Exhaust systems and hidden moisture: kitchens, laundries, dryers and the ducts nobody checks

Every household makes moisture: showers, cooking, washing, drying clothes, even breathing. In a house built to modern energy standards, that moisture has few places to go except where we deliberately send it. The exhaust system is the way out.

Our article on bathroom exhaust fans covers the code for bathrooms. This one looks at the whole system, including kitchens, laundries and clothes dryers, and at the places where exhaust is most often done wrong and where the mistakes are hardest to correct. It is the second of three on condensation, alongside external walls and roof space ventilation.

What the code requires

For houses, clause 10.8.2 of NCC 2022 Housing Provisions Part 10.8 provides that:

• an exhaust system in a kitchen, bathroom, sanitary compartment or laundry must have a minimum flow rate of 25 L/s for a bathroom or sanitary compartment and 40 L/s for a kitchen or laundry;
• exhaust from a kitchen, kitchen range hood, bathroom, sanitary compartment or laundry must discharge directly, or via a shaft or duct, to outdoor air;
• a venting clothes dryer must also discharge directly, or via a shaft or duct, to outdoor air. The ABCB's explanatory notes say this does not apply to condensing dryers;
• a bathroom or sanitary compartment fan that is not run continuously, and where the room is not naturally ventilated, must be interlocked with the light switch and run on for 10 minutes after the light is turned off; and
• except for naturally ventilated rooms, the room needs make-up air: openings to an adjacent room with a free area of 14,000 mm², or a design in accordance with AS 1668.2. For a venting clothes dryer, make-up air must be provided in accordance with AS 1668.2.

Apartments are covered by Part F8 of NCC Volume One. These provisions are from NCC 2022, and the edition that applies depends on when the work was approved, so confirm with your certifier.

Why it matters more now

Houses that were once leaky dried out by accident. As the ABCB explains, exhausting air at the moisture source is one of the most effective condensation controls, and sealed buildings with poor ventilation have a higher condensation risk. A tightly built home with a weak or ineffective exhaust holds the moisture in, and it settles on the coldest surface it can find: ceilings, window reveals, external wall corners and the underside of the roof.

Where exhaust goes wrong, and what that does

The duct ends in the roof space

A fan that discharges into the ceiling cavity moves the moisture from the room to the roof space. There, it meets cold surfaces and condenses. The outcome is damp insulation, stained ceiling linings, rusting fixings and wet timber. It was once common practice and is now not compliant, but you can still find it in older houses and in poor-quality renovations. See roof space ventilation for what happens next.

The duct is long, kinked or crushed

Flexible duct is easy to install and easy to ruin. Long runs, tight bends and sections squashed under insulation or trusses cut the airflow dramatically. A fan marked 25 L/s on the box may deliver a fraction of it at the outlet. A fan with a high rating and a bad duct is a noisy fan that does little.

The duct sags and holds water

Where warm, moist air travels through a duct in a cold roof space, water can condense inside the duct. If the duct sags or runs uphill away from the fan, that water collects in the low point, and can drain back into the fan or leak through joints onto the ceiling below. Manufacturers' installation instructions commonly call for ducts to be supported, kept straight and sloped toward the outlet, and for insulation in unconditioned spaces. Check the instructions for the fan and duct being installed.

The outlet is wrong

The outside end of the duct matters as much as the fan:

• no outlet cover or a cover that lets in rain, birds or vermin;
• a termination under the eaves, close to an eave vent, so the moist air is drawn straight back into the roof space; and
• a roof termination without a proper flashing, which becomes a leak point (see roof plumbing, gutters and downpipes).

The controls do not work in real life

A fan on its own switch will be forgotten. A fan that stops the moment the light goes out leaves the steam behind. The code's interlock and 10-minute run-on exist for these reasons where the room is not naturally ventilated. Continuous low-rate fans with a boost are another solution.

There is nowhere for the replacement air to come from

Fans cannot move air out of a sealed room. A new, tight-fitting door with no gap, grille or undercut starves the fan, which then runs loud and moves little. The code's make-up air requirement is there for this reason.

Kitchen and laundry are forgotten

Range hoods that recirculate rather than exhaust, laundries with no fan, and venting dryers that blow into a cupboard or roof space all put moisture into the building. A hood that simply filters and returns air to the kitchen controls grease and smell, but it does not remove moisture.

Fans are shared, or tied into other systems

Connecting several fans into one duct, or into a ventilation system not designed for it, can reverse airflow or push moisture back into other rooms.

Why it is hard to fix later

• The duct is above the ceiling. Moving a termination, replacing a crushed duct or re-sloping a run means access through the roof space, or opening the ceiling.
• Evidence is out of sight. Water staining on a ceiling may be blamed on a roof leak when the real source is the bathroom fan's duct. Fixing the roof does not fix it.
• Mould feeds on what you cannot see. Damp insulation and the upper side of ceiling linings can be affected for a long time before it shows. See what a damp patch is telling you.
• Related finishes get damaged. The cost of an exhaust fault is rarely the fan. It is the ceilings, insulation, paint and sometimes timber that moisture damages over several seasons.
• Strata makes it slower. Exhaust ducts in apartments often run through shared shafts or the building's common property, where responsibilities, access and approval are more complicated. See renovating a strata apartment.

A checklist for owners and committees

Before the ceiling is closed, or when inspecting an existing house:

1. Where does each fan discharge? To outdoor air through a proper outlet, not to the roof space.
2. What is the duct? Rigid or well supported, short, with few bends, not crushed, sloped toward the outlet and insulated where required by the manufacturer.
3. Is the airflow adequate? The installer should confirm the fan, duct and outlet combination achieves the required flow rate, not just that the fan's label does.
4. Are the controls right? Interlock and run-on where required, or a continuous system.
5. Is there make-up air? A door undercut, grille or designed solution.
6. What about the kitchen and laundry? Range hoods that exhaust, and dryers that vent outside or are condensing models.
7. Is the outlet protected and flashed?
8. Is it recorded? Photographs of the ducts and terminations before the ceiling is lined, and product data in the handover record. See anatomy of a handover record.

Exhaust installation usually involves licensed electricians, and in some cases mechanical services contractors. Ask who is responsible for the duct and the airflow, not just the wiring.

Maintenance

Fans and ducts collect dust and grease. Clean grilles, filters and range hood filters regularly, and have the fan checked if it becomes noisy or the room stays steamy. Add it to your maintenance schedule.

Working with us

We plan exhaust ducts, outlets and make-up air as part of the design, and check and photograph them before the ceiling goes up. See How We Build, or get in touch to discuss your project.

Sources

• NCC 2022 Housing Provisions, Part 10.8 Condensation management, Australian Building Codes Board
• NCC 2022 Volume One, Part F8 Condensation management, Australian Building Codes Board
• Improved condensation management in NCC 2022, Australian Building Codes Board

This article is general information, not technical advice for your project. Requirements depend on the building class, the NCC edition and the products used, so confirm with your certifier and a licensed electrician or mechanical services contractor. Follow the manufacturer's installation instructions for fans and ducts. Full website disclaimer