Air Tightness – The Next Big Shift

A major theme is the growing importance of air tightness in residential energy performance.

Key points discussed:

• Air tightness is already improving across the industry

• Typical assumptions (e.g. ~10 air changes per hour) are often outdated

• Many homes are now performing closer to 6 air changes per hour or better

The issue:
NatHERS still often assumes higher leakage rates, meaning:

• Builders may be undervalued in ratings

• Real-world performance isn’t fully reflected in modelling

Performance-Based Opportunity

Because NatHERS is performance-based, there’s potential to:

• Test actual air tightness (e.g. blower door testing)

• Input real data into modelling

• Achieve improved star ratings based on actual performance

This opens the door to:

• More accurate assessments

• Recognition of better construction practices

But it also introduces complexity:

• Additional testing requirements

• Cost and time implications

• Risk if performance targets aren’t achieved

The Risk of Verification

A key concern raised is post-construction verification:

• What happens if a home doesn’t meet its air tightness target?

• Could this delay certification or occupancy?

• How much tolerance (if any) should be allowed?

The challenge:
Moving toward measurable performance creates a compliance risk at the end of construction, not just during design.

Industry Learning Curve

Air tightness isn’t something that can be adopted overnight.

• Builders will need time to adjust

• Some will lead, others will learn through trial and error

• Performance will improve over time through experience

Similar to past changes (e.g. star rating increases), this will likely be a gradual industry shift, not an instant transformation.

Under Slab Insulation – The Big Debate

The episode then shifts to one of the most contentious issues in NCC 2022:

Is under slab insulation actually required?

The problem:

• Conflicting advice from different authorities

• Different interpretations of Specification 42

• Lack of clear written guidance

Examples discussed:

• Some regulators say yes, it’s required

• Others say only slab edge insulation is required

Result:
Assessors are left navigating uncertainty — with real project risk if interpreted incorrectly.

Real World Risk

This ambiguity creates serious practical issues:

• If insulation is omitted and later required → major construction problem

• If included unnecessarily → cost and pushback from builders/clients

Most conservative approach:
Assessors often choose to include under slab insulation to avoid compliance risk

Jurisdictional Differences

The confusion is further complicated by state-by-state variations:

• Some states have amended requirements

• Others have effectively removed under slab insulation

• Victoria remains one of the key jurisdictions where the issue persists

Reinforcing a familiar theme:
National rules, local interpretation

NCC 2025 – A Fix on the Way

There is good news:

• NCC 2025 is expected to clarify the wording

• Likely confirming that requirements apply to slab edge insulation only

However:

• Uncertainty remains for current NCC 2022 projects

• Transition between versions may create further inconsistency

Industry Insights – Real Questions from the Field

The episode also includes a review of real questions from assessors, highlighting everyday challenges:

Improving ratings

• Some designs are inherently difficult to get over the line

• Small changes (e.g. floor type, thermal mass) can make a difference

Orientation accuracy

• Even small errors in north alignment can impact results

• “North arrows” on plans are often unreliable

Key takeaway:
Accuracy in inputs is critical — small details can drive big outcomes

Broader Industry Themes

Across all topics, a consistent pattern emerges:

• Regulations are evolving

• Software is improving

• Construction practices are changing

But, alignment between them is still catching up

This creates:

• Grey areas

• Interpretation challenges

• Opportunities for more advanced practitioners

Final Takeaway

This episode highlights a turning point in the industry:

• Air tightness is becoming a major driver of performance

• Verification and real data are the future

• But current regulations still leave gaps and inconsistencies

At the same time, practical issues like under slab insulation show that:
Even small wording differences can have big real-world consequences

Ultimately, success in this space comes down to:

• Understanding the intent

• Managing risk

• And staying ahead of a rapidly evolving system

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