Subfloor Ventilation FAQ — Shoalhaven

A musty smell inside, particularly stronger after river flooding or heavy rain, is the clearest sign. Under the house, look for pooling water, dark staining on the bearers, and vents that are less than half open to outside air. A subfloor inspection with a moisture meter will confirm it.

A basic vent replacement or unblocking job might run $200-$500. A single mechanical fan installed is usually $800-$1,500, and a full multi-fan system for a larger or particularly damp home is typically $1,500-$3,000. A moisture barrier over exposed soil adds roughly $15-$25 per square metre.

Usually, yes, provided the fan is sized correctly and the vents feeding it aren't blocked. On flood-prone flats around Nowra, a humidistat-controlled fan that responds automatically to rising moisture tends to work better than one that relies on someone remembering to switch it on.

Replacing existing vents or adding extra vents in a wall generally doesn't need council approval, since it's maintenance rather than structural work. The National Construction Code sets minimum cross-ventilation requirements for suspended timber floors, which is the standard any new or replacement vents should meet.

Every couple of years as a baseline, and always after any river flooding or a particularly wet season, since ground moisture on the Shoalhaven flats can stay elevated for weeks and put extra strain on existing airflow.

Moisture doesn't cause termites directly, but a damp, poorly ventilated subfloor is exactly the environment they and timber rot prefer, and river-flat suburbs with elevated groundwater carry that risk more than most. Restoring airflow is a genuine part of protecting the structure.