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- Transforming An Old Cold House into Warm Desirable Home
How Adding Airtightness and Ventilation Unlocked a Warmer, Healthier, and More Energy-Efficient Home
The Challenge:
From Cold and Damp to Warm and Comfortable
This 1970s Kāinga Ora home was typical of New Zealand’s aging housing stock: hard to heat, draughty, affected by condensation and poor air quality. Like many similar homes, it was uncomfortable and unhealthy to live in.
The goal was clear—transform the house into a warm, dry, energy-efficient home that could meet Homestar 7 certification, setting a new benchmark that exceeds most new builds.
What made this renovation stand out was the decision to go beyond the standard approach by integrating airtightness and mechanical ventilation—delivering results far beyond what insulation alone can achieve.
The Strategy:
Include Airtightness and Ventilation in the Renovation
This was a full deep renovation: wall linings were removed, new insulation was installed in walls, ceilings, and floors, and double glazing was retrofitted.
What set this project apart was the decision to also prioritise airtightness and mechanical ventilation—a strategy supported by IECC and BRANZ research showing that air leakage is the biggest source of heat loss in homes.
Using AeroBarrier, the building envelope was sealed to reduce uncontrolled air leakage. A balanced heat recovery ventilation (MVHR) system was added to provide fresh, filtered air and control indoor humidity year-round.
ENERGY STAR
System % of impact
Air leakage accounts for between 25 percent and 40 percent of the energy used for heating and cooling in a typical residence.
The Air Leakage:
The Leading Cause of Heat Loss in Homes
Air leakage is the biggest source of heat loss in homes; more than walls, ceilings, or windows. Warm air escapes through gaps in the building envelope, bypassing insulation and reducing heating efficiency. IECC and ENERGY STAR data shows that ignoring airtightness can mean losing up to 40% of potential energy savings. At One Tree Hill, airtightness was made a central part of the strategy—delivering performance that exceeded expectations.
Source: Based on IECC data from NEHERS presentation, ANSI/RESNET Conference, November 2023
The Result:
Certified Performance, Measurable Gains
- Initial airtightness: 19 ACH50
- Final airtightness: 1.7 ACH50
- Homestar rating achieved: Homestar 7
- Construction completed: Within the school term by student apprentices
This outcome is not just impressive—it’s transformative. The Trade Academy apprentices delivered airtightness results better than most new homes in New Zealand.
“We’ve taken a 1970s Kāinga Ora home from 19 air changes down to just 1.7 ACH50—a level of performance well beyond most new homes. Absolutely amazing.”
Charlotte McKeon, Project Leader, One Tree Hill, Trade Academy
Energytight™ Retrofit – An alternative pathway
While the One Tree Hill project followed a full deep renovation, an alternative approach was modelled using the ECCHO system to assess a more affordable retrofit pathway. The goal was to compare traditional renovation methods with the Energytight™ Retrofit, which focuses on airtightness and ventilation without removing wall linings.
| System | Original State | Deep renovation | Energytight™ Retrofit | Traditional renovation |
|---|---|---|---|---|
| Wall insulation | None | R2.5 | None | R2.5 |
| Glazing | Timber single | T/B ali, IGU | T/B ali, IGU | T/B ali, IGU |
| Ceiling insulation | None | R4.0+R2.9 | R4.0+R2.9 | R4.0+R2.9 |
| Under floor insulation | None | R2.8 | R2.8 | R2.8 |
| MVHR | No MVHR | MVHR | MVHR | No MVHR |
| Airtightness | 19 ACH50  | 1.7 ACH50 | 1.7 ACH50 | 19 ACH50 |
| Heating (kWh/m2)/year | 291.6 | 30.6 | 78.7 | 75.6 |
Key: Original state systems / specification Upgraded system / specification T/B ali, IGU: Thermal break, Aluminium, double glazed MVHR: Mechanical ventilation, Heat Recovery (90%) ACH50: Level of building envelope airtightness
| Process | Traditional Renovation | Energytight™ Retrofit |
|---|---|---|
| Add ceiling and floor insulation |  |  |
| Remove wall linings |  | |
| Remove skirting and architrave |  | |
| Create wall cavity |  | |
| Install wall insulation |  | |
| Fix and stop new wall linings |  | |
| Fix skirting and architrave |  | |
| Remove windows |  | |
| Fit new frames and windows |  | |
| Retrofit new glazing in existing frames | |  |
| Airseal house | |  |
| Install ventilation system | |  |
| ECCHO Model: Heating (kWh/m2)/year | 78.7% | 75.6% |
A Lower-Cost, Simpler Pathway
ECCHO modelling shows that the Energytight™ Retrofit, which avoids removing wall linings and instead focuses on airtightness and mechanical ventilation offers a highly effective alternative to traditional renovations.
This approach delivers nearly the same level of energy efficiency as a full renovation, but with far less cost, disruption, and time. It’s an ideal solution for upgrading homes between tenancies, before moving in, or when a deep renovation isn’t feasible.
The Energytight™ Retrofit achieved nearly the same level of energy efficiency as the full traditional approach, but with significantly less cost, disruption, and time.
Energy-Efficient Homes Sell Faster—and for More
Energy efficiency has become a top priority for homebuyers—and it’s paying off. According to Domain’s latest report, homes marketed as “energy-efficient” sold for an average of $118,000 more in early 2025 and moved faster in the market. As living costs rise, buyers are actively seeking homes that promise lower energy bills and year-round comfort. Energy efficiency is no longer a nice-to-have—it’s a key driver of property value and buyer demand.
Key Benefits of Airtightness
Improving airtightness does more than reduce energy use—it enhances overall home performance in multiple ways:
- Lower Heating Demand → Less heat loss means smaller, more efficient heating systems and lower energy bills
- Healthier Indoor Air → Controlled ventilation delivers fresh, filtered air and helps eliminate condensation, mould, and dampness
- Greater Comfort → Consistent temperatures year-round—warm in winter, cool in summer
- Practical and Scalable → Easily implemented using proven methods, even by apprentices—not just specialist teams
Why It Matters:
A Blueprint for Retrofit Success in New Zealand
The One Tree Hill project proves that great results are feasible for old homes.
By prioritising airtightness and ventilation, this renovation achieved performance that rivals—or exceeds—most new builds.
This isn’t just a home that looks better—it performs better, feels better, and supports healthier living. The takeaway is clear:
- Deep Renovations: Don’t stop at insulation—without airtightness, up to 40% of energy savings may be lost.
- Energytight™ Retrofit: A faster, lower-cost alternative that delivers nearly the same performance as a traditional renovation, with far less disruption.
This project sets a new benchmark for what’s possible—and practical—in upgrading New Zealand’s existing homes.
