Key takeaways
- Local exhaust removes moisture and contaminants at the source, but only if the duct runs continuously to outside air and terminates outside the building envelope.
- Ducting that discharges into a roof space or subfloor moves moisture from a wet room into a cold, poorly ventilated cavity where it condenses on structure.
- Flexible duct is the most common failure point because it is easy to install with crushed sections, long runs and multiple bends that strangle airflow.
- Terminating under a soffit, into a carport, or into a wall cavity leaves the moisture inside the building or pushes it into an adjoining space.
- The certifier and the homeowner both look at the termination, so the cost of getting it wrong surfaces at inspection or later as a moisture complaint.
Every wet room in a house generates moisture that has to leave the building. A rangehood, a bathroom exhaust fan, a laundry exhaust fan and a dryer duct all do the same job: they capture moist air at the source and push it out before it can spread through the house. That is local exhaust, and it is the most efficient form of moisture control you can install because it deals with the air while it is still concentrated.
The catch is that local exhaust only works if the duct actually terminates outside the building envelope. An exhaust fan connected to a duct that stops in the roof space is not a ventilation system. It is a machine that moves moisture from a wet room into the coldest, least ventilated cavity in the building, where it condenses on timber, insulation and metal. The fan runs, the homeowner hears it, and the moisture ends up exactly where you do not want it.
The roof ventilation and condensation risk in NSW article covers what happens in that roof space once moisture arrives. This article stays on the ducting itself: how to route it, what to terminate it with, and what to check before handover.
Why local exhaust is the first line of moisture control
Moisture in a home comes from cooking, showering, washing and drying. Each of those activities releases water vapour into the air in a specific room. If that vapour is not removed at the source, it drifts into the rest of the house, raises the internal humidity, and eventually finds a cold surface to condense on. The ABCB condensation in buildings handbook sets out this mechanism: moisture moves as vapour, condenses on cold surfaces, and the resulting liquid water drives mould, corrosion and timber decay.
Whole-home ventilation dilutes and replaces internal air across the whole dwelling. Local exhaust is different. It captures the moisture before it spreads, which means the volume of air it needs to move is far smaller and the energy cost is lower. A rangehood running during cooking removes the steam directly above the cooktop. A bathroom fan running during and after a shower removes the vapour before it leaves the bathroom.
That efficiency depends entirely on the duct. A fan rated to move a certain volume of air will only move that volume if the duct it discharges into allows it. The appliance manufacturer specifies a duct diameter and maximum run for a reason, and the installation has to respect those numbers. Where the specification is not to hand, check the manufacturer’s installation instructions or the current requirement before you commit to a route.
Where ducts go wrong on site
The most common failure is a duct that terminates inside the building. Flexible duct is run up into the roof space and left there because it is quick and no one checks. The fan discharges into the roof space, the moisture condenses on the underside of the roof lining or on the top plate, and the problem shows up as staining, mould or rusted fixings months later.
The second failure is a duct that terminates in the wrong place outside. Discharging under a soffit, into a carport, into a wall cavity or into an adjoining room does not remove the moisture from the building. It relocates it, often to a space that is even harder to ventilate. A duct that discharges into a subfloor is the same problem in a different location, and the subfloor ventilation for suspended timber floors article explains why that space is already vulnerable.
The third failure is a duct that is technically connected to outside air but is so restricted that almost no air moves. Flexible duct crushed behind a bulkhead, bent through a tight radius, or run for an excessive length will strangle the fan. The fan still runs and still draws current, but the airflow at the termination is negligible. The moisture stays in the room.
The mistake that costs the most. A duct that terminates in the roof space rather than outside the building. It passes a casual look, it is cheap to do, and it deposits moisture directly onto the structure above the ceiling. By the time it surfaces as staining or mould, the ceiling lining, insulation and framing may all need work, and the cost lands on the builder.
