New homes should not have mould. But they often do, and in the Illawarra, where estate developments in Albion Park, Shell Cove, Calderwood, and the Dapto growth corridor are expanding the housing stock, newly-constructed homes are presenting with mould problems earlier than most owners expect. The causes are not mysterious: they are a combination of construction sequence errors, specification shortcuts, and design decisions that do not account for the coastal humidity this region regularly delivers. This guide explains the most common mistakes and how to avoid them, whether you are buying off-the-plan, building, or moving into a recently-completed home. See our mould prevention for coastal homes guide for ongoing maintenance advice.
Quick Answer (BLUF)
The most common causes of mould in new Illawarra builds are: construction moisture trapped in the building envelope before lock-up; insufficient mechanical ventilation in wet areas; plumbing defects in wet area membranes; and building envelope air leakage that allows humid outdoor air into wall and ceiling cavities. Most of these are preventable with correct specification and supervision.
Mistake 1, Construction Moisture Enclosed Before Drying
All new builds accumulate moisture during construction: concrete slabs emit moisture as they cure, framing timber absorbs moisture from rain exposure, and wet trades (plastering, screed) add significant water to the building. If the building is closed up and air-conditioned before this moisture has fully dissipated, that moisture has nowhere to go, it becomes elevated moisture content in the slab, framing, and plasterboard.
In the Illawarra, where construction programs are often compressed and builders push to practical completion, it is not uncommon for houses to be painted and carpeted while the slab is still curing and the framing moisture content is above the 15% threshold required for finishing.
What to check:
- If you are building, ask for moisture meter readings of the framing and slab before drywall is installed
- If you are buying a recently completed house, use a moisture meter in floor-adjacent walls and in lower-ground areas to check for elevated readings
- A new home with a musty smell, even slight, is a red flag
Mistake 2, Inadequate Bathroom and Kitchen Ventilation
The Building Code of Australia (NCC Volume One) requires mechanical exhaust ventilation in bathrooms and laundries. However, the minimum specification is often exactly that, minimum. A 90mm exhaust fan rated at 35L/s in a typical Wollongong bathroom is adequate for a mild climate; it is often insufficient for the sustained high humidity of a summer coastal day when the household runs two showers in succession.
Common specification mistakes:
- Undersized fans rated below the recommended 25 air changes per hour for wet areas
- Fans that duct into the ceiling space instead of directly outside (technically non-compliant, still done)
- Fans without timers, a fan that stops when the light turns off stops ventilating when the bathroom is most humid (immediately after a shower)
- Kitchen rangehoods that recirculate rather than exhaust, recirculating rangehoods remove grease but return all the moisture from cooking to the kitchen air
The correct specification for Illawarra conditions:
- 100mm minimum duct diameter, exhausting directly through the soffit or roof to exterior
- Timer-run fans rated for at least 30-40 air changes per hour in the wet area
- Consider continuous low-level exhaust ventilation (a low-speed continuous exhaust) in addition to the demand-run fan
Mistake 3, Wet Area Waterproofing Defects
Waterproofing membrane failures in shower recesses and bath surrounds are the leading cause of structural moisture problems in new homes. AS 3740 (Waterproofing of domestic wet areas) provides the specification, but defects in application are common:
- Membrane applied too thinly, below the 0.8-1.0mm wet film thickness requirement
- Upturns not carried to the required height (minimum 150mm above finished floor level)
- Membrane not continued to the underside of the hob and across the shower floor-to-wall junction
- Curing time not observed before tiling
- Membrane punctured by tile adhesive screed applied before full cure
A waterproofing membrane defect may not manifest for 12-18 months, until the first period of intensive use following occupancy. The first sign is usually mould on the wall adjacent to the shower or bath, or damp in the room below on a two-storey home.
Builder’s warranty note: In NSW, a statutory warranty for major defects (including waterproofing) in new residential construction runs for 6 years. If waterproofing fails within this period, it is a builder’s warranty claim, document with photographs and dates and contact NSW Fair Trading if the builder is unresponsive.
Mistake 4, Air Leakage Into the Building Envelope
Modern new builds are more airtight than their predecessors, but air sealing at penetrations, particularly around service penetrations (pipes, conduit, exhaust fans, recessed lights) through the building envelope, is still poorly executed in most residential builds.
Air that leaks into wall cavities from the exterior brings moisture with it. In coastal Illawarra conditions, this infiltration air is often near saturation point. When it meets the relatively cooler interior surface of the wall lining, it condenses. This process is invisible and accumulates over time into elevated moisture content in wall cavities and ceiling spaces.
Minimum air sealing measures for new builds in coastal NSW:
- Foam seal all service penetrations through top plates, bottom plates, and external wall framing before plasterboard installation
- Use an air-permeable but vapour-resistant building wrap on external wall framing before cladding
- Seal recessed light fittings with sealed LED downlights or airtight caps
Mistake 5, Inadequate Subfloor Ventilation in Raised Floor Sections
In split-level and partially-raised homes, common in the Illawarra’s sloped terrain in suburbs like Figtree, Keiraville, and Keira, the subfloor of raised sections must be adequately ventilated per AS 3600. Inadequate subfloor ventilation in coastal conditions leads directly to elevated humidity in the subfloor space, mould on the underside of floor sheeting, and timber bearer/joist decay.
See our subfloor ventilation guide for the specifics.
FAQs
I just moved into a new home and smell mould, what should I do?
Act quickly. Contact the builder in writing documenting the issue. Request a moisture assessment be carried out. Do not allow the builder to simply paint over any mould staining, this masks the problem and wastes your warranty period. An independent mould inspection gives you an objective report to use in any warranty discussion.
Does a new home warranty cover mould?
Major defects (structural) are covered for 6 years under the NSW Home Building Act 1989. Minor defects (finish and workmanship issues) are covered for 2 years. Waterproofing failure that causes mould is typically argued as a major defect with a 6-year warranty. Evidence of a specific defect (not owner neglect) is required.
How do I ensure a builder addresses moisture properly during construction?
Engage an independent building inspector at key stages, before concrete slab pour, before plasterboard installation (to check framing moisture), and before practical completion. An independent inspector has no commercial relationship with the builder and can document any moisture concerns objectively.
Do new builds need mould remediation?
Yes, occasionally. Mould growth in partially-completed buildings exposed to rain, or in buildings enclosed before construction moisture has dissipated, can require professional mould remediation even before the first occupant moves in.