The practical answer
The assessment should define the enclosure, its use, the permanent openings, effective free area, cross-ventilation path and external obstructions.
Operational openings such as roller doors may help in practice but should not be relied upon casually if the ventilation basis depends on them always being available.
Where natural ventilation is not reliable for all zones or operating conditions, a mixed-mode or mechanical solution may be more defensible.
When this question usually matters
A warehouse or workshop has louvres and large doors but no mechanical outdoor-air system.
An office uses split-system air-conditioning and relies on windows for outdoor air.
A building alteration blocks or encloses part of an existing naturally ventilated space.
A car park relies on permanent perimeter openings.
A designer wants to avoid new mechanical ventilation by using façade openings.
Red flags that make the question more significant
The openings are only available when occupants choose to open them.
The nominal louvre size is used without considering free area.
Racking or partitions block the air path.
A nearby building now obstructs previously open façades.
The building use or occupancy has changed since the original approval.
How to screen a natural-ventilation strategy
Start with the air path and the permanence of the assumptions.
Define the enclosure and use
Identify the exact room or zone, occupancy and ventilation need.
Separate permanent from operational openings
Do not automatically count doors or windows that may remain closed.
Determine effective opening area
Consider screens, louvres and other restrictions.
Map cross-ventilation
Check entry, movement through the space and discharge, including internal obstructions.
Review external obstructions
Adjacent buildings and recessed areas can change the natural pathway.
Confirm the compliance pathway
Document the AS 1668.4 and NCC basis or use a performance solution where appropriate.
Detailed practical examples
These examples show how the question changes a real engineering decision. They are not universal answers; they show what needs to be clarified before the right answer becomes obvious.
Split-system cooling with windows that staff never open
A converted building uses split systems for cooling. The approval history assumes natural ventilation through openable windows, but road noise and security mean the windows stay shut.
Cooling is being provided, but the operational outdoor-air strategy may not match the natural-ventilation assumption.
What made the job difficult
- The windows are technically openable.
- Occupancy has increased.
- The split systems recirculate room air.
How the review should proceed
- Recover the approval basis.
- Assess the openings under AS 1668.4.
- Compare the assumed natural strategy with actual operation.
- Consider dedicated mechanical outdoor air if the natural pathway is not dependable.
The building may need a mechanical ventilation solution even though the cooling system performs well.
New internal room has no direct external openings
A warehouse with wall louvres adds an enclosed packing room in the middle of the floor plate.
The warehouse’s natural ventilation cannot automatically be assigned to a new enclosed room without demonstrating the transfer air path.
What made the job difficult
- The room is separated from external louvres.
- Doors may remain closed for noise control.
- More people work in the room than in the surrounding warehouse.
How the review should proceed
- Assess the new room as its own enclosure.
- Determine whether a valid permanent natural air path exists.
- Do not rely on occasional door opening.
- Provide mechanical ventilation if the natural pathway is not robust.
A mixed solution is often appropriate: naturally ventilated warehouse, mechanically ventilated internal room.
Information to gather before deciding
Building and room use
Plans showing permanent openings
Opening free-area data
Internal partitions and obstructions
Adjacent building geometry
Current occupant behaviour
Existing mechanical systems
NCC approval basis