Does opening a window help prevent mould?

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 (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 houses. If this is happening regularly, it creates the ideal conditions for mould to form.

Many older houses are built of lighter materials, so retain less heat. Even with the heater on it is likely things like insulation and double-glazing will be needed 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 water vapour out of houses. But ventilation isn’t as simple as keeping a window open in winter.

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

That sudden drop in temperature 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.

But in the afternoon the temperature outside is warmer, making it a good time to open windows.

Which is why Unitec school of architecture’s Professor Bin Su argues preventing mould depends on more than just open windows: insulation and heating also matter.

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