Why New Zealand homes get damp and mouldy

How common it is

In the 2023 Census, about one in five New Zealanders (20.6%) said their home was damp at least some of the time. One in six (16.5%) lived with mould larger than a sheet of A4 paper. For children under 15 the figures were higher: 26% in damp homes, 21% with mould. Things are improving, since all of those figures fell from 2018, but it's still more than 850,000 people. (Environmental Health Intelligence New Zealand, EHINZ, from 2023 Census data)

Why it matters

EHINZ links damp and mouldy homes to coughing, wheezing and asthma, and to stress and anxiety, with children and vulnerable people most affected.

Where the moisture comes from

Much of it comes from everyday living. Showers, cooking, drying clothes inside and unflued gas heaters all put water into the air. When that air meets a cold surface, like a window or a poorly insulated wall, it turns back into water. If the room stays humid, mildew grows. (Marlborough District Council, mildew and condensation fact sheet)

The damp you can't see

Then there's water that gets into the walls. Government guidance on leaky buildings is blunt: “Damage is usually hidden and may not be obvious during a visual examination.” Between 1996 and 2004, untreated kiln-dried timber framing was common in outside walls, and it made the damage from leaks worse. (Weathertightness: Guide to the Diagnosis of Leaky Buildings, building.govt.nz)

What helps in any house

• Open windows a little, often; several open slightly beats one wide open.
• Use extractor fans for showers and cooking, and vent the dryer outside.
• Keep the house heated at a steady, low level, rather than in bursts.
• Unflued gas heaters add almost a litre of water to the air for every litre of fuel, so use them with ventilation or not at all.
• Insulate the ceiling, walls and floor where you can.

How we approached it

Mould can grow on almost any surface if a room stays humid, so heating and ventilation matter in every house, including ours. What a wall can do is give rot and decay nothing to feed on. Our walls have no timber in them: a steel frame, steel on both faces, and a core that barely takes up water.

Reviewed October 2026.