The importance of airtight construction in passive houses
The passive house, or Passivhaus, standard is an internationally recognised benchmark for energy-efficient building. While it originated in Germany, its principles are increasingly relevant in Australia, where extreme weather events demand a higher level of performance from our homes.
While insulation, orientation and glazing all play crucial roles in passive house design in Australia, it’s the building envelope’s ability to prevent unwanted air leakage that separates a high-performance home from an ordinary one.
What is airtightness in passive house design?
Airtight construction refers to how well a building’s outer shell – its walls, roof, windows and floors – prevents air from escaping or entering unintentionally. In a passive design house, this is achieved through a carefully sealed building envelope that limits air leakage to less than 0.6 air changes per hour (ACH) at 50 Pascals of pressure, according to international passive house standards. This is typically 20 to 25 times tighter than a conventional Australian home.
This is not about sealing a house completely. Passive houses have ventilation that is controlled and managed by a mechanical system. Instead, it’s about preventing uncontrolled draughts and leaks through gaps, cracks and unsealed junctions in the structure.
Why is airtightness so critical?
Airtightness is one of the five core principles of the passive building design standard, alongside super-insulation, thermal-bridge-free construction, high-performance windows and a mechanical ventilation system with heat recovery (MVHR). If a building is not airtight, the entire system fails.
1. Superior energy efficiency
The most significant benefit of an airtight home is the massive reduction in energy demand. Uncontrolled air leakage allows cooled air to escape in summer and warm air to escape in winter. This forces air conditioning and heating systems to work harder, wasting energy and money.
By creating an airtight building envelope, a passive design house dramatically cuts heat loss and gain. This allows the home to maintain a consistent, comfortable temperature with minimal energy use – up to 90% less energy for heating and cooling compared to conventional homes.
2. Thermal comfort
Draughts are a major source of discomfort. Even with good insulation, a leaky home will have cold spots near windows, doors and walls where outside air infiltrates. An airtight structure eliminates these cold and hot spots, providing uniform thermal comfort across the entire living space, year-round. This makes designing a passive house an investment in a significantly better quality of life.
3. Excellent indoor air quality and health
The idea that an airtight house is ‘stuffy’ is a myth. In fact, a passive home building is far healthier. Because the building envelope is sealed, all the air entering the home comes through the MVHR system. This system continuously supplies fresh, filtered air, while removing stale, moisture-laden air. The MVHR filters out external pollutants such as dust, pollen, and, more critically in the Australian context, bushfire smoke and pollution. This ensures a clean, healthy indoor environment that is a vast improvement over standard buildings.
4. Moisture control and structural durability
Air leakage is a primary cause of moisture-related issues within the structure. Warm, moist air from inside can escape through gaps in the wall or roof and condense on cold surfaces within the building fabric, leading to mould, rot and structural degradation over time. An airtight home prevents this uncontrolled movement of moist air, protecting the insulation and ensuring the longevity of the structure. This attention to detail by passive house builders protects the long-term value and health of your home.
How to achieve airtight construction
Building an airtight home requires both design precision and quality workmanship. Here’s how experienced passive house builders in Australia achieve it:
- Continuous airtight layer: The airtight layer must be continuous throughout the building envelope, wrapping the house completely. It’s often made from membranes, tapes and sealants applied to internal surfaces.
- Attention to junctions and penetrations: Corners, window frames, pipe entries and electrical fittings are common leakage points. Skilled passive home builders carefully seal these areas using specialised materials.
- Testing and verification: A blower door test is performed during and after construction to measure air leakage. This ensures the home meets the strict passive house airtightness standard.
Airtightness should be considered from the very start of designing a passive house. Collaboration between architects, builders and energy consultants helps identify and solve potential weak points early in the process.
