A damp subfloor is the most common structural moisture problem in older Australian housing, and it is also one of the few a homeowner can genuinely fix themselves. This guide covers how to tell whether you have a problem, what actually causes it, how to size a system, what it costs done properly, and where the money is usually wasted.
We sell fans, so read the recommendations with that in mind. We have tried to be straight about the cases where a fan is the wrong answer — there are several, and they are near the top.
1. Do you actually have a subfloor problem?
Worth establishing before spending anything. The signals, roughly in order of how seriously to take them:
- A musty smell in rooms over the subfloor, strongest on still days or after rain, and worst near floor vents and skirting gaps.
- Cupping or springiness in timber floorboards, or boards that have lifted at the edges.
- Mould on the underside of floorboards, joists or bearers — white, grey or black bloom. This is the definitive one.
- Visibly damp or dark soil under the house that never dries out, or standing water after rain.
- Rust on metal stumps, ant caps, pipework or ducting under the house.
- Salt staining or damp patches on the bottom courses of brick piers or external walls.
If you want a number rather than an impression, a cheap hygrometer left under the house for a week will tell you more than any of the above. Sustained relative humidity above roughly 80% at the surface is where mould growth becomes likely, and timber above about 20% moisture content is where decay fungi become possible. A subfloor that sits in the 60s and dries out between rain events is doing its job.
Those two thresholds are widely used general guidance rather than a code requirement, and surface conditions in a cold corner can be considerably worse than a reading taken in the middle of the space. Treat them as a trigger to investigate, not a pass mark.
2. Fix the water before you fix the air
This is the step most often skipped, and skipping it is why some systems disappoint. A fan moves air. It does not stop water arriving. If liquid water is entering the subfloor faster than ventilation can carry the vapour away, the fan will run permanently and the space will stay wet.
Check these first, in this order:
| Leaking plumbing | A weeping supply line or a cracked waste pipe under the house will keep a subfloor saturated indefinitely. Look for a patch that is wetter than everywhere else. |
| Blocked or misdirected stormwater | Downpipes discharging at the base of a wall, or a blocked line backing up, put roof water straight into the subfloor. Common and cheap to fix. |
| Ground falling toward the house | Garden beds, paving or fill built up against the house over the years send surface water underneath. The ground should fall away from the building. |
| Air conditioner condensate | A split-system drain running under the house delivers litres a day in summer. Easily missed. |
| Bare, permanently damp soil | Ground moisture evaporating upward is a continuous vapour source. A polythene ground membrane over the soil cuts it substantially and is the highest-value thing you can do after drainage. |
If the subfloor is visibly wet rather than merely humid, deal with the source first. If it is humid, stale and slow to dry but not wet, ventilation is the correct tool and the rest of this guide applies.
3. Check your existing vents against the minimum
Most Australian houses on a suspended floor were built with passive wall vents. A large share of damp subfloors are simply houses whose vents have been painted over, rendered across, blocked by garden beds, covered by a deck or an extension, or were never adequate to begin with.
The National Construction Code sets minimum subfloor ventilation opening areas per metre of external wall. From NCC Housing Provisions Part 6.2:
| Climatic zone | Bare ground | With impervious membrane |
| Zone A | 2,000 mm² per metre of wall | 1,000 mm²/m |
| Zone B | 4,000 mm² per metre of wall | 2,000 mm²/m |
| Zone C | 6,000 mm² per metre of wall | 3,000 mm²/m |
Two things that table makes obvious. Laying a ground membrane halves the ventilation you need — that is how significant ground evaporation is. And the requirement is per metre of wall, so a long house needs proportionally more vent area, not a fixed number of vents.
The NCC also sets minimum ground clearance: 150 mm under the floor generally, and 400 mm where a termite inspection zone is required. A subfloor with less clearance than that has both a ventilation problem and an inspection problem.
