“The building has windows, louvres and roller doors. Can we rely on natural ventilation instead of installing mechanical ventilation?”
Natural ventilation is often treated casually because the openings are visible. The difficult questions are whether the openings are permanent, sufficiently free and unobstructed, connected to the enclosure being assessed and valid under the chosen building approval pathway.
The practical answer
AS 1668.4:2024 covers natural ventilation of occupied enclosures and car parks or vehicle facilities. It provides prescriptive procedures and also guidance relevant to performance-solution development.
The assessment should consider free opening area, opening location, cross-ventilation, external obstructions, internal partitions and whether the openings remain available during normal building use.
A building alteration can invalidate an old natural-ventilation assumption by enclosing an area, blocking louvres, adding adjacent construction or changing the use and ventilation demand of the space.
Where this Standard fits
- Warehouses with permanent louvres
- Workshops and industrial buildings with large wall openings
- Naturally ventilated occupied rooms
- Car parks relying on permanent natural openings
- Existing buildings where openings have been blocked or altered
- Mixed-mode systems using both natural and mechanical ventilation
- Any room with an openable window is adequately ventilated
- Roller doors will always be open when ventilation is needed
- Gross louvre size equals effective free ventilation area
- Internal partitions have no effect on the natural air path
- A naturally ventilated car park can be assessed only by total façade opening area
- Natural ventilation eliminates the need to check the NCC pathway
Can the space genuinely rely on natural ventilation?
The key is the complete air path and the permanence of the assumptions, not just the existence of openings.
A good result needs the equipment definition, loads, design method, fabrication, inspection and operating limits to describe the same real system.
Key engineering concepts to understand
Permanent openings matter
A design assumption should remain valid without relying on someone remembering to open a door at the right time.
Free area is not façade area
Louvres, grilles, screens and geometry can reduce the effective opening available for ventilation.
Cross-ventilation is a path
Air must be able to enter, move through the enclosure and leave. Internal partitions and stored goods can break that path.
External obstructions matter
Nearby buildings, recessed levels and barriers can affect whether an opening arrangement performs as assumed.
Car parks have their own geometry
Natural ventilation of vehicle facilities depends on opening arrangement and equivalent ventilation, not a casual ‘open-sided’ description.
Mixed-mode design needs clear controls
Where fans supplement natural ventilation, the design should state when each mode is relied upon and how the changeover works.
Detailed engineering case studies
The examples below show how the Standard changes a real engineering decision. They are not clause summaries or universal answers; each case starts with the equipment, task and evidence available.
New racking and internal enclosure block an old louvre strategy
A warehouse has long wall louvres on two façades and has historically been described as naturally ventilated. A refurbishment adds high-bay racking and encloses a packing room inside the building.
AS 1668.4 requires the natural-ventilation path to be assessed as it exists after the alteration. The old façade opening area does not automatically ventilate the new enclosed room or deep zones behind racking.
What made the job difficult
- The original design calculations cannot be found.
- Racking reduces clear paths to one louvred wall.
- The new packing room has only internal openings to the warehouse.
- Heat and occupant loads have also increased.
How the engineering review should proceed
- Map the post-refurbishment enclosures rather than treating the warehouse as one volume.
- Determine the effective permanent openings serving each zone and whether cross-ventilation remains available.
- Check the new packing room separately and identify whether transfer through the warehouse can form a valid basis.
- Introduce mechanical ventilation where the natural pathway is no longer robust.
The likely result is a mixed solution: the main warehouse may retain a natural-ventilation pathway while the enclosed packing room receives mechanical ventilation. The refurbishment documents should state the boundary clearly.
Small office relies on openable windows that remain shut
A converted residential-style building is used as an office. The compliance narrative refers to natural ventilation through openable windows, but staff keep them shut because of traffic noise, security and air-conditioning operation.
The presence of windows does not by itself prove the operational ventilation strategy is dependable. The engineering review should distinguish an approval assumption from how the building is actually used.
What made the job difficult
- The windows are technically openable but rarely opened.
- Split-system air-conditioners provide cooling but no outdoor air.
- Traffic noise and security make natural ventilation unattractive.
- Occupancy has increased since the office opened.
How the engineering review should proceed
- Recover the building approval basis and determine whether natural ventilation was relied upon formally.
- Assess the opening arrangement and effective area, then compare that basis with actual operating behaviour.
- Do not count recirculating split-system cooling as outdoor-air ventilation.
- Consider a mechanical ventilation solution if the natural strategy is not realistically available during occupancy.
The practical answer may be that the original natural-ventilation basis exists on paper but is not a robust operational strategy for the current office. The remedy should address outdoor air separately from comfort cooling.
New adjacent development changes the opening context
An existing multi-level car park relies on natural ventilation through perimeter openings. A new building is constructed close to one side and the owner asks whether the original ventilation basis is still valid.
AS 1668.4 considers the arrangement of permanent openings and nearby obstructions. The original calculation may no longer describe the current external geometry.
What made the job difficult
- The closest façade was previously open to a large setback.
- The new building reduces the clear space in front of several openings.
- Some openings also have new security mesh.
- No post-development ventilation review was performed.
How the engineering review should proceed
- Recover the original natural-ventilation design basis and map the current geometry.
- Reassess affected openings using effective free area and obstruction conditions rather than gross dimensions.
- Check whether the remaining opening arrangement still provides the required equivalent ventilation.
- Consider mechanical assistance or other alterations if the natural pathway has been materially weakened.
The review should state whether the existing natural strategy remains valid after the neighbouring development. The conclusion may range from no material impact to targeted mechanical supplementation.
Common mistakes
A ventilation basis should not depend on behaviour that cannot be assured.
Screens, louvres and geometry can materially reduce effective free area.
Racking, partitions and enclosures can break the cross-ventilation path.
External obstruction conditions can change after the original building is completed.
Recirculating air-conditioning may control temperature without providing outdoor air.
Natural ventilation should be tied to the NCC and project compliance basis.
Information to gather before making the decision
Building classification and approval documents
Current space use and occupancy
Plans and elevations showing openings
Opening type and effective free area
Louvre and screen data
Internal partitions and racking layout
Adjacent buildings and external obstructions
Whether doors/windows remain available during occupancy
Existing mechanical ventilation systems
Car-park geometry where relevant
Recent alterations or neighbouring development
Measured conditions if a performance concern exists
What should happen next?
Related AS Applied content
Source basis & limitations
- The page does not reproduce opening-area formulae, coefficient tables or the Standard’s figures.
- AS 1668.2 is used as an interface where required ventilation rates or mechanical supplementation need to be investigated.
Need the Standard applied to real equipment?
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