The Hidden Impact of NatHERS Default Values

You're looking at a NatHERS rating that's 0.3 stars below target, wondering where the performance disappeared. The insulation specifications look right, the windows match your schedule, and the shading you proposed seems appropriate. What you might not realise is that somewhere in your documentation, missing details forced the assessor to apply default values—and those conservative assumptions are quietly eroding your star rating.

Right now, across Australia, thousands of homes are receiving lower NatHERS ratings than their actual specifications warrant. The culprit, in some cases, isn't poor design—it's incomplete documentation that triggers NatHERS software's built-in conservative defaults. At Makao, we've seen projects lose entire star points simply because critical specifications weren't clearly documented.

Let's examine how these defaults work and, more importantly, how to avoid them.

Understanding NatHERS Default Philosophy

NatHERS software is fundamentally an energy rating model that includes approximations and assumptions based on decades of science and consultation with stakeholders. However, when specific information isn't provided, the software doesn't guess optimistically—it defaults to conservative values that often represent worst-case scenarios.

The NatHERS Assessor Handbook explicitly states that where documentation does not specify required details, assessors must choose provisional values that represent the worst performing option. For instance, if roof colour isn't specified in a tropical climate, the software defaults to 'dark'—the worst performing option for cooling loads.

Understanding what these defaults actually are reveals their impact. The Chenath engine, which underpins NatHERS calculations, operates with specific built-in assumptions:

Temperature settings

• Bedrooms: 18°C from 7am-9am and 4pm-midnight, dropping to 15°C from midnight-7am
• Living spaces: 20°C minimum heating thermostat setting
• Cooling settings vary by climate zone using the Effective Temperature method

Default lighting density

• Downlight density: 1 per 5m² (updated in October 2024)
• Lighting power density: 4W/m² for benchmark homes

Occupancy and Heat Loads

• Heat loads from appliances, cooking, lighting, and occupancy based on zone type
• Standard occupancy for 160m² dwelling: two adults and two children

This conservative approach serves an important purpose: it ensures that buildings perform at least as well as their rating suggests. Nevertheless, it can significantly penalise projects where specifications aren't fully documented.

The Most Common Default Traps

Thermal bridging specifications

Under NCC 2022, thermal bridging must be modelled for steel-framed elements, with default values applied when precise framing specifications aren't provided. If you don't specify thermal break details, the software assumes standard steel framing without adequate thermal separation.

Default steel framing specifications and thermal breaks are applied automatically, often resulting in significantly higher thermal bridging penalties than what's actually being constructed.

Insulation documentation

When edge batt insulation values aren't specified, NatHERS applies a default of R3.0 with 450mm width. Similarly, if ceiling penetration details aren't documented, the software assumes standard downlight installations without proper sealing or spacing requirements.

Moreover, if insulation installation methods aren't clearly specified, assessors cannot account for improved installation techniques that reduce thermal bridging and air gaps.

Window performance defaults

The window defaults are particularly problematic. Window opening percentages default to conservative values if specific hardware isn't documented. Furthermore, security restrictors automatically trigger a default opening percentage of just 10% for each window unit, dramatically reducing natural ventilation benefits.

Frame specifications also matter enormously. Without detailed window schedules, assessors must use generic frame categories rather than specific WERS-rated products that often perform significantly better.

Surface Colours and Finishes

Colour specifications represent one of the most common default applications, with 'medium' values applied when wall, roof, or ceiling colours aren't specified. In practice, this means:

• Roof colours default to medium in temperate climates, regardless of actual selection
• Wall colours assume medium solar absorptance
• Internal surface colours default to medium reflectance values

These defaults can impact both heating and cooling loads, particularly in extreme climates where light or dark colours provide significant performance advantages.

HVAC and Appliance Specifications

When heating and cooling appliance specifications aren't provided, default performance levels and star ratings from prescribed tables must be applied. These defaults typically represent minimum performance levels rather than the efficient equipment often specified in modern homes.

For Whole of Home assessments, the benchmark assumes:

• 3-star ducted reverse cycle air conditioner (under GEMS 2019 standards)
• Electric resistance hot water system
• 4W/m² lighting power density across all internal spaces

Additionally, if ducted system zoning isn't clearly documented, the software cannot accurately model the efficiency benefits of properly designed zoning strategies.

Air Sealing and Construction Details

Construction methodology documentation proves crucial for air infiltration modelling. Without specific details about sealing strategies, window installation methods, and junction treatments, NatHERS assumes standard construction practices that may significantly overestimate air leakage rates.

