As buildings get tighter, ventilation stops being something that happens by accident. NCC 2022 tightened the exhaust provisions for exactly that reason, and they are the easiest part of the condensation requirements to comply with.
An old, leaky, poorly insulated house ventilated itself. Air moved through gaps in the envelope continuously, carrying moisture out with it. It was energy inefficient and uncomfortable, but it rarely accumulated moisture.
Better insulation and reduced air leakage removed that accidental ventilation without necessarily replacing it. A household generates a significant moisture load every day through cooking, showering, laundry, and simply breathing. If that moisture is not removed, it accumulates, and it condenses on the coldest available surface.
This is why airtightness and ventilation are two halves of one strategy. Tightening the envelope without addressing ventilation moves the problem rather than solving it.
The exhaust provisions apply alongside the vapour permeance requirements as part of the condensation management package.
Range hoods must comply with the specified flow rates for kitchens.
Venting clothes dryers must exhaust directly to outdoor air. This applies to venting dryers specifically, not to condensing dryers, which do not discharge moist air.
Exhaust discharge. Many exhaust systems commonly used in homes must discharge to the outside of the building rather than into a roof space. Exhausting a bathroom fan into the roof cavity relocates the moisture rather than removing it, and roof spaces are exactly where you do not want it.
Make-up air. Rooms with exhaust systems that are not naturally ventilated need provision for replacement air. The code gives a worked example: an opening with a free area of 14,000 mm² can be achieved by a 20mm undercut on a 700mm wide door. Without make-up air, the exhaust fan cannot move its rated flow.
Roof space ventilation requirements apply, scaled by roof pitch, with specified free open areas at eaves and high level.
This is worth calling out because it is common in existing stock and still happens in new work.
A bathroom fan discharging into a roof space takes humid air from one part of the building and puts it into another part, usually a cooler one with cold surfaces and no drying capacity. In a subtropical climate the roof space is already humid. Adding shower moisture to it produces exactly the conditions mould needs.
The fix is ducting to outdoor air with a proper terminal, correctly sealed where it passes through the ceiling plane, which is also a control layer penetration.
Sealing penetrations and services
The code sets a minimum. In a well sealed building, particularly in a humid climate, mechanical ventilation is worth considering as a system rather than as a set of individual fans.
Balanced mechanical ventilation supplies fresh air and extracts stale air at matched rates, so the building is not pressurised or depressurised. Depressurisation matters in a humid climate because it draws outdoor air through whatever gaps exist in the envelope, bringing humidity with it.
Heat recovery adds a heat exchanger so outgoing air pre-conditions incoming air. In an Australian climate the energy case is modest compared to Europe, but the moisture and air quality case stands on its own.
Ventilation and envelope sealing are the same conversation.
If the building is not sealed, ventilation is uncontrolled and moisture moves wherever pressure differences take it. If the building is sealed and not ventilated, moisture accumulates. Both need addressing, and they need addressing together.
The practical sequence on a job is: seal the envelope properly, then size ventilation for the actual moisture load, then verify both. A blower door test tells you whether the first part worked.
