, SHGC, frame, window area, orientation, shading, room exposure and local climate.
Some homes may need double glazing across most openings. Others may benefit from targeted upgrades, another glazing specification or changes to solar control. Some designs may reach the required outcome without double glazing.
The most reliable approach is to test the proposed whole-window systems in the project model before the window schedule and supplier order are finalised.
Frequently Asked Questions
Double Glazing and 7 Star NatHERS FAQs
Is double glazing required to achieve a 7 Star NatHERS rating?
Not in every project. The required window performance depends on climate, design, window area, orientation, shading, construction and the contribution of other parts of the dwelling.
Does all double glazing perform the same?
No. Double-glazed systems can use different glass, coatings, gaps, gases, spacers and frames. Their whole-window U-values and SHGCs can therefore vary substantially.
Is a lower U-value better?
A lower whole-window U-value indicates greater resistance to non-solar heat flow and therefore stronger insulating performance. The effect on the dwelling should still be tested within the complete model.
Is a lower SHGC always better?
No. A lower SHGC reduces solar heat transmission and may help cooling performance. It can also restrict useful solar gain during colder periods. The appropriate value depends on climate, orientation and shading.
Does the window frame affect the NatHERS result?
Yes. NatHERS uses whole-window performance values that account for the glazing and frame. Different frames can produce different U-values and SHGCs even where similar glass is used.
Can only selected windows be double glazed?
Potentially. An assessor can test targeted systems on selected rooms or elevations. A mixed window specification must be clearly documented and coordinated with the supplier and builder.
Does low-e glass mean the window is double glazed?
Not necessarily. A low-emissivity coating can be used in different glazing configurations. The complete system description and performance values should be checked rather than inferred from the phrase “low-e”.
Can better glazing replace external shading?
Not automatically. A lower-SHGC system can reduce solar heat transmission, while external shading changes the amount and timing of sunlight reaching the glass. The two measures should be assessed together.
What happens if the window product changes after certification?
The replacement system may have different whole-window U-value and SHGC characteristics. The proposed substitution should be reviewed by the assessor before it is accepted for the project.
Related Guidance
Continue Planning the 7 Star Window Strategy
Window Option Review
Need to Test Whether Double Glazing Will Improve the Project?
Certified Energy can review the available plans, window schedule, shading and supplier performance information to compare suitable whole-window systems within the NatHERS model.
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