Consequently, homes designed with superior air sealing details but poorly documented can receive similar infiltration penalties to conventionally built homes.

Shading and Orientation Details

All fixed and non-fixed shade features shown in documentation must be modelled, but default values apply when dimensions or materials aren't specified. For external shading devices, missing details about:

• Shading depth and width
• Material specifications and colours
• Seasonal operation for adjustable systems
• Mounting details and thermal bridging

...can result in reduced shading benefits or, in some cases, shading elements being excluded entirely from the assessment.

The Documentation Solution

Creating comprehensive specifications

Effective NatHERS documentation requires thinking like the software. Every element that influences thermal performance needs explicit specification, not just general notes or typical details.

For thermal bridging, this means providing:

• Specific steel framing dimensions and spacing
• Thermal break material specifications and R-values
• Installation details at critical junctions
• Cavity insulation and continuous insulation details

Window schedules that work

Comprehensive window schedules should include:

• WERS ratings for specific products (not just generic categories)
• Frame material and thermal performance specifications
• Hardware details affecting opening percentages
• Installation methods affecting air sealing

Surface and material specifications

Rather than general finish schedules, provide:

• Specific colour selections with solar absorptance values where known
• Roof material specifications including colour and finish type
• Internal surface finishes affecting thermal mass and reflectance

Quality Control and Review Processes

Pre-Assessment Documentation Review

Smart project teams now include NatHERS documentation review as a standard design development milestone. This involves checking that all thermal performance-related specifications are complete and clearly documented before submitting for assessment.

Assessor collaboration

Early engagement with your energy assessor during design development helps identify potential default applications before they impact ratings. Experienced assessors can flag missing specifications that will trigger defaults, allowing time for clarification.

Real-World Impact: Case Study

Recently, we reviewed a townhouse project in Melbourne that initially rated 6.8 stars despite specifying premium insulation and high-performance windows. The documentation review revealed several default applications:

• Steel framing details: Missing thermal break specifications resulted in maximum thermal bridging penalties
• Window hardware: Unspecified security hardware triggered opening  and frame colour defaults
• Roof colour: "Colorbond" specification without specific colour defaulted to medium
• Insulation installation: Missing installation method details prevented air gap benefits

After documentation clarification—requiring no design changes—the rating improved to 7.4 stars. The time investment: approximately four hours of documentation review and specification clarification.

The Economic Reality

Default applications don't just affect star ratings—they affect real project economics. When defaults push ratings below compliance thresholds, projects face expensive retrofits or design changes that could have been avoided with better documentation.

Furthermore, homes that receive conservative ratings due to defaults may underperform market expectations, particularly as energy performance becomes increasingly important for resale values.

Technology Trends and Future Considerations

The Window Energy Rating Scheme (WERS) Link portal is now complete and will underpin new custom and default window libraries, improving how assessors can model windows. These improvements will reduce some window-related defaults, but they also emphasise the importance of specifying actual WERS-rated products rather than generic window categories.

Additionally, as NatHERS software continues evolving, default categories are becoming more sophisticated but also more numerous. This trend emphasises the growing importance of comprehensive documentation.

Best Practice Documentation Checklist

Thermal Envelope Elements

✅ Complete insulation specifications with R-values and installation methods

✅ Thermal bridging details and thermal break specifications

✅ Air sealing strategies and penetration treatments

✅ All surface colours and finishes with specific product names

Fenestration Details

✅ WERS ratings for specific window products

✅ Frame specifications and thermal performance

✅ Hardware details affecting operation and sealing

✅ Installation methods and flashing details

Mechanical Systems

✅ Specific equipment models and performance ratings

✅ Zoning strategies and control methods

✅ Ductwork specifications and sealing requirements

✅ Hot water system details and controls

Shading and Solar Control

✅ Dimensions and materials for all shading devices

✅ Seasonal operation strategies for adjustable elements

✅ Neighbouring building and vegetation details

✅ Mounting and structural details

Regulatory Context and Compliance

Where provisional values represent worst-case scenarios, assessors must test the choice to ensure the rating provided with the provisional value is the minimum that can be achieved. This requirement means that multiple simulations may be needed to determine appropriate defaults, potentially extending assessment timelines.

Moreover, clients should be advised when provisional values are used, and that some provisional values represent worst-case scenarios that may adversely affect the rating. This notification requirement creates additional documentation and communication overhead for both assessors and project teams.

