A raised timber floor on piers or stumps depends on air moving under the house to carry away moisture rising from the ground. When perimeter vents are buried, painted over or closed off by later additions, damp air sits trapped under the floor and mould grows on joists, bearers and the underside of the boards. The fix is to restore airflow, deal with any water reaching the subfloor, and treat growth that has already taken hold; ventilation alone does not remove existing mould.
Why airflow under the floor matters
Much of the Illawarra’s older housing, from Corrimal through Woonona and Bulli to the coastal villages and older parts of Dapto, sits on a raised timber floor rather than a slab. Soil gives off moisture continuously, and more so on sloping blocks that take runoff from the escarpment. Vents around the base of the house let outside air sweep that moisture away. The Australian Government’s YourHome ventilation and airtightness guide is blunt about it: when sealing a home, do not block subfloor vents or wall cavities.
Slab-on-ground homes have no subfloor cavity, so their moisture problems show up instead as damp at the slab edge or condensation on walls; rising damp vs condensation covers those.
What blocks subfloor vents
Decades of sensible-looking changes tend to choke the airflow:
- Buried or built-over vents. Garden beds, retaining walls, paving and decks laid over openings nobody remembers were there.
- Painted or rendered shut. A repaint or render job carried straight across an old airbrick.
- Additions. Verandas, extensions, sunrooms and enclosed carports that closed a run of perimeter wall without new vents elsewhere.
- Draught-proofing. Some cottages relied on general leakiness; sealing gaps later removed that airflow.
- Vegetation and debris. Overgrown plants, leaf litter and stored items against the base of the house.
Water makes it worse. Downpipes discharging at the wall, gutters overflowing near vents, and soil that slopes towards the house all put water under the floor. Low, flat ground that stays wet after rain, such as pockets near creeks and lagoons around Corrimal or slow-draining clay soils around Dapto, keeps subfloor humidity up even where vents are clear.
Warning signs
Subfloor mould is usually smelled before it is seen:
- A persistent musty or earthy smell, worse after rain or when the house has been shut up
- Cupping, lifting or springy floorboards, or carpet that feels cool and damp
- Mould returning at skirting level or low on walls after cleaning
- Damp soil, standing water or white fungal growth visible from the access hatch
- A building or pre-purchase report noting subfloor damp or inadequate ventilation
If you are not sure where a smell is coming from, musty smell in the house walks through narrowing it down to subfloor, roof void or walls.
What fixing it involves
Once an inspection has confirmed the cause, the work usually combines:
- Clearing existing vents. Removing soil, paving, plants or debris from original openings. Often the single most useful step.
- Adding vents. Reinstating airflow lost to additions, ideally so air can cross the subfloor rather than pocket in one corner. Cutting new openings in brickwork is builder’s work.
- Dealing with water. Fixing downpipes and gutters, regrading soil away from the house, and clearing stormwater or subsoil drains (a licensed plumber or drainage contractor). NSW Health’s flood clean-up advice likewise recommends draining water away from under the house to increase airflow and help drying.
- Ground cover. A sealed vapour barrier laid over bare soil can cut the moisture coming off the ground in persistently damp subfloors.
- Mechanical subfloor ventilation. Fans that move air through the space where passive vents cannot, such as very low clearance, poor cross-flow or a block that stays wet. Wiring is a job for a licensed electrician.
- Treating established growth. Cleaning or removing mould on timbers, and removing mouldy debris and old insulation.
The subfloor and roof void mould service covers the treatment side once growth is established. Roof voids have a parallel problem: bathroom fans ducted into the ceiling space instead of outside push steam into the roof, which is a separate fix.
Can you clear vents yourself?
Often, yes. If a vent is simply buried under soil, mulch or leaf litter and there is no sign of mould or damp timber, clearing it costs nothing but time. Keep garden beds below vent level and well away from the wall. It becomes trade work when a vent has been rendered or bricked over, when timber is visibly mouldy or soft, or when the subfloor is too tight to see into properly. Do not crawl into a cramped, damp subfloor if you are unsure what is down there; an inspection will tell you.
Frequently asked questions
Can blocked vents cause mould without any leak?
Yes. Moisture rising from soil, still air and cold timber are enough to grow mould on joists and bearers over time, with no plumbing or roof fault involved. That is why some owners are surprised by subfloor mould in an apparently dry house.
How many subfloor vents does a house need?
It depends on the size of the subfloor, how enclosed it is and whether air can cross from one side to the other, so a generic count is not much use. An inspection checks whether the existing vents actually move air through the whole space.
When is mechanical ventilation needed instead of more vents?
When clearing and adding passive vents still leaves the subfloor damp, or when low clearance, poor cross-flow or slow-draining soil means natural airflow cannot keep up. A fan system is then added alongside treatment.
Will fixing ventilation remove mould that is already there?
No. Airflow stops conditions that let mould keep growing, but existing growth on timber, insulation and debris still has to be cleaned or removed. The two are done together, because treatment without airflow usually sees the mould return.
Should I get the subfloor checked before buying an older home?
It is worth it for older raised-floor houses, where blocked vents and subfloor damp are common and easy to miss in a walk-through. See pre-purchase mould inspection for what an independent check covers.