Bathroom Ventilation Guide: Exhaust Fans, Mould Prevention & Ducting

Bathroom ventilation is one of the most important but least glamorous aspects of bathroom design. Without proper ventilation, moisture from showers and baths accumulates, leading to mould, mildew, paint damage, timber swelling and even structural issues over time. Planning effective ventilation during construction prevents costly problems later.

Building Code Requirements

Australian building codes require mechanical ventilation (an exhaust fan) in bathrooms that don't have operable windows, or where the window doesn't provide adequate ventilation. Even in bathrooms with windows, an exhaust fan is strongly recommended — windows alone are rarely sufficient to remove shower steam quickly, particularly in winter when windows are closed.

Exhaust Fan Types

• Ducted exhaust fan — vents moist air outside through ductwork; the most effective option; requires ducting through the roof or wall; recommended for all bathrooms
• Recirculating fan — filters and returns air; less effective; not recommended for bathrooms; sometimes used where ducting isn't possible
• Combo fan/light/heater — exhaust fan combined with a ceiling light and/or heat lamps; common in Australian bathrooms; convenient all-in-one unit
• Sensor-activated fan — turns on automatically when motion is detected; runs on a timer after you leave; ensures the fan always runs; highly recommended
• Continuous run fan — runs continuously at low speed; premium option for constant ventilation; typically used in high-end builds

Ducting

Ducted exhaust fans vent moist air outside, which is far more effective than recirculating fans. The duct runs from the fan in the ceiling through the roof cavity to a roof vent or eave vent. If your bathroom is on the ground floor of a two-storey home, the duct runs through the upper floor's wall cavity or ceiling to the roof. Ensure your builder includes ducted exhaust fans — not just ceiling fans that vent into the roof space.

• Roof venting — duct runs from fan to a roof cowl or tile vent; the most common and effective method
• Eave venting — duct runs to a vent in the eaves/soffit; used when roof access is difficult
• Wall venting — duct runs through a wall to an external vent; less common; used for ground-floor bathrooms
• Avoid venting into the roof space — moisture in the roof cavity can cause timber rot and mould

Mould Prevention

• Run the exhaust fan during and after every shower — for at least 5–10 minutes after
• Ensure the fan is adequately sized — measured in litres per second (L/s); bathrooms typically need 50–90 L/s
• Keep the bathroom door slightly open when not in use — allows air circulation
• Wipe down shower screens after showering — removes water that would otherwise evaporate slowly
• Use mould-resistant grout and silicone — particularly in showers and wet areas
• Consider a heated towel rail — helps dry towels quickly and adds gentle warmth

Heated Towel Rails

Heated towel rails serve a dual purpose — they warm and dry towels (preventing musty smells) and provide gentle background heat that helps reduce bathroom humidity. They're a relatively affordable upgrade ($200–$500) and require a power point planned at the electrical appointment. Many buyers consider them a worthwhile quality-of-life upgrade.

Frequently Asked Questions