When is mechanical ventilation required?

Mechanical ventilation is required under the NCC and AS 1668.2 for any space that cannot achieve natural ventilation. This includes rooms with no openable windows, bathrooms, toilets, commercial kitchens, car parks, server rooms, and any room deeper than about 2.5 times the ceiling height from an external wall. If natural ventilation cannot meet the minimum outdoor air rates, mechanical ventilation must be provided.

What is the NCC openable area requirement for natural ventilation?

For Class 1-4 buildings (houses and apartments), the NCC requires openable windows, doors, or vents with a total area of at least 5% of the floor area of the room being ventilated. For Class 5-9 buildings (offices, shops, factories), the openable area increases to 10% of the floor area. The opening must face directly to outdoor air.

Can I use natural ventilation for a commercial building?

Yes, but the requirements are stricter. Class 5-9 commercial buildings need openable area equal to at least 10% of the floor area, compared to 5% for residential. The opening must face outdoor air, not into another enclosed space. Rooms deeper than about 2.5 times the ceiling height from the window will struggle to get effective cross-ventilation, and mechanical ventilation may be needed.

What does AS 1668.2 require for mechanical ventilation?

AS 1668.2 sets minimum outdoor air rates for mechanically ventilated spaces. For offices, the rate is 10 L/s per person with a minimum of 0.35 L/s per square metre of floor area. The standard covers supply air, exhaust air, filtration, and system design for all building types. It also sets specific exhaust rates for toilets, kitchens, car parks, and other special-use spaces.

Natural vs Mechanical Ventilation: NCC Part F6 Rules

Section 1

Two Paths, Same Goal

The NCC requires all occupied spaces to have fresh air. You can provide it naturally through openable windows, or mechanically with fans and ducts. Natural ventilation costs nothing to run. Mechanical ventilation costs money but works in spaces without windows.

The NCC sets minimum requirements for both paths. You must meet one or the other. NCC Part F6 covers the rules for natural ventilation. AS 1668.2 covers mechanical ventilation. Part J6 covers energy efficiency for air-conditioning and ventilation systems. Every habitable room needs at least one of these paths to comply.

Section 2

Natural Ventilation

1

How It Works

Openable windows or doors let outdoor air into the room. Wind and temperature differences drive the airflow. No fans. No ducts. No energy use.

2

NCC Openable Area Requirements

For Class 1–4 buildings (houses, apartments, hotels), openable windows, doors, or vents must have a total area of at least 5% of the floor area of the room. (NCC 2025, F6D7)

For Class 5–9 buildings (offices, shops, factories), the openable area increases to 10% of the floor area. (NCC 2025, F6D7)

The opening must face directly to outdoor air. Not into another room. Not into an enclosed corridor. Not into a lightwell that does not meet minimum dimensions.

Borrowed ventilation is allowed. A room can ventilate through an adjoining room if the opening between them is at least 5% of the ventilated room's floor area, and the adjoining room's window covers 5% of the combined floor areas of both rooms.

3

Where It Works

Offices, retail shops, and residential units with external walls. Any room where you can put a window that opens to outside air. No running cost. No maintenance. No ductwork needed.

Natural ventilation is limited by building depth. As a practical guide, rooms more than about 2.5 times the ceiling height from an external wall will not get effective cross-ventilation. For a typical 2.7 m ceiling, that is roughly 6–7 m from the window.

Section 3

Mechanical Ventilation

1

How It Works

Fans and ducts supply fresh outdoor air into the space and extract stale air out. The system is designed to AS 1668.2. A mechanical engineer sizes the fans, ductwork, and air intakes to meet the required outdoor air rates.

2

Outdoor Air Rates

AS 1668.2 Table A1 sets the minimum outdoor air rate for each space type. Key rates:

Offices: 10 L/s per person, with a minimum of 0.35 L/s per m² of floor area.

Retail: 10 L/s per person.

Classrooms: 12 L/s per person.

Gyms and nightclubs: 15 L/s per person.

The per-person rate and the per-area rate both apply. You use whichever gives the higher total outdoor air volume.

3

What It Costs

Mechanical ventilation uses energy. Fans run during occupied hours. The air may need heating or cooling before it enters the space. Filters need replacing. Ducts need cleaning. Budget for ongoing maintenance and electricity.

A mechanical engineer must design the system. This adds to your consultant fees, but it is required by the NCC for any space that relies on mechanical ventilation.

Section 4

When You Must Use Mechanical

Internal rooms with no external walls. No window means no natural ventilation. Mechanical is the only option. (NCC F6D7)
Bathrooms and toilets. Exhaust ventilation is required to remove moisture and odour. (AS 1668.2)
Commercial kitchens. Exhaust hoods must capture grease, heat, and smoke at source. (AS 1668.1)
Car parks. Enclosed car parks need mechanical ventilation for carbon monoxide control, unless the car park qualifies as open-deck under AS 1668.4. (AS 1668.2, Section 4)
Rooms deeper than about 2.5 times the ceiling height from an openable window. Natural ventilation cannot reach the back of the room effectively.
Server rooms and electrical switchrooms. These spaces generate heat and have no occupants opening windows. Mechanical cooling and ventilation are required.
Any space where the certifier determines natural ventilation is inadequate. If the building surveyor is not satisfied the openings provide enough air, mechanical ventilation must be provided.

Section 5

Combining Both

Many buildings use both systems. An office might have openable windows for the open-plan areas and mechanical exhaust for the toilets and kitchens. This is common and practical.

Mixed-mode systems go further. They switch between natural and mechanical ventilation depending on outdoor conditions. When the weather is mild, windows open and the mechanical system shuts down. When it is too hot, too cold, or too humid, the windows close and the mechanical system takes over.

The NCC allows mixed-mode if each mode meets its respective requirements independently. The natural ventilation path must comply with F6D7 on its own. The mechanical path must comply with AS 1668.2 on its own. You cannot rely on one to make up for a shortfall in the other.

Section 6

Who Needs to Know What

Section 7

References

National Construction Code 2022, Volume One, Part F6 — Light and ventilation (F6D7: Natural ventilation)
AS 1668.2:2012, The use of ventilation and air-conditioning in buildings — Part 2: Mechanical ventilation in buildings
National Construction Code 2022, Volume One, Part J6 — Air-conditioning and ventilation (energy efficiency requirements)
AS 1668.1:2015, The use of ventilation and air-conditioning in buildings — Part 1: Fire and smoke control in buildings
AS 1668.4:2012, The use of ventilation and air-conditioning in buildings — Part 4: Natural ventilation of buildings

Natural Ventilation vs Mechanical Ventilation: NCC Requirements

Two Paths, Same Goal

Natural Ventilation

How It Works

NCC Openable Area Requirements

Where It Works

Mechanical Ventilation

How It Works

Outdoor Air Rates

What It Costs

When You Must Use Mechanical

Combining Both

Who Needs to Know What

Need this engineered for your project?

References

Related design memos

Natural Ventilation NCC Compliance

Minimum Outdoor Air Rates for Commercial Offices

Demand Control Ventilation with CO2 Sensors