The A/B/C zones here are the NCC's subfloor climatic zones, which are not the same as the energy-efficiency climate zones people usually mean. Check which zone your address falls in rather than assuming, and note these are new-build provisions — an older house is not required to comply retrospectively. They are still the best available benchmark for whether your passive ventilation is anywhere near adequate.
The cheapest fix in this entire guide is unblocking vents you already have, adding more passive vents, and clearing the soil and garden beds back from them. Do that before buying a fan. Plenty of subfloors need nothing else.
4. When passive ventilation is not enough
Passive ventilation depends on wind. On a still, humid day it does approximately nothing, and those are exactly the days the subfloor needs help. Mechanical ventilation is worth adding when:
- The subfloor has dead corners — areas enclosed by additions, infilled verandahs or internal dwarf walls, with no path for cross-flow.
- One or more sides are below ground or against a retaining wall, so vents are impossible on that elevation.
- The site is sheltered — close neighbours, fencing, or a hollow — so there is rarely enough wind to drive passive flow.
- You have already maximised passive vents and readings are still high.
- The ground is persistently damp for reasons you cannot economically fix, such as a high water table or a spring line.
The principle is simple: draw fresh air in on one side of the subfloor and exhaust stale, humid air out the other, so the whole void is swept rather than just the perimeter.
5. Fan or dehumidifier?
This comes up constantly, so here is the direct answer: in an ordinary vented subfloor, a dehumidifier is the wrong tool.
A dehumidifier works by removing moisture from a fixed body of air. That is effective in a sealed room. A subfloor is not a sealed room — it is deliberately open to outside air through vents, and it sits on damp ground that evaporates continuously. A dehumidifier placed there is trying to dry the atmosphere of New South Wales. It will run constantly, draw several hundred watts doing it, need its tank emptied or a condensate line run, and it will not win.
A fan takes the opposite approach: rather than removing moisture from the air, it replaces the humid air with drier outside air. Against a continuous ground-moisture source, replacement beats extraction, and it does so at a fraction of the running cost.
| Ventilation fan | Dehumidifier | |
| Power draw | 10–90 W | Typically 200–600 W |
| Maintenance | Effectively none | Empty tank or plumb a drain; filters |
| Works in a vented space | Yes — that is the design intent | Poorly — fights unlimited outside air |
| Handles ground evaporation | Yes, by replacing the air | Only if the ground is sealed first |
The one case where a dehumidifier makes sense is a fully encapsulated subfloor — ground membrane sealed and taped, vents closed off, perimeter sealed — which is a deliberate and relatively expensive building strategy rather than something you retrofit with an appliance from a hardware store. If your subfloor still has open vents, ventilate it.
6. Solar or mains power?
Solar subfloor fans are marketed heavily, and the appeal is obvious: no wiring, no running cost. The trade-offs are less obvious.
| It runs on sunshine, not humidity | The fan is at full output on a bright dry afternoon and stopped on a still, overcast, humid day — which is precisely when the subfloor needs it. That is backwards. |
| Output is usually modest | A panel-powered fan is limited by what the panel delivers. Compare its rated m³/h against what your subfloor volume actually needs before assuming it is enough. |
| Harder to control | Running a humidistat means running the fan only when humidity is high. Most solar units simply run when the sun is up. |
| The saving is small | A mains EC fan on a humidistat costs roughly $10–$25 a quarter to run. Solar is not saving you a meaningful amount of money. |
Where solar genuinely wins is when there is no power available near the subfloor and getting an electrician to run a new circuit would cost more than the whole system. That is a real situation and a fair reason to choose it. If there is a usable outlet, mains with humidity control is the better system for less money.
7. How to size the system
Sizing works from the volume of the subfloor void and how many times an hour you want that volume replaced.
