Clause D.1.13.1 is limiting the effectiveness of H1. The building code is out of balance and the industry is seeing the consequences.
The Problem Hidden in the Code
New Zealand’s H1 clause is designed to support energy efficiency in buildings, but in practice, it has become narrowly focused on insulation and windows. One of the most powerful tools we have for improving performance — airtightness — is currently locked out of the compliance pathway.
This is due to a technical restriction in Clause D.1.13.1 of the H1 Verification Method, which states:
“Infiltration assumptions for the proposed building and the reference building shall be the same…”
The result? Even when airtightness is measured and proven via blower door testing, it cannot be modelled as part of energy compliance. That restriction is skewing the entire H1 system.
H1 Is Out of Balance
The three main drivers of building envelope energy efficiency are:
- Airtightness – sealing gaps and reducing uncontrolled air leakage
- Insulation – in walls, ceilings, and floors to slow heat flow
- Windows and glazing – to manage heat loss and gain through joinery and glazing
These elements have roughly equal impact on a building’s energy performance and all three must work together. But by removing airtightness from the equation, H1 overloads the burden onto insulation and glazing.
This forces over-specification; R8 ceilings, expensive thermal break joinery, and excessive window performance all in an effort to make up for something that could be fixed more affordably by simply sealing the gaps.
Worse still, it creates unintended performance issues:
- Overheating in summer due to over specified ceilings
- Moisture and mould from trapped air
- High build costs without real-world comfort gains
Removing Clause D.1.13.1 would restore balance to the code. It would let airtightness, insulation, and glazing all play their role and allow designers to choose the most cost-effective, practical path to compliance.
Source: Based on IECC data from NEHERS presentation, ANSI/RESNET Conference, November 2023 and ENERGY STAR publications.
Air leakage accounts for between 25 percent and 40 percent of the energy used for heating and cooling in a typical residence.
The data alongside is from BRANZ Bulletin 698 – Residential Mechanical Ventilation Systems.
What BRANZ Research Tells Us
BRANZ tested the energy impact of airtightness in a typical Wellington home: Improving airtightness from 9 to 1 ACH50 reduced heating energy use by ~50% Adding mechanical ventilation with heat recovery (MVHR) boosted savings to ~90%
This aligns with international research from ENERGY STAR and RESNET in the US, which consistently shows that air leakage is one of the biggest energy losses in homes.
And yet, New Zealand’s code assumes every house leaks the same, no matter how well it’s sealed.
“We are still seeing mould and damage in the wall cavity even after the exterior is made watertight. You need to work just as hard to keep moisture travelling into the wall cavity from the inside – and that’s where envelope sealing comes in.”
MATTHEW CUTLER-WELSH, NZ GREEN BUILDING COUNCIL
Airtightness and ventilation
Airtightness and ventilation work together as an integrated system, enabling buildings to breathe in a controlled, energy-efficient way. This combination improves comfort, manages moisture, protects building durability, and ensures healthy indoor air — all without relying on occupants to open windows in foul weather or when they are not at home.
Why Airtightness Matters for Health and Durability Poor airtightness allows warm, moist air to enter wall cavities, where it condenses on cooler surfaces. This trapped moisture can lead to mould growth, timber rot, structural damage, and increased risks of respiratory illnesses such as asthma.
New Zealand has some of the worst respiratory health statistics in the OECD, largely due to cold, damp, and draughty homes. Simply adding wall insulation won’t fix this problem—preventing moist air from entering wall cavities through airtightness is essential.
The Health Cost of Draughty, Damp Homes
48% of New Zealand homes suffer from mould
Potential Energy Use Reduction – 73%
Potential Energy Use Reduction – 100%
Global Practice, Local Roadblock
Airtightness is standard in energy modelling in most developed countries:
- UK, US and Europe all require airtightness testing to count in compliance
- Builders are rewarded for achieving high performance
- Design teams can trade off insulation or glazing where appropriate
New Zealand is the outlier. Clause D.1.13.1 actively blocks this practice despite the fact that blower door testing is now widely available, affordable, and already in use by quality builders.
Builders Are Feeling the Pressure
The H1 changes were meant to support smarter, more energy-efficient building, but with airtightness removed from the equation, many builders are facing:
- Overdesign and higher material costs
- Moisture and overheating issues
- No recognition for high-performance detailing
Allowing airtightness in modelling would:
- Reduce excessive insulation where it can cause performance problems
- Allow standard joinery in more homes
- Reduce heating loads and system costs
- Improve building durability and reduce call-backs
The fix is simple
All we’re asking is to remove the restriction in Clause D.1.13.1 a low-risk, zero-cost change that simply allows airtightness to be modelled when tested.
- No new regulations
- No mandatory targets
- Just permission to reflect what’s already being delivered on-site
“Once the basic joinery is taken care of, this technology allows the guys to dial in whatever level of airtightness they want, then confirm that they’ve met it. It’s that easy”
MATTHEW CUTLER-WELSH, NZ GREEN BUILDING COUNCIL
Supported Across the Sector
- BRANZ, EECA, NZGBC all back airtightness as a major energy lever
- Modellers already have the tools, they just need permission to use them
- Builders want to be able to use airtightness modelling to reduce costs
MBIE supports the idea in principle, but unless there is strong industry feedback, it may delay action until a future code cycle.
Let’s Get H1 Working Properly
H1 was meant to support energy-efficient design. Instead, it’s unintentionally forcing unnecessary cost and complexity while ignoring one of the most effective tools we have.
Removing Clause D.1.13.1 would:
- Bring H1 back into balance
- Cut unnecessary building costs
- Improve energy outcomes
- Support healthier, more durable homes
