Damp under the floor on the Nowra flats, a musty weatherboard at Berry, salt-humid air at Huskisson working into the boards: under a Shoalhaven house the air needs help. We clear and cut vents, fit fans where the breeze never reaches, lay a barrier over wet ground and check the timber while we are there. Tell us what the house is doing and where it is.
Get a Quote →A lot of the older weatherboard homes through Nowra, Berry and Shoalhaven Heads stand on a timber floor with a subfloor space underneath, and that space is built to ventilate itself: air in through vents on one side, across under the floor, out the other. Subfloor ventilation is making sure that still happens, so the moisture rising off the ground leaves with the air instead of settling on the timber.
The Shoalhaven River floodplain keeps the water table high across the flats around Nowra, and coastal pockets like Huskisson, Vincentia and Culburra Beach add salt-laden humidity. When vents get blocked by landscaping, extensions or old renovations, that ground moisture has nowhere to escape. It condenses under the floor, grows mould and, over time, softens the bearers and joists.
We do this work because the fix is simple and the damage is not. A blocked vent costs almost nothing to clear; a bearer that has rotted through on a river-flat block means jacks, new timber and a floor lifted. Most of what we see could have been a morning's work a few years earlier, so we would rather look under a house while the smell is the only symptom.
On the day we go under first: moisture meter on the timber, torch over the soil and piers, and a check of which vents still breathe. Blocked vents are dug out or cut back in, rusted grilles replaced, new openings made on the side the breeze arrives. A fan sized to the floor goes on the far wall to pull the air across, with the cable clipped clear of the ground to a switch or humidistat outside. Where the ground stays wet, heavy plastic goes down over it, overlapped and pinned. Expect a bit of dust and noise, then a low hum, and give the timber a few weeks to dry before you judge it. The chart below shows the path the air should take.
Multi-fan systems that pull moist air out and push fresh air through, wired to a discreet switch or humidistat. Cost depends on whether one fan covers the space or a larger Shoalhaven home needs a full multi-fan system, plus how much wiring and ducting the job involves.
Cutting in new brick or timber vents, replacing rusted or crushed metal vents, and unblocking vents covered by decking or garden beds. Priced per vent, supplied and fitted — a simple swap costs less than cutting a brand new opening.
Heavy-duty polythene sheeting over exposed subfloor soil, particularly useful on the low-lying river flats around Nowra and the coastal pockets near Culburra Beach. Priced per square metre of ground covered, so it scales with the size of the subfloor.
A crawl-through with a moisture meter and torch to check vent coverage, timber condition and mould before it becomes structural. There's usually a modest inspection fee, and many operators credit it back if you go ahead with the work, so it's worth asking when you book.
Prices are a general guide only — your exact quote comes after an under-house look.
There isn't one product called "subfloor ventilation" — there are a few different setups, and which one suits your place depends on how big the space is, how much air already moves through it, and how wet the ground underneath is. Here's what a contractor is choosing between when they get under your house.
No power, no fans — just openings in the base brickwork letting the breeze do the work. It's the cheapest fix, and it's the right one when the house simply hasn't got enough vents or the ones it had have been covered over. It only works if air can get in one side and out the other. If your subfloor is wide, boxed in, or the vents all face the same way, passive on its own usually won't shift it.
One fan pulling damp air out, with the existing vents letting fresh air in behind it. Suits smaller subfloors, or a house where the airflow is nearly there and just needs a nudge. Simplest powered option and the least to go wrong.
Fans pushing fresh air in on one side of the house and pulling stale air out on the other, so the whole space gets swept instead of just the bit near the vent. This is the setup for wider homes where the air in the middle sits still and never dries. Most multi-fan jobs end up being some version of this.
Flexible ducting run to the corners air never reaches — behind a bathroom addition, under an enclosed deck, or wherever piers and walls break the space into pockets. Usually added to a cross-flow system rather than used on its own.
A small solar panel runs the fan instead of mains power, so there's nothing on your electricity bill and often no switchboard work. The neat part is the timing — it runs hardest on hot, bright days, which is exactly when the air is driest and moisture actually evaporates. Some setups add a mains or battery backup to keep going through a run of grey weather.
A plain switch means you have to remember. A timer runs set hours. A humidistat is the set-and-forget option — it starts the fans only when humidity under the house climbs past a set point and stops them when it drops. Worth asking about, because it's the difference between a system you manage and one you forget about.
If you get more than one quote you'll hear different fan types named, and they aren't interchangeable. What suits your place depends mostly on whether the air has to travel down a duct or just move across an open space.
Propeller-style, the type usually fitted straight into a vent opening or through the base wall. They shift a lot of air cheaply, which is what you want across an open subfloor. What they don't do well is push air along a length of ducting — the resistance stops them.
Squirrel-cage style, sat in the ductwork rather than in the wall. They move less air than an axial for the same size, but they'll push it a long way against resistance — so they're the choice when air has to be carried to a far corner or around obstacles. Usually the quieter option too, since the unit sits away from the vent.
