The hidden mould risk when opening your windows

One of the most common mould prevention tips we hear is to open your windows to let outside air in to help evaporate moisture and prevent condensation. But an expert has warned this could be doing more harm than good.

Jenny Smith, an indoor air quality expert with 45 years in the HVAC (heating, ventilation and air conditioning) industry and the founder of Mould Consultancy, highlighted the problem in an Instagram Reel after spotting fungal spores on security fly screens while doing an assessment at a client’s home.

“All [those contaminants] would go into her house and all over that child’s bedroom and then that child would get sick. They’d close the windows, they’d go to sleep, we breathe so we expire moisture and the relative humidity in the room elevates and we now have mould growing because it only takes 48 hours and mould is off again on its cycle,” she told nine.com.au

To prevent the mould risk coming from air blowing contaminants on the screen into our homes, Smith recommends cleaning them at least once a year, but checking them every few months.

The best way to do this is with soapy water made with just a small amount of dishwashing liquid and cold water and a single-use microfibre cloth. The cloth should be damp, but not wet, and you should change over to a fresh cloth regularly rather than using the same one to clean the entire screen.

Smith also encourages going outside to look around your house for water damage and dust on the building so you can keep on top of things before they become a bigger problem.

She suggested going around with a white cloth or paper towel and rubbing it along the outside of the building and if it comes up dirty, it needs cleaning.

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“If you have dust, you have mould,” she said.

But that’s not the only reason to think twice when opening your windows. If the humidity is higher outside than it is inside you’re inviting moisture into the home. Setting up a hygrometer in your home and outside will allow you to keep an eye on humidity.

“A hygrometer tells you the temperature and the humidity, just by looking at it. There are cheapies around, but even if you got a cheapie, you would soon get used to [interpreting it]... You’d still know, well, the one inside says it’s 60 per cent humidity and the one outside says it’s 75 per cent [so] I’d be saying don’t open your windows [in that case] because you’ve got a 15 per cent difference that will naturally migrate inside.”

When we do open our windows to let better quality air in – provided we’ve checked the above – there is one more thing to consider.

“You need another window open for the air to actually flow out the back end or to flush [it out], if that’s why you’re opening up a house. If you were opening up a room, does it have an exit? And where is that exit? You’d like it to be on the other side of the building, not, let’s say, letting dirty air out of your bedroom go into, let’s say, the hallway or the lounge,” she said.