Duct material and diameter
Rigid duct and flexible duct behave differently, and the choice affects how much air actually reaches the outside. Rigid duct has a smooth internal bore, so air moves along it with less resistance. It also holds its shape, which means it cannot be accidentally crushed during the fit-off. The trade-off is that it needs more planning and more space, and bends have to be formed with fittings.
Flexible duct is faster to install and can be threaded through awkward spaces, but its corrugated bore adds resistance even when it is fully extended. If it is compressed, kinked or bent too tightly, the resistance rises sharply and the airflow drops. Flexible duct is acceptable for short, straight runs where it is fully extended and properly supported. It is a poor choice for long runs or for any installation where the fan is already working against resistance.
Duct diameter matters just as much. The appliance manufacturer specifies a diameter for the fan, and reducing it to fit a tight penetration increases resistance and reduces airflow. If the specified diameter will not fit through the available space, the answer is to change the route or the appliance, not to neck the duct down. The same applies to the termination: a cowl or cap that is smaller than the duct restricts the discharge and undoes the work of the run.
Rigid duct versus flexible duct
| Rigid duct | Flexible duct |
|---|---|
| Smooth internal bore, so airflow is not disrupted along the run. | Corrugated bore adds resistance, especially when compressed or bent. |
| Holds its shape and is harder to crush during installation. | Easily crushed against framing, services or ceiling lining. |
| Bends are formed with fittings, so the radius is controlled. | Bends are formed by curving the duct, so the radius can be too tight. |
| Requires more planning before framing is closed. | Faster to install and can be threaded through tight spaces. |
| Preferred where the run is long or the fan is working against resistance. | Acceptable for short, straight runs where it is fully extended and supported. |
Routing the duct through the building
Duct routing is a framing-stage decision, not a fit-off decision. Once the ceiling lining is up and the bulkheads are closed, your options shrink to whatever cavity happens to be left, and that is how ducts end up crushed or terminated in the roof space. Plan the run before the framing is closed so the route is short, direct and inside the building’s thermal envelope where possible.
Keep the run as short as the structure allows, because every metre of duct and every bend adds resistance. Use gentle bends rather than tight ones, and avoid multiple bends in quick succession. Where the duct passes through a cold space, insulation around the duct reduces the chance of condensation forming on the duct itself before the air reaches the outside.
Support the duct at intervals so it does not sag, and keep it clear of ceiling lining, insulation and other services. A duct resting on a ceiling battens or pressed against a sharp edge can be damaged during the fit-off or during later trade work. Where the duct penetrates a fire-rated element or a separating wall, the penetration must be detailed to maintain the rating, and that detail should be resolved before the lining goes on.
Routing a local exhaust duct
- 1Locate the applianceSet the rangehood or fan position so the duct run is as short and direct as the structure allows.
- 2Plan the runDecide the route through the building before framing is closed up, including any bulkhead or penetration.
- 3Select the ductMatch the duct diameter and material to the appliance manufacturer’s specification for the fan.
- 4Terminate outsideDischarge through an external wall or roof to outside air, clear of openings and with a suitable cowl or cap.
- 5Support and sealSupport the duct at intervals, keep bends gentle, and seal every penetration through the envelope.
- 6CommissionRun the fan and confirm airflow at the termination before handover.
Termination: the detail that decides whether it works
The termination is where the duct meets the outside. It has to be outside the building envelope, it has to be weatherproof, and it has to be clear of openings that would let the discharged air straight back in. A duct that terminates under a soffit or beside an openable window is discharging into a space that connects back to the building, so the moisture returns.
Terminations through an external wall are common for rangehoods and bathroom fans where the appliance is on an external wall. The duct passes through the wall, the penetration is sealed, and an external cowl or cap with a backdraft flap is fixed to the outside face. The cowl protects against wind-driven rain and stops air from being pushed back down the duct when the fan is off. The flap must be free to open and close, so it cannot be painted shut or blocked by render.