Fan and panel supplied as a package, with the panel mounted where it catches sun and the fan under the house. Runs on daylight rather than mains, and no ongoing cost. Sizing matters more here than with mains fans, because output rises and falls with the light.
Bureau of Meteorology records from the Nowra RAN Air Station put mean relative humidity at 66% at 9am and 55% at 3pm. At the same station February (133.9 mm) and March (139.2 mm) are the wettest months, with a second peak in June (102.3 mm), the month with the shortest days and the least light for a panel.
If you're leaning towards a solar fan, ask how it would cope with a wet June.
Most people notice it inside the house first and never connect it to what's underneath. Here's what to look for, upstairs and down.
You don't have to crawl under to learn a lot. Get down at a vent with a decent torch on a still morning and look for these.
Tape a square of clear plastic flat onto the soil under the house, sealed on all four edges, and leave it a couple of days. Water forming underneath the plastic means the ground is still giving off moisture and a vapour barrier is likely part of the answer. Water forming on top means the air down there is damp and needs shifting.
Chapter G9 of Council's Development Control Plan 2014 sets site-specific flood controls for 12 areas, among them Kangaroo River, Broughton Creek, Currambene and Moona Moona Creeks, St Georges Basin, and Conjola, Burrill and Tabourie lakes. If your place is on flood-prone land in one of them, run the torch along the piers and the base brickwork for a silt line or tide mark, and look for flood debris caught under the floor.
The NSW Government's estuary listing puts the Shoalhaven River's catchment at about 7,086 square kilometres, one of the larger on the South Coast, reaching inland from Nowra to grazing land around Braidwood. Bureau of Meteorology records for the Nowra RAN Air Station average about 1,030 mm of rain a year (2000–2026), but that station sits at 109 m on the naval air station, so the river flats may not match it.
Shoalhaven City Council has adopted floodplain risk management plans for Terara Village and the Riverview Road area (2002), the lower Shoalhaven River floodplain (2007), and Bomaderry Creek and Nowra and Browns Creeks (2016). Under Chapter G9 of its Development Control Plan 2014, habitable floors in infill development and reconstruction in Terara and the Riverview Road area must sit at least 0.5 m above the pre-levee 1-in-100-year flood level. For development in the Bomaderry Creek area, any part of a building below the flood planning level must be flood proofed, and properties with long-duration flooding need flood evacuation plans.
Airflow is what dries a subfloor after water gets in, and G9 reaches under the floor too. Where its flood control matrix applies, any part of a building below the flood planning level must be built from flood-compatible materials and electrical installations must sit above that level, which affects where a subfloor fan's wiring and controller go. On the lower river floodplain, landowners also need Council's written approval to modify existing flood mitigation drains, and G9 warns that enlarging them the wrong way can remove wetlands and expose acid sulfate soils.
Fans are sold in hardware stores and plenty of people have a go themselves. Sometimes that works. Here's what a contractor is actually being paid to get right.
Musty smells and mould on the walls don't always come from underneath. A leaking shower waste, a blocked downpipe, a roof leak tracking down a cavity, or rising damp in the brickwork all look much the same from inside. Ventilating a subfloor that was never the problem is money gone.
Two fans mounted the wrong way round will short-circuit — pulling air straight back out the vent beside them and leaving the middle of the house untouched. Where the fans go matters as much as how many there are, and that gets judged by looking at the actual space, not a floor plan.
A fan won't outrun water that keeps arriving. Ground sloping back toward the house, a downpipe emptying beside the footings, a garden bed built up over the vents, or a dripping pipe underneath all need sorting or the damp comes straight back.
Mains-powered fans are electrical work and need a licensed electrician. Solar setups avoid a good deal of that, which is part of why they suit older homes with tight switchboards.
Bearers and joists are what hold your floor up. Ventilation work is cheap next to replacing structural timber, and the gap between those two problems is usually a couple of wet seasons.
Termites want moisture as much as they want timber, and a damp subfloor is an invitation. Anyone going under with a torch and a moisture meter is looking at both at once.
If the fix means regrading soil or trenching a drain away from the footings, check first whether the block is on the Shoalhaven LEP 2014 Acid Sulfate Soils Map. Council's DCP 2014 Chapter G26 applies to mapped land and points to clause 7.1 of the LEP, and to clause 7.2 where earthworks are proposed. Where a development application is involved and the site sits in a probability area on the NSW acid sulfate soil risk map, an acid sulfate soil management plan has to go in with it, and it should follow the ASSMAC Guidelines (1998).
On flood-prone land, Chapter G9 doesn't require a hydraulic impact report for a building raised on piers that let floodwater through unobstructed in a 1% AEP flood (a 1-in-100 chance in any year). If your subfloor is open like that, ask how cladding or enclosing it would change that. Boxing it in cuts the airflow too.
Enquiries are welcome from Nowra, Terara, Berry, Shoalhaven Heads, Huskisson, Vincentia, Culburra Beach and the rest of the Shoalhaven.
Tell us what you need and where the property is.