Who is responsible for mould in a rental?

It depends on where the moisture is coming from, which is why this argument is so common. The landlord's side of the deal is a house that can be kept dry: weathertight, drained, insulated, with working extraction and adequate heating, the Healthy Homes hardware. The tenant's side is using the house reasonably: ventilating, heating it to a sensible degree, cleaning mould that appears in the normal course of living, and reporting problems early.

Signs it is the building

• Mould returns quickly after cleaning, in the same places, especially ceilings and wall corners.
• There is a leak, failed flashing, blocked gutter or wet subfloor anywhere near it.
• The bathroom or kitchen has no working extraction to the outside.
• Rooms are cold even when heated, suggesting missing insulation doing quiet damage.

Signs it is the living pattern

• Washing dried indoors through winter, unvented dryers, long showers with the fan off.
• Windows never opened, furniture hard against cold external walls.
• The mould is light surface growth that cleaning and airing keep away.

How to resolve it instead of arguing it

Investigate before assigning blame: check the hardware first, because a tenant cannot ventilate their way out of a leaking roof. Fix what is the building's, write down what was found and done, and if habits are the issue, put the request in writing kindly and specifically. If it still cannot be resolved, the Tenancy Tribunal decides these cases on exactly the kind of dated evidence a shared maintenance record produces.