Openable area (ventilation)
Openable area is the minimum openable window or door area a habitable room needs for natural ventilation under the NCC, at least 5% of floor area on the DTS path.
Ask Chalkline about this →Openable area is the minimum area of openable windows or doors that a habitable room must have for natural ventilation under the NCC: at least 5% of the room’s floor area on the deemed-to-satisfy path. It is checked at the occupancy-certificate stage.
The rule is one of the simplest in the NCC and one of the easiest to trip on at the finish. A habitable room that is naturally ventilated (rather than mechanically ventilated) must have openable windows, doors, or other devices that open directly to outdoor air, with a combined openable area of at least 5% of the floor area of the room. “Openable” is the operative word: a large fixed picture window contributes nothing to this figure, only the part that actually opens counts.
There are a couple of traps:
- A room with lots of glazing but mostly fixed panes can still fail, because the openable portion is too small.
- Borrowing ventilation through another room, or ventilating to an enclosed space rather than outdoor air, generally does not satisfy the requirement.
Where natural ventilation cannot be achieved, the alternative is compliant mechanical ventilation.
For a builder the practical move is to check the openable area against floor area at the window-schedule stage, room by room, not at occupancy when it is expensive to fix. Make sure each habitable room’s openable sashes add up to at least 5% of its floor area, and watch rooms with large fixed glazing, walk-in robes converted to studies, and the like, which is exactly where a certifier picks up a shortfall.
Also known as: Ventilation openable area, 5% ventilation rule.
Category: Ventilation / NCC.
Related
See also
References
- NCC ventilation and drainage (Chalkline) (verified 2026-06-01)
Last updated: 2026-06-01. Verified: 2026-06-01. Quarterly review for currency.