Subfloor mould in raised-floor Illawarra homes is most commonly attributed to inadequate ventilation, and while ventilation is important, it is only half the picture. In many cases, the primary driver of subfloor moisture is ground moisture rising as vapour from the soil beneath the building. In Wollongong’s coastal and Escarpment-adjacent suburbs, where the water table is relatively shallow after rain events and many sites have clay-rich soils with poor drainage, ground vapour is a major contributor to high subfloor relative humidity year-round. Managing it requires a vapour barrier strategy, not just ventilation. Our subfloor and roof void mould service addresses both. See our subfloor ventilation guide for the ventilation component.
Quick Answer (BLUF)
Ground-sourced moisture vapour rising into the subfloor is a major mould driver in Illawarra raised-floor homes. Installing a ground cover membrane (polyethylene vapour barrier) over the exposed soil reduces subfloor relative humidity significantly and reduces the load on subfloor ventilation. In many homes, this single intervention reduces subfloor mould recurrence dramatically.
How Ground Vapour Drives Subfloor Mould
Bare soil in a subfloor space is a continuous source of moisture vapour. Even soil that feels dry at the surface is releasing moisture as water evaporates from below the surface layer. In a typical Illawarra subfloor:
- Bare soil continuously emits moisture vapour into the enclosed subfloor space
- The subfloor air becomes humid (often exceeding 80-90% RH during rain periods)
- This humid air contacts the underside of the floor sheeting (particle board, plywood, or tongue-and-groove hardwood), timber joists, bearers, and piers
- Timber absorbs moisture, once moisture content of timber exceeds 16-19%, mould can establish; above 20%, timber decay (rot) begins
In Illawarra coastal suburbs, Thirroul, Bulli, Corrimal, Wollongong CBD, many homes are built on sites where the water table is within 0.5-1.5m of the surface during peak wet season. These sites emit enormous quantities of ground moisture, and ventilation alone cannot keep pace.
What a Vapour Barrier Is and How It Works
A subfloor vapour barrier (also called a ground cover membrane) is a sheet of plastic, typically polyethylene in thicknesses from 200 to 500 microns, laid over the exposed soil in the subfloor space. It physically prevents soil moisture from evaporating into the subfloor air.
How it reduces subfloor humidity: The membrane does not stop all moisture, it is a vapour barrier, not a perfect seal, but it dramatically reduces the rate of moisture evaporation from the ground. Studies and field measurements consistently show that a properly installed ground cover membrane can reduce subfloor relative humidity by 10-30 percentage points compared to bare soil in similar conditions.
This reduction is often enough to bring the subfloor from a mould-growth environment (above 70% RH) to a non-growth environment (below 60% RH), even with existing ventilation levels.
Vapour Barrier Specifications
Not all ground cover membranes are equal. For an Illawarra subfloor:
Minimum specification:
- 200 micron (0.2mm) polyethylene sheeting, the minimum for this application
- UV-stabilised if the subfloor receives any direct or indirect light
Better specification:
- 300-500 micron polyethylene, more durable, less likely to be punctured during any future subfloor access
- Reinforced or woven polyethylene (e.g., Sisalation 450 or similar), significantly higher puncture resistance
Coverage: The membrane should cover the entire soil area of the subfloor. This sounds obvious but in practice, a badly installed membrane with significant gaps or insufficient coverage at the edges provides much less benefit than a fully covered installation.
Lapping and edges:
- Sheets should overlap at joints by at least 300mm, taped with adhesive tape rated for polyethylene
- Edges should run up the face of piers and footings by at least 150mm and be secured, this prevents ground moisture wicking up the face of footings and bypassing the membrane
- The membrane does not need to be perfectly sealed (a subfloor is not a vapour-proof room), but minimising gaps at joints and edges significantly improves performance
Drainage provision: If the site has a history of water pooling in the subfloor after heavy rain, the ground should be graded before membrane installation to direct any pooled water toward a drainage point. The membrane on a flat wet site can trap pooled water beneath it, accelerating decay of the membrane itself.
Installation Considerations for Illawarra Subfloors
Access and site conditions: Many Illawarra subfloors are shallow, 300-600mm clearance is common. Installation requires working in a confined, often dusty space. PPE is essential: P2 respirator (there may be mould, asbestos-containing soil, or rodent droppings), knee pads, and coveralls.
Existing mould: If existing mould is present on timber framing, this must be treated before membrane installation, a membrane over untreated mould does not fix the biological problem. Professional subfloor mould treatment should be completed before membrane installation.
Pest management: Ground cover membranes can displace soil-dwelling insects. It is worth combining membrane installation with a pest inspection, particularly in older Illawarra homes where termite activity is a concern.
Is a Vapour Barrier Enough on Its Own?
A ground cover membrane works best in combination with adequate cross-ventilation. The ventilation provides air exchange that carries moisture-laden air out; the membrane reduces the rate at which new moisture enters the subfloor air. Neither alone is as effective as both together.
If ventilation is also inadequate (fewer than the AS 3600 minimum vents, blocked vents, no cross-flow pattern), adding ventilation openings at the same time as installing the membrane provides the best outcome.
FAQs
Can I install a subfloor vapour barrier myself?
For a simple rectangular subfloor with good access (400mm+ clearance), a DIY installation is feasible. The main risks are inadequate coverage, insufficient overlap at joints, and disturbing any mould or asbestos-containing material that may be in the subfloor. In an Illawarra home built before 1990, have an asbestos assessment done before entering the subfloor.
How long does a subfloor vapour barrier last?
A quality 300-500 micron polyethylene membrane in a subfloor with no standing water and reasonable access protection can last 20-30 years before needing replacement. Cheaper 200 micron material can degrade and become brittle, particularly if UV exposure is an issue, choose UV-stabilised material.
Will a ground cover membrane stop mould in my subfloor completely?
It significantly reduces mould risk by lowering humidity. Combined with adequate ventilation and treatment of any existing mould, most subfloors remain mould-free after a combined ventilation and vapour barrier upgrade. However, if there is a plumbing leak, drainage issue, or water ingress from a separate source, the membrane alone will not address it.
My subfloor already has concrete piers, do I need to go around them?
Yes, cut the membrane to fit around each pier or footing and turn the edge up the face of the footing by at least 150mm. This prevents capillary moisture movement up the concrete footing face bypassing the membrane. Tape the edges to the footing with appropriate adhesive tape.