Step 2. Required airflow (m³/h) = volume × air changes per hour
Step 3. Divide by your derate factor to get the fan rating you need to buy
Worked example. A house with a 10 m × 12 m footprint and an average clearance of 0.5 m:
- Volume = 120 m² × 0.5 m = 60 m³
- At 10 air changes per hour = 60 × 10 = 600 m³/h required
- Allowing for ducting, bends and grilles at roughly 60% of the free-air rating: 600 ÷ 0.6 = 1,000 m³/h of rated fan
That points to one VIND 200 mm at 995 m³/h, or a pair of VIND 150 mm fans at 480 m³/h each, which is often the better layout because it lets you put one at each end of the house.
Both the 10 air changes per hour and the 60% derate are working rules of thumb, not standards. Published subfloor ACH recommendations vary, and real duct losses depend entirely on your run length, bend count and grille type — a short straight run will do considerably better than 60%, a long bendy one worse. If the numbers put you between two fan sizes, take the larger one and run it slower: it is quieter, draws less power at reduced speed, and lasts longer than a small fan held at maximum.
Our exhaust fan sizing calculator works through the same maths in more detail, including how much airflow each bend and each metre of duct costs you.
8. Where to put the fans
Layout matters more than raw airflow. The goal is cross-flow — air entering on one side and leaving on the other, sweeping the whole void.
- Exhaust from the dampest side, typically the shaded or south-facing elevation, or the side against a retaining wall.
- Draw intake from the driest side, usually the sunny, open elevation. Existing passive vents can serve as the intake — you do not necessarily need a second fan.
- Do not put intake and exhaust close together. The fan will short-circuit, recycling the same air and leaving the rest of the subfloor untouched. This is the most common layout error.
- Duct into dead corners. An enclosed section behind an addition will not ventilate on its own no matter how good the fan is. Run ducting to it.
- Discharge outside, away from openings. Not into another enclosed space, and not next to a window or an air intake.
- Hang the fan, do not bolt it hard to a joist. A rigidly mounted fan transmits vibration into the floor structure and you will hear it in the room above. Use the supplied straps.
Fit a backdraft damper on the exhaust so wind cannot blow back down the duct when the fan is off, and screen every opening against vermin.
9. Run it on humidity, not on a timer
A fan on a timer runs for a fixed period whether or not it is achieving anything — including through humid nights when it is pulling damper air in than the air already under the house.
A humidistat starts the fan when subfloor humidity crosses your threshold and stops it when it drops back. The fan runs a fraction of the hours, the noise is intermittent rather than constant, power use falls, and the fan lasts longer. On most installs this is the difference between a system that works and one that is merely running.
Options: the VIND Temperature & Humidity Controller at $79.95 adds it to any fan, direct-wired with no app or account. The AIRTITAN T8 and the T-series Cloudline fans include a controller and probe in the box, with WiFi and data logging if you want to see what the space is actually doing over a season.
Place the sensor probe in the problem area — the damp corner, not next to the vent where the air is already fresh.
10. What it costs
Published Sydney installation pricing from specialist installers puts a professionally installed subfloor ventilation system at roughly $2,500–$5,000 for an average home, rising above that for large houses and difficult access. Those figures include the assessment, the hardware, cutting and fitting vents, wiring and a workmanship warranty.
The hardware itself is a small fraction of that. Competitor DIY kits are commonly listed at $399–$549. Ours:
| 150 mm Subfloor Kit — Cloudline T6, Controller 69 Pro, 7.6 m ducting, clamps | $386.95 |
| 200 mm Subfloor Kit — Cloudline T8, Controller 69 Pro, 7.6 m ducting, clamps | $500.00 |
| AIRTITAN T8 — wall-mounted, IP55, controller included | $269.95 |
| VIND 150 mm Silenced — acoustic housing built in | $229.95 |
| VIND 200 mm Silenced — 995 m³/h | $329.95 |
The installed figures above are drawn from Sydney installers' own published price guides and are indicative only — they are not quotes, they vary with access difficulty and house size, and we have no commercial relationship with those installers. The honest comparison is not $400 against $4,000 for the same outcome: the difference buys you an on-site assessment, the labour, and someone else's warranty on the workmanship. Whether that is worth it depends on your access, your confidence and your time.
