Subfloor Ventilation FAQ — Batemans Bay

A musty smell indoors is usually the first sign. If it's safe to get under the floor, look for mould or dark staining on bearers and joists, rusty nails and brackets, damp soil, and vents covered by garden beds, paving or decking. Standing water after heavy rain is a different problem: the water is getting in from somewhere, and vents or fans won't fix that on their own. A moisture meter reading on the timbers shows how damp the subfloor really is.

There is no set price, because it depends on what the subfloor needs. Clearing or replacing vents sits at the low end. One mechanical fan is a step up, and a system of several fans with ducting for a bigger house is a step beyond that. A moisture barrier over bare soil is priced by the square metre. If water is getting under the house, drainage adds to the job, and on low-lying land the digging can trigger Eurobodalla Council's acid sulfate soil rules, which may mean soil testing and a management plan before work starts.

Humid air doesn't stop a fan working, but it means more air has to pass under the floor to carry the same amount of moisture away. Bureau of Meteorology figures for its Batemans Bay (Catalina Country Club) station put mean 9am relative humidity at 79% across the year and 85 to 86% from May to July (1991–2010). A fan keeps air moving on still days, when vents alone do little. It has to be sized for the space, with clear vents letting fresh air in, and a humidistat or timer saves anyone having to remember the switch.

It depends on the work. New vents should meet the National Construction Code's cross-ventilation requirements for suspended timber floors. Digging is different. Eurobodalla Council says acid sulfate soils occur in low-lying areas near the coast less than 5 m above mean sea level. Depending on the land's class (1 to 5 on the acid sulfate soil maps) and the type of work, digging there may need development consent. If work is likely to disturb those soils and triggers clause 6.3 of the Eurobodalla Local Environmental Plan 2012, Council says a qualified environmental scientist must test the soil and a management plan must be prepared before consent is granted.

If your home is in or near one of the seven catchments in Eurobodalla Council's Batemans Bay Urban Creeks Flood Study, look under the house after heavy rain. They are Maloneys Lagoon, Long Beach Lagoon, Surfside Creek, the Batemans Bay Water Garden, Hanging Rock Creek, Joes Creek and Short Beach Creek. The study looks at flooding from rainfall and allows for projected sea-level rise. Council lists it as adopted; before major subfloor or floor-level work near them, check Council's flood information. As a baseline, look under the house every couple of years, and after any work that raises garden beds, adds paving or builds a deck against the walls, because vents sit low and are easy to cover.

It can make both more likely. Subterranean termites need moisture, and so do the fungi that rot timber, so a damp, still subfloor suits both. Wet timber also softens, and damage is harder to spot in a dark crawl space. Getting air moving and keeping the space clear enough to inspect helps protect the bearers, joists and floorboards. If you find soft or crumbling timber, have it looked at before any ventilation work, because airflow won't repair timber that is already damaged.