Terminations through the roof are used where an external wall is not available. The duct passes through the ceiling, through the roof space, and out through the roof covering with a flashing and a weatherproof cap. The flashing has to match the roof material and be installed so it does not rely on sealant alone. The duct must be continuous through the roof space; a duct that ends at the ceiling line and relies on the roof space to carry the air away is not a termination.
Every penetration through the envelope is a potential leak, so seal it properly. This is the same principle that applies to any service penetration, and it matters more here because the duct is carrying moist air under pressure. A poorly sealed penetration lets water into the wall or ceiling assembly and lets air leak out, which undermines the building’s thermal performance as well.
Commissioning and handover checks
A local exhaust system is only finished when it has been run and confirmed. Before handover, run each fan and confirm that air is actually moving at the termination. On a rangehood this may be as simple as holding a tissue near the external cowl and watching it move. On a bathroom or laundry fan, confirm the flap opens when the fan runs and closes when it stops.
Check the duct is continuous from the appliance to outside air with no open ends inside the building. Check flexible duct is fully extended, uncrushed and supported. Check the duct diameter matches the manufacturer’s specification and that the termination is not restricted. Check the penetration is sealed and the cowl is fixed and weatherproof.
These checks take minutes and they are the difference between a system that controls moisture and one that moves the problem into the roof space. They also protect you commercially, because a moisture complaint that traces back to a duct terminating inside the building is a defect you will be asked to fix.
Commissioning checks before handover
- Duct is continuous from the appliance to outside air with no open ends inside the building.
- Termination is outside the building envelope, not in a roof space, subfloor or wall cavity.
- Flexible duct is pulled tight with no crushed or kinked sections and bends are kept to a minimum.
- Duct diameter matches the appliance manufacturer’s specification.
- Duct is supported so it does not sag, rest on ceiling lining or sit against services.
- Penetration through the envelope is sealed and the external cowl or cap is fixed and weatherproof.
- Fan is running and airflow can be felt at the termination.
How this interacts with BASIX and whole-home ventilation
BASIX is the NSW planning framework that sets sustainability requirements for new dwellings and alterations, and it deals with water, energy and thermal comfort. Ventilation and moisture management interact with those requirements because the way a building is sealed and insulated affects how moisture behaves inside it. The BASIX requirements are set out on the NSW Planning Portal, and the specific commitments for a project are recorded in the certificate.
Local exhaust does not replace whole-home ventilation, and whole-home ventilation does not replace local exhaust. A heat recovery ventilation system or a whole-home exhaust system dilutes and replaces air across the dwelling, but it is not designed to capture a burst of steam directly above a cooktop. Local exhaust handles the source; whole-home ventilation handles the background. The heat pump dryer and laundry ventilation article covers the laundry side of that balance.
The connection to BASIX is that the building envelope and the ventilation strategy have to work together. A tightly sealed, well-insulated house holds moisture more effectively than a leaky one, which makes correctly terminated local exhaust more important, not less. If the ducting is wrong, the moisture has nowhere to go except into the structure.
Worked example: a rangehood duct in a new kitchen
A builder in western Sydney is fitting out a new kitchen in a dwelling on a sloping site. The cooktop sits on an island bench, away from the external wall. The kitchen has a raked ceiling with a bulkhead over the island, and the roof space above is insulated at ceiling level.
The quick option is to run flexible duct from the rangehood up into the bulkhead and leave it discharging into the roof space. It is fast, it needs no penetration through the roof covering, and no one will see it once the ceiling lining is fixed. It is also the option that fails. The moisture from cooking discharges into the roof space, condenses on the underside of the roof lining, and shows up as staining above the kitchen within a couple of years.
The correct option is to route the duct through the bulkhead, across the roof space and out through the roof covering with a flashing and a weatherproof cap. The run is longer than the manufacturer’s maximum for flexible duct, so the builder uses rigid duct with formed bends to keep the resistance down. The duct is insulated through the roof space because the air inside it is warm and moist and the roof space is cold. The penetration is flashed to match the roof material and sealed, and the cap is fixed clear of any opening.
