Ventilation and air leakage

Two different thingsVentilation and leakage

Ventilation is fresh air you choose to let in: an open window, a rangehood, a bathroom fan, a ventilation system. It comes in where and when it's needed, and you can turn it off.

Leakage is air that finds its own way through gaps around doors, windows, downlights, skirtings and pipes. It comes in hardest on cold, windy nights, when you least want it, and it carries your heating out with it. It also goes wherever the gaps lead, which may not be where the moisture or smells are.

Older houses rely on leakage for fresh air, by accident. A modern house should be sealed tight and ventilated on purpose. That gives you more fresh air when you need it and less heat loss when you don't.

HealthWhy fresh air matters

Fresh air removes what a household puts into the indoor air:

• Moisture. A family of four adds 10 to 14 litres of water to the air every day. If it isn't removed, it condenses on cold surfaces and feeds mould, including inside walls and roofs. See Condensation and building breathability. Household dampness is linked to about 8% of childhood asthma in Australia.
• Cooking fumes. About 12% of childhood asthma in Australia has been attributed to cooking on gas stoves. A high-efficiency rangehood ducted outside could cut that share to around 3%. A recirculating rangehood, which filters and returns the air to the kitchen, does far less.
• Everything else: odours, the carbon dioxide we breathe out, and chemicals released by new furniture, paints and finishes.

In the ACT, new homes have been built all-electric since new gas connections were stopped on 8 December 2023, so gas cooking is now mainly an issue in existing houses. The rest still applies: extract moisture and cooking fumes at the source, and bring in fresh air to replace them.

The rulesWhat the code requires

The National Construction Code sets minimums for both sides of the equation.

Ventilation

• Habitable rooms need openable windows or other openings with a ventilating area of at least 5% of the room's floor area.
• Bathrooms, laundries and kitchens can be ventilated with exhaust fans instead. Since NCC 2022, continued in NCC 2025, these must generally discharge outdoors, not into the roof space. A bathroom fan without enough natural ventilation must switch on with the light and run for ten minutes after it goes off, and the room needs a path for replacement air, such as a gap under the door.

Sealing, for habitable rooms in climate zones 4 to 8, which includes Canberra:

• External doors need a draught seal at the bottom and seals around the edges. Openable windows need seals too, unless they meet the Australian Standard's air infiltration rating.
• Exhaust fans need a self-closing damper or similar, so they don't leak when switched off.
• Open fireplaces need a damper that closes the flue. Roof lights must be sealed or capable of being sealed.
• Ceilings, walls and floors must be built to minimise air leakage, with joints and penetrations sealed.

LeaksFinding and fixing them

The common leaks in a Canberra house:

• Downlights and ceiling penetrations
• Gaps where skirtings and cornices meet the walls
• Around window and door frames
• Plumbing and electrical penetrations, especially under sinks and behind appliances
• Exhaust fans without dampers, and open fireplaces
• Gaps between the floor and the subfloor in timber-floored houses
• Access hatches and attic stairs

A blower door test finds them. A fan in the front door depressurises the house, and a smoke pencil or thermal camera shows where air is coming in. The result is expressed in air changes per hour at 50 pascals (ACH50). The Putting design decisions together guide compares typical results. Older homes are often around 15, and Passive House requires 0.6.

On a new house, the best time to test is at lock-up, before the plasterboard goes on, while leaks are still easy to reach. On an existing house, a test before and after draught-proofing shows what the work achieved.

SummerCooling with the night air

Canberra's dry summers and cool nights are ideal for natural ventilation. Shut the house during a hot day, then open it in the evening to flush out the heat:

• Cross-ventilation. Windows on opposite or adjacent walls let air pass through a room rather than just in and out of one opening.
• Stack ventilation. A high window, clerestory or open stair lets warm air rise and escape while cooler air comes in low.
• Secure night openings. Windows that can be left safely open overnight, using awning windows with restrictors, lockable vents or security screens, get used. Windows that can't, don't.
• Insect screens on every opening that will be used at night.
• Thermal mass inside the house stores the night's coolness for the next day. The Thermal mass guide explains how.

MechanicalHeat-recovery ventilation

In a very tight house, opening windows in winter throws away heat. Mechanical ventilation with heat recovery (often called MVHR or HRV) solves this. A quiet fan unit draws stale air out of the kitchen, bathrooms and laundry and supplies fresh air to the bedrooms and living rooms. On the way, the outgoing air warms the incoming air through a heat exchanger. The best units recover up to about 95% of the heat.

The system also filters incoming air, which helps with pollen and bushfire smoke, and runs continuously at low speed, so the air stays fresh with the windows shut. It's essential in Passive House designs and worth considering in any very airtight home. In a house with ordinary airtightness, good exhaust fans, a ducted rangehood and openable windows are usually enough.

Plan the ductwork at design stage. It needs space in ceilings or bulkheads that is hard to find later.