Running cost. A 38 W EC fan running eight hours a day uses roughly 27 kWh a quarter — on the order of $10 at typical Australian retail rates. A humidistat cuts that further. This is a genuinely cheap system to own.
11. What to buy for your situation
| Small cottage, simple access, one damp side | One AIRTITAN T8 through the wall on the damp elevation. Simplest possible install, controller included, 32 dBA. |
| Average house, want one order that covers everything | The 150 mm kit — fan, humidity controller, 7.6 m of ducting and clamps together. |
| Large footprint or long duct runs | The 200 mm kit, or a Cloudline PRO S8 at 746 Pa where the run is long or bendy. |
| Bedroom directly above the subfloor | A VIND silenced fan — the acoustic chamber is part of the fan, not an extra component in the duct. |
| Dead corners behind an addition | Any inline fan plus ducting run into the enclosed section, rather than a second wall fan. |
| Already own a fan, want it on humidity | The VIND Temperature & Humidity Controller, $79.95. |
Everything above is in the Quiet Home Ventilation & Exhaust Fans collection. We are in Punchbowl in Sydney, we hold the stock here and ship Australia-wide. If you send us your subfloor dimensions, clearance height and a note on which sides you can vent, we will tell you what we think you need — including if the answer is that your existing vents just need clearing.
FAQ
How many subfloor fans do I need?
Work it from volume rather than from a rule about house size. Floor area times average clearance gives volume; multiply by your target air changes per hour; divide by roughly 0.6 to allow for duct losses. Most average homes land on one 200 mm fan or two 150 mm fans. Two smaller fans at opposite ends often outperform one larger fan in the middle, because cross-flow matters more than total airflow.
Should the fan blow air in or suck it out?
Exhaust — pull air out of the dampest part of the subfloor and let fresh air draw in through existing vents on the opposite side. Exhausting puts the void under slight negative pressure, which stops damp subfloor air being pushed up through floor gaps into the house. Blowing in does the opposite.
Do I need an electrician?
Every fan we sell comes with an Australian three-pin plug and a lead, so if there is a suitable outlet within reach the work is carpentry — cutting the opening, running duct, weathering the outside. Running a new circuit or outlet, or hard-wiring a unit, is licensed electrical work in Australia and is not a DIY job.
Will a subfloor fan fix mould inside the house?
It fixes mould that is being caused by subfloor moisture, which is a common cause but not the only one. Mould in a bathroom with no exhaust fan, condensation on cold windows, or a roof leak are separate problems with separate fixes. If your mould is at floor level, near skirtings, and worse in rooms over the subfloor, the subfloor is a likely contributor. If it is on ceilings or around windows, look elsewhere first.
How long before I see a difference?
Air humidity in the subfloor responds within days. Timber that has absorbed moisture over years takes considerably longer to dry — weeks to months depending on how wet it got and the season. Put a hygrometer under the house before you start so you have a baseline to measure against, rather than relying on whether it smells better.
Will I hear it in the house?
Not if it is installed properly. The three things that make a subfloor fan audible upstairs are running it at full speed, bolting it rigidly to a joist so vibration transfers into the floor, and using bare foil ducting that carries noise along the run. Run a larger fan at a lower speed, hang it on the supplied straps, and put acoustic ducting on the first metre off the outlet. A VIND silenced fan handles the first part in the fan itself.
Is it worth laying plastic on the ground as well?
Yes — it is arguably the best value item in the whole exercise. The NCC halves the required ventilation opening area where an impervious ground membrane is present, which tells you how much of the moisture load is coming up out of the soil. Polythene sheet lapped, weighted and run up to the piers is inexpensive and makes whatever ventilation you have work considerably harder.
Do you install these?
No — we supply the equipment and ship Australia-wide from Sydney. The installs are well within reach of a competent DIYer or any general handyman or carpenter, and we are happy to talk through the layout and sizing with you or with your tradesperson before you buy.