Working With Assessors: Communication Strategies

Early Engagement Protocols

Engage your energy assessor during developed design, not just for compliance checking. Share draft documentation and ask specifically about potential default applications. Experienced assessors can identify specification gaps that will trigger conservative defaults.

Specification Clarification Processes

When clarification has been sought but not received, assessments must be undertaken using provisional values in the NatHERS Technical Note. This creates time pressure for specification clarification—waiting too long can force default applications that could have been avoided.

Documentation Standards

Different assessors may interpret specification ambiguities differently. Working with the same assessor across multiple projects allows you to understand their documentation preferences and avoid recurring default applications.

Industry Trends and Market Response

As energy performance becomes increasingly important in residential markets, the gap between conservative NatHERS defaults and actual building performance becomes more problematic. Homes that receive conservative ratings due to documentation issues may struggle in markets where energy performance influences buyer decisions.

Additionally, as building codes continue requiring higher performance levels, the margin for error created by default applications becomes smaller. Projects that previously achieved compliance despite some defaults may fall short under stricter requirements.

Future-Proofing Documentation Practices

Digital Documentation Systems

Consider documentation management systems that link thermal performance specifications directly to NatHERS assessment requirements. These systems can flag incomplete specifications before they reach the assessor.

Specification Databases

Maintain databases of previously specified products with their thermal performance characteristics. This prevents repeated specification errors and ensures consistent documentation across projects.

Process Integration

Integrate NatHERS documentation requirements into standard design development processes, rather than treating energy assessment as a separate compliance exercise.

In Summary

In today's energy-conscious market, precise documentation isn't just about compliance—it's about accurately representing your design's performance. Every default application represents lost performance, potentially affecting not just regulatory approval but long-term project value. The solution isn't complex, but it requires discipline.

Comprehensive specification, early assessor engagement, and systematic documentation review can eliminate most default applications. What's more, the documentation improvements that prevent defaults often improve construction quality and reduce site ambiguities.

As building performance standards continue rising and market awareness of energy efficiency grows, projects that master precise documentation will have significant competitive advantages. They'll achieve higher ratings for equivalent designs, complete assessments faster, and deliver buildings that perform as intended.

Remember:

NatHERS software doesn't reward incomplete documentation with optimistic assumptions. Every missing specification becomes a conservative default that quietly erodes your star rating. However, with systematic attention to documentation completeness, you can ensure your ratings accurately reflect your design's true performance potential.

The question isn't whether you can afford to improve your documentation practices—it's whether you can afford not to.

References and Further Reading

These defaults are codified in several key NatHERS documents that provide the authoritative basis for all energy rating assessments:

Primary NatHERS Documentation:

• Software Accreditation Protocol (SAP) for new homes (NCC 2025) - Sets out required inputs, assumed behaviour, and software accuracy tolerances. Available at: nathers.gov.au/publications/software-accreditation-protocol
• NatHERS Technical Note (Version 20241023) - Provides specifications for defaults including appliances, shading, colours, and window libraries. Current version available at: nathers.gov.au/publications/nathers-technical-note
• User Interface Protocol (UIP) for existing homes - Includes Approved Settings covering default assumptions for consistent rating. Access via: nathers.gov.au

Supporting Technical Documents:

• NatHERS Assessor Handbook (Version 1.1, April 2024) - Comprehensive guide to assessment processes and zoning requirements
• Chenath Repository - Open source library of algorithms and methodologies. Available at: hstar.com.au/Chenath
• NatHERS Whole of Home Calculation Method (January 2023) - Detailed calculation procedures for appliance and energy modelling
• Thermal Bridging Implementation in AccuRate Report (2023) - CSIRO methodology for thermal bridging calculations

Regulatory Context:

• National Construction Code 2022 - Building Code of Australia energy efficiency provisions
• NatHERS Climate Files and Star Band Recalibration - Updated climate data from 1990-2016 Bureau of Meteorology records
• Window Energy Rating Scheme (WERS) Link - Custom and default window libraries integration

Quality Assurance Documents:

• A Guide to the NatHERS Certificate - Understanding certificate components and verification processes
• Internal Heat Loads guidance - Occupancy patterns and thermal comfort assumptions

All documents are maintained by the NatHERS Administrator and available through the official NatHERS website. Software-specific guidance should be obtained directly from accredited software providers.