Indoor air quality — IAQ — is one of those topics that gets discussed a lot and understood rarely. It tends to surface during specification discussions as a filter grade selection or a CO₂ sensor requirement, and it tends to disappear again once the mechanical system is designed.

But genuine indoor air quality is a whole-building outcome. It’s the result of decisions made about the building envelope, the ventilation strategy, the mechanical system, the materials used inside the building, and the way the building is occupied. Getting any one of these wrong can compromise the others — and in high-performance airtight buildings, the consequences are more acute than in leaky ones.

What IAQ Actually Means

Indoor air quality refers to the condition of the air inside a building as it relates to the health, comfort, and wellbeing of the occupants. It’s affected by:

• CO₂ concentration — a proxy for ventilation adequacy. Elevated CO₂ causes drowsiness, reduced concentration, and headaches. Most building codes target below 1000 ppm in occupied spaces.
• Volatile organic compounds (VOCs) — emitted by paints, adhesives, flooring, furniture, and cleaning products. Some are harmless; others are carcinogenic. In airtight buildings, they accumulate without adequate ventilation.
• Particulate matter (PM2.5 and PM10) — fine particles from outdoor air, combustion sources, and building materials. PM2.5 is particularly concerning as it penetrates deep into the lungs.
• Humidity — too low causes respiratory irritation; too high promotes mould growth and dust mite proliferation. The target range for occupied buildings is generally 40–60% RH.
• Temperature — directly affects comfort and productivity. Thermal comfort is an IAQ outcome even though it’s usually treated as an HVAC performance metric.
• Biological contaminants — mould spores, bacteria, and allergens that proliferate in buildings with inadequate ventilation or moisture control.

Why Airtight Buildings Change the Equation

A leaky building is self-ventilating to a degree. Air infiltrates through gaps in the envelope, diluting pollutants and refreshing the indoor air — inadvertently, inefficiently, and uncontrollably, but it happens.

A high-performance airtight building doesn’t have this accidental ventilation. Every litre of fresh air that enters the building enters through the mechanical ventilation system. Every pollutant generated inside the building stays inside until the ventilation system removes it. The system is no longer a comfort amenity — it’s a health system.

This means the ventilation design decisions that matter most in a high-performance home are:

• Fresh air delivery rate and distribution — ensuring adequate fresh air reaches all occupied zones
• Extract rates from wet areas — kitchens, bathrooms, and laundries are the primary sources of moisture and VOCs
• Filtration grade — what particle sizes the system captures before they reach the indoor air
• Heat recovery efficiency — how much of the energy in the exhaust air is recovered before it leaves the building

Filtration: What the Grades Actually Mean

Ventilation system filters are graded using the ISO 16890 standard, which classifies filters by their efficiency in capturing particles of different sizes. The most common grades you’ll see in high-performance residential ventilation are:

• ISO Coarse (G-class) — captures large particles. Used as pre-filters in commercial systems. Not appropriate as the primary filter in occupied residential spaces.
• ePM10 >50% — captures a significant proportion of particles between 1–10 microns. Acceptable for standard residential use in low-pollution environments.
• ePM2.5 >50% — captures a significant proportion of fine particles (PM2.5). Recommended for high-performance homes in areas with bushfire risk or urban air pollution.
• ePM1 >50% — captures a significant proportion of the finest particle category. Used in passive house and premium high-performance residential projects.

Filter selection should be matched to the local air quality environment and the sensitivity of the occupants. A household with asthma or allergy sufferers has different requirements than a standard family home.

IAQ and Building Materials

One of the most frequently overlooked sources of indoor air quality problems is the building itself. Paints, adhesives, sealants, flooring products, and furniture all emit VOCs — some intensely in the first months after installation, some for years.

In a high-performance airtight building, these emissions are concentrated by the lack of infiltration. The combination of a high-VOC building interior and inadequate ventilation is a reliable recipe for poor IAQ regardless of how good the mechanical system is.

The solution is to approach IAQ as a design brief — selecting materials with low VOC emissions, specifying adequate purge ventilation in the first weeks after handover, and designing the ventilation system around the actual internal load profile of the building.

How Air Theory Approaches IAQ

At Air Theory, indoor air quality is a design outcome we work toward from concept stage — not a specification checkbox at documentation stage. This means selecting filter grades based on local conditions and occupant needs, designing fresh air distribution to serve all occupied zones adequately, sizing extract rates correctly for wet areas, and specifying HRV or ERV systems that maintain humidity balance as well as heat recovery.

For Passive House projects, IAQ targets form part of the certification criteria — and our Passive House consulting add-on covers the full IAQ design brief. For high-performance residential projects outside the Passive House pathway, we apply the same rigour as a matter of course.

If IAQ is a priority on your project, get in touch.