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Airtightness and the Building Envelope
What Is the Building Envelope?
The building envelope is the protective shell that separates the inside of your home from the external environment. It includes the walls, roof, floor, windows, and doors, essentially everything that encloses the interior space.
A high-performing building envelope plays a critical role in maintaining thermal comfort, reducing energy use, and protecting the structure from moisture and pollutants. In New Zealand, Aero is the specialist when it comes to airtightness, offering expert assessment and solutions to improve the performance and integrity of the building envelope.
The Two Key Control Layers
Within the building envelope, there are two main control layers that perform distinct and complementary functions:
1. Weathertight Layer
This is the home’s outer defence against the elements. It protects the interior from wind, rain, snow, heat, cold, pollen, and other outdoor pollutants. Common components include roofing, cladding, windows, flashing and sealants’.
2. Airtight Layer
This internal layer keeps the inside air in—and the outside air out—unless deliberately ventilated. The interior lining includes the continuous layer formed by the walls, ceilings, floors, windows and doors. It:
- Reduces heat loss and energy bills
- Controls indoor moisture from daily activities like cooking and washing
- Protects against mould, mildew, and interstitial condensation
- Enhances indoor air quality and thermal comfort
The airtight layer can be located on the interior lining (e.g., wall linings and windows) or exterior lining (e.g., rigid air barriers), but in either case, its continuity is essential.
What Makes Up the Building Envelope?
The envelope comprises the major structural systems:
- Walls (framing, insulation, interior linings, cladding)
- Ceilings and Roof (roof structure, insulation, linings)
- Floors (particularly suspended timber floors)
- Windows and Doors (including their frames and seals)
Airtightness depends on how these elements are joined and how
penetrations are treated. That includes everything from windows, framing,
joinery, downlights and power points to plumbing, ducts, and cable runs.
“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
Improving airtightness means treating any part of the building where air could leak through. The main risk areas are at junctions between components (e.g., walls and floors) and around penetrations. Here are key areas to address:
Making the Building Envelope Airtight
Windows
Windows are a core part of the airtight layer in the building envelope—and they often play a bigger role in air leakage than people realise. Gaps around frames, sashes, and seals can allow significant heat loss and uncontrolled airflow if not addressed properly.
New homes often feature well-performing joinery, but airtightness still needs to be explicitly specified and tested to ensure performance.
Older windows, especially those with timber joinery, can warp over time and leak air. While timber has good insulation value, its airtightness is often poor unless upgraded or carefully maintained.
Improvement options include:
Retrofitting double glazing into existing timber or aluminium frames (if structurally sound)
Adding or replacing draught-stopping seals around the operable parts of windows
In some cases, opting for full window replacement may be the most effective solution
Addressing windows is essential for improving airtightness, energy efficiency, and indoor comfort—especially in older homes.
Downlights
Older recessed downlights are problematic:
They can’t be insulated over
They open into the roof space, allowing heat loss and moisture transfer
Replacing them with modern, IC-F rated sealed LED fittings improves airtightness and energy efficiency
Airsealing
Airtightness relies on sealing all the hidden gaps:
AeroBarrier is an advanced solution that pressurises the home and distributes a mist that seals gaps as small as 0.5 mm. It finds leaks you can’t see and seals them uniformly.
If AeroBarrier isn’t feasible due to budget or site conditions, manual sealing (using foams, gaskets, and tapes) can also yield reasonable gains, particularly when targeted at known leakage points.
The Energytight™ Retrofit Solution from Aero is a tailored process that helps you improve your home’s airtightness, comfort, andefficiency—without the disruption of a full rebuild or can be added as part of a renovation.
The Energytight™ Retrofit Approach
Step 1: Audit
We start with a professional Energytight™ audit, using blower door testing and diagnostics to measure your home’s current airtightness (ACH50) and identify the biggest sources of air leakage.
Step 2: Retrofit Options
Based on your home’s condition and budget, we recommend a mix of upgrades, typically including:
- AeroBarrier airsealing or manual sealing
- Ventilation with heat recovery (MVHR)
- Retrofit double glazing or window sealing
- Door draught-stoppers
- Ceiling and underfloor insulation
You choose which improvements to proceed with—and even small upgrades can make a big difference. These options can be a stand-alone retrofit or added as part of a renovation project. The option chosen will affect the cost and also the level of improvement that may be achieved.
Step 3: Verification
Every Energytight™ Retrofit is designed to deliver meaningful improvements in airtightness and energy performance, with results depending on the home’s condition and the upgrades selected. While Aero can’t guarantee a specific target like below 10 ACH50, substantial gains are typical. The key difference with Energytight™ is the independent final airtightness test, recorded in the Energytight™ Certificate. This verified result provides credible, third-party proof of performance—offering real value for homeowners and future buyers alike.
Airtightness and Ventilation Work as a System
Mechanical ventilation and airtightness are designed to work as a system—one simply doesn’t perform well without the other.
In a leaky home, air escapes through gaps—bypassing the ventilation system and wasting heating and cooling energy. In an airtight home, most air flows through the system, allowing it to filter air properly and recover heat efficiently. This means better air quality, lower energy use, and smaller, more effective systems.
Airtightness makes the whole home, work better.
Feel the Difference
A house needs to breathe—but it should do so in a controlled, consistent way, not just when the windows are open. In an airtight home with proper ventilation, fresh air can flow continuously without relying on open windows or losing valuable heat. This manages internal moisture, reducing condensation, musty smells, and the risk of hidden mould—creating a home that feels fresher, more stable in temperature, and noticeably more comfortable every day.
