Subfloor Ventilation FAQ — Forster

A musty smell is usually the first sign, often strongest when you open the house up after it has been shut for a while. Under the floor, look for pooling water or damp, dark ground, staining or soft spots on bearers and joists, and vents blocked by garden beds, paving or decking. Damp, still air also suits timber rot and termites. A tradesperson with a moisture meter can confirm how wet the timber is. After any paving, decking or garden work near the house, check that the vents are still clear.

It mostly depends on where the moisture is coming from. If vents are blocked or broken, clearing or replacing them is the smallest job. If the space needs more air than passive vents can move, fans add the cost of the units, any ducting and a power supply, and a multi-fan system costs more than a single fan. If the ground itself is wet, as it can be over the Tuncurry aquifer, a moisture barrier is priced by the square metre of ground it covers. A house that sits shut up between visits may suit fans on a timer or controller.

They can, but the humidity changes how the job is set up. Bureau of Meteorology records for the Forster-Tuncurry Marine Rescue station show mean relative humidity of 76% at 9am and 68% at 3pm, reaching 73-74% on January and February afternoons (1999-2010). Humid air carries away less moisture as it passes under a house, so passive vents alone have less to work with. Fans push more air through the space, and they do more good when the ground is dealt with too: a moisture barrier over exposed soil, and clear vents feeding the fans. A fan pulling against blocked vents just stirs stale air around.

On the foreshore, it can. The 2012 flood study review for the Wallis Lake foreshore, prepared by WMAWater and listed under MidCoast Council on the NSW Flood Data Portal, modelled floods dominated by ocean levels as well as floods dominated by rain runoff, and used whichever came out higher. So a water mark under a foreshore house may not match a local downpour. Vents and fans won't keep floodwater out, but they help the subfloor dry once the water drops.

It can. Great Lakes Council's 2014 Wallis Lake Estuary and Catchment Management Plan (Great Lakes Council merged into MidCoast Council in 2016) noted historic spear-point access to the Tuncurry aquifer on properties in the Tuncurry area. So under a suspended floor in Tuncurry, moisture can come up from the ground below, not only down with the rain. A moisture barrier over exposed ground slows that moisture, and vents or fans carry off what gets through.

Drainage that stops surface runoff reaching the subfloor is one thing. Pumping groundwater down is another. A 2003 report prepared by WetlandCare Australia for Great Lakes Council describes the Darawakh Creek and Frogalla Swamp wetland, about 10 km north of Tuncurry, as entirely underlain by acid sulfate soils. It says floodgates, drains and lift pumps lowered the groundwater enough to release severely acid groundwater, which drained seasonally into Darawakh Creek and on to the Wallamba River. The report calls this a major concern for local fisheries and the Wallis Lake oyster industry. On low-lying land, ask whether acid sulfate soils have been checked before any sump or deep trench goes in, and whether MidCoast Council needs to be involved.