Before handover, the builder runs the rangehood and confirms airflow at the cap. The flap opens when the fan runs and closes when it stops. The run is supported and clear of the ceiling insulation. The total cost is more than the quick option, but it is a fraction of the cost of rectifying a moisture-damaged roof space later.
Where to check the current requirements
The specifics of duct diameter, maximum run, termination height and clearance vary by appliance and by project. The appliance manufacturer’s installation instructions give the duct diameter and the maximum run for the fan you are installing, and those instructions are the first place to check. The ABCB condensation in buildings handbook explains the moisture mechanism and the design principles behind local exhaust. BASIX sets the broader sustainability requirements for the project, and the certificate records the commitments.
Where a figure genuinely matters and you do not have it, check the current requirement in the relevant document rather than working from memory or from what was done on the last job. Getting the ducting right is not complex, but it is a detail that is easy to lose once the lining goes on. Treat it as a framing-stage decision, terminate it outside the envelope, and confirm it works before handover.
If you want to keep your knowledge of moisture control, ventilation and the NCC current, the PointCPD guide to how NSW builder CPD works explains what is required and how to record it. The PointCPD course catalogue lists the courses available, including sustainability and moisture topics for licensed builders.
Questions builders ask
Can a rangehood duct terminate in the roof space?
No. A duct that ends in the roof space discharges moist air into a cold, poorly ventilated cavity where it condenses on the roof lining, insulation and framing. The duct must run continuously to outside air and terminate outside the building envelope with an appropriate cowl or cap. If the run cannot reach an external wall, it should be taken through the roof covering with a flashing and weatherproof cap.
What is the maximum length for a flexible duct run?
The maximum run depends on the appliance manufacturer’s specification for the fan, which also sets the duct diameter. Flexible duct adds more resistance than rigid duct because of its corrugated bore, and that resistance rises if the duct is compressed or bent tightly. Check the manufacturer’s installation instructions for the run and diameter that apply to the fan you are installing.
Why does my exhaust fan run but not clear the steam?
The most common cause is a restricted duct. Flexible duct that is crushed behind a bulkhead, bent through a tight radius, or run for an excessive length will strangle the airflow even though the fan is running. A termination that is undersized or blocked by a painted-shut flap will do the same. Confirm airflow at the termination rather than relying on the sound of the fan.
Where should a bathroom exhaust fan terminate?
It should terminate outside the building envelope, either through an external wall or through the roof covering. Terminating under a soffit, into a carport, into a wall cavity or into the subfloor leaves the moisture inside the building or pushes it into an adjoining space. The termination should be clear of openable windows and other openings so discharged air is not drawn straight back in.
Does local exhaust replace whole-home ventilation?
No. Local exhaust captures moisture and contaminants at the source, while whole-home ventilation dilutes and replaces air across the dwelling. The two work together: local exhaust handles the burst of steam from cooking or showering, and whole-home ventilation manages background humidity. A tightly sealed, well-insulated house needs both, because it holds moisture more effectively than a leaky one.
What should I check before handover on a local exhaust system?
Run each fan and confirm air is moving at the termination, and that the backdraft flap opens when the fan runs and closes when it stops. Check the duct is continuous from the appliance to outside air, fully extended, uncrushed and supported. Check the duct diameter matches the manufacturer’s specification and that every penetration through the envelope is sealed and the external cowl is fixed and weatherproof.
Sources
- WoodSolutions: timber and moisture · Forest and Wood Products Australia
- WoodSolutions: timber and moisture · Forest and Wood Products Australia
- BASIX · NSW Planning Portal
- Condensation in buildings handbook · Australian Building Codes Board
Published 13 August 2026. General information for builders, not legal, financial or engineering advice for a specific job. Check current requirements with the relevant regulator or standard before relying on them.
