Mould is a fungal growth that forms and spreads on damp or decaying organic matter. Outdoors, it plays a vital ecological role in breaking down dead plant material. Indoors, it is a building defect indicator, a health hazard, and — in severe cases — evidence of construction failure that can result in significant remediation costs and legal liability.
For builders and building professionals, understanding mould is about more than aesthetics. It’s about the conditions that allow it to grow, what those conditions tell you about a building, and how to design and build in ways that prevent it.
Why Mould Grows Indoors
Mould requires three things: moisture, a food source, and the right temperature. In buildings, the food source is almost always present — timber, gypsum board, insulation, and many other building materials support mould growth when wet. Temperature is rarely a barrier in most of New Zealand’s climate. That leaves moisture as the controllable variable.
Moisture gets into buildings in several ways:
- Water ingress: Rain penetrating through failed flashings, cladding, window seals, or roof details
- Rising damp: Groundwater wicking up through floors and lower walls that lack adequate damp-proof membranes
- Condensation: Warm, moisture-laden indoor air meeting cold surfaces — particularly windows, exterior walls, and in cold, unventilated roof spaces
- Plumbing leaks: Hidden leaks in walls and under floors that persist undetected
The Health Consequences
Mould produces spores that become airborne and are inhaled by occupants. The health effects range from mild (nasal irritation, watery eyes) to serious (respiratory conditions, exacerbation of asthma, immune responses in susceptible individuals). New Zealand has among the world’s highest rates of childhood asthma, and research has established a clear link between damp, mouldy housing and respiratory illness in children.
What This Means for Builders
The Healthy Homes Standards — which set minimum requirements for rental properties including moisture ingress and drainage — have raised the visibility of mould-related issues in the housing sector. For new builds, the Building Code’s requirements for weathertightness (E2) and moisture control are the primary standards.
The most important construction practice for preventing mould is correct installation of the building envelope — particularly flashings, waterproofing membranes, and ventilation systems. Cutting corners on these elements creates conditions for mould growth that may not manifest for months or years after completion. When it does appear, the trail leads back to the builder.