Home ventilation: fixing damp, mould and roof-space heat

What ventilation involves

At the small end, it's a single unit installed: a bathroom exhaust fan ducted to outside, or a roof whirlybird. In the middle sits subfloor ventilation — ducted fans and vents under the house to clear damp, musty air. At the top is a whole-home ducted system that manages moisture, heat and air quality across the main rooms. Small jobs are priced per unit; systems are quoted on rooms and duct runs.

The right fix follows the problem. Condensation and mould usually point to exhaust and subfloor ventilation, while summer heat trapped in the roof is a job for roof ventilators or a heat-extraction system. Getting the diagnosis right matters more than the size of the system.

One rule underpins all of it: exhaust fans must duct to outside air. Venting into the roof cavity just moves the moisture problem somewhere you can't see it, where it does quiet damage over years.

How the cost works

An exhaust fan or roof vent installed sits at the low end, a subfloor ventilation system lands in the low-to-mid thousands, and a whole-home ducted system runs higher again. These are indicative bands; the estimate on this page adjusts for the system type, the number of outlets and access.

System type is the biggest lever — a single fan costs a fraction of a ducted multi-room system. Beyond that, each extra room adds ducting and an outlet, long duct runs and tight roof or subfloor spaces slow the install, and two-storey homes or metal roofs complicate venting to the outside. Fixing the moisture source first — leaks, drainage — is cheaper than paying ventilation to manage the symptom forever.

Choosing the right installer

Ventilation is a mixed trade for licensing. The ducting and passive vents are general work, but any hard-wired fan or system must be installed by a licensed electrician — that part is non-negotiable, so confirm who does the wiring before the job starts.

A good installer diagnoses before selling: they'll check where the moisture is actually coming from, insist that exhausts vent outside, and tell you when the real fix is stopping a leak rather than adding a fan. One who quotes a big system without asking about the source of the problem is selling hardware, not solving it.

Mistakes to avoid

Ventilation regrets are usually about treating the symptom, or venting moist air to the wrong place.

• Letting an exhaust fan vent into the roof cavity instead of outside air
• Paying for ventilation to manage moisture when a leak or drainage fix was the real problem
• Letting anyone but a licensed electrician install a hard-wired fan or system
• Over-buying a whole-home system when a couple of well-placed exhaust fans would do
• Ignoring the diagnosis and treating roof heat and bathroom damp as the same problem
• Forgetting that two-storey homes and metal roofs make venting outside more involved