Originally published on 2 May, 2022 on Stuff.co.nz
By Dileepa Fonseka

Nearly one-third of houses may have a mould problem. Surely you should just open the window? Not so fast.

Mould is a big problem in New Zealand. Nearly one-third of houses are estimated to have an issue with it, but is the solution just as simple as opening a window?

In short: no — and sometimes this can even make it worse.

The main reasons mould is so prevalent in New Zealand are our cold and wet winters, combined with the light materials we traditionally use for housing.

To understand why opening a window may not work, we need to understand why mould forms in the first place.

The temperature of the air affects its ability to absorb moisture. Water takes on a gas form (known as a vapour) at higher temperatures, but condenses as a liquid when the atmosphere is cooler.

When we are indoors we produce a lot of water vapour just by breathing. When the temperature is high, the water vapour stays in that form, but not when it is cold.

At colder temperatures the water vapour condenses, gathering on hard surfaces inside our houses instead. If this is happening regularly, it creates the ideal conditions for mould to form.

Many older houses are built of lighter building materials, which means they retain less heat. Even with the heater on you will likely need things like insulation and double-glazing to keep the temperature at a high enough level to prevent water condensing on indoor surfaces.

That said, ventilation is still very important, because good ventilation can cycle all that water vapour out of your house.

But ventilation isn’t as simple as keeping a window open in the middle of winter.

Picture the typical Kiwi routine of leaving the heater running all night with the windows closed, then opening a window up early in the morning to ‘air’ the room.

That sudden drop in temperature dramatically reduces the air’s ability to absorb moisture, and can cause the water to gather on surfaces before it is cycled out of the house.

A similar principle might apply if you were to leave a window open during the night, thus dramatically reducing the temperature within your house.

But in the afternoon the temperature outside is warmer, making it a good time to open those windows and get some ventilation in.

All of which is why Unitec school of architecture’s Professor Bin Su argues preventing mould depends on more than just cracking open a window:  insulation and heating also matter. “It depends on the situation and the combination of those three things.”

Reporting disclosure statement: Unitec school of architecture’s Professor Bin Su provided expert advice for this post. It was reviewed by The Whole Truth: Te Māramatanga expert panel member Professor Philippa Howden-Chapman.

This story is part of a fact-checking project on the health and well-being of Kiwis.