Subfloor Ventilation FAQ — Shoalhaven

A musty smell inside the house is the usual first sign, often stronger after heavy rain or a flood. Under the house, look for pooling water, damp soil, dark staining on the bearers and joists, and vents that are blocked or less than half open to outside air. A moisture meter reading confirms it. Have the vents and subfloor checked every couple of years, and again after any flood, especially if your home is in an area covered by one of Shoalhaven City Council's floodplain risk management plans, such as those for Terara Village or Bomaderry Creek.

It depends on what the subfloor needs. Clearing or replacing blocked vents is the smallest job. A single mechanical fan costs more to install, and a multi-fan system with ducting for a larger or wetter subfloor costs more again. A moisture barrier over bare soil is priced by the square metre. If you are in Terara Village, the Riverview Road area or the Bomaderry Creek area, say so when you enquire, because Shoalhaven City Council's flood controls there can affect the job.

Usually, if the fan is sized for the space and the vents feeding it are clear. A fan pulling against blocked vents just stirs stale air around. A humidistat that switches the fan on as moisture rises saves anyone having to remember. On flood-prone land around Nowra, Shoalhaven City Council's DCP Chapter G9 requires electrical installations to sit above the flood planning level where its controls apply, so settle where the fan, its wiring and its controller will go before the job starts.

Clearing or replacing existing vents is maintenance, but ask Council before new openings are cut. New or replacement vents should meet the National Construction Code's cross-ventilation requirements for suspended timber floors. Bigger changes on flood-prone land are another matter. Shoalhaven City Council's DCP Chapter G9 doesn't require a hydraulic impact report for a building raised on piers that let floodwater pass through in a 1% AEP flood, a flood with a 1% chance of being reached or exceeded in any year. Closing in an open pier subfloor could change that, so check with Council before the piers are enclosed.

Check with Council first. On the lower Shoalhaven River floodplain, landowners need Shoalhaven City Council's written approval to modify the existing flood mitigation drains, and DCP Chapter G9 warns that enlarging them the wrong way can remove wetlands and expose acid sulfate soils. Where drainage work needs a development application and the site is in an area the NSW acid sulfate soil risk map shows as possibly containing those soils, Chapter G26 requires an acid sulfate soil management plan to be submitted with the application. Fans, vents and moisture barriers don't normally involve digging.

Moisture doesn't cause termites directly, but a damp, still subfloor is the kind of space termites and timber rot prefer, and wet timber is harder to inspect. Restoring airflow is about protecting the structure, not just the smell. If damage means bearers or joists need replacing on flood-prone land, note that where Shoalhaven City Council's DCP Chapter G9 applies, any part of a building below the flood planning level has to be built from flood-compatible materials. Ask what the replacement will be made of before the work starts.