This is a question I hear all the time.
“Why does my brand-new house already have mould?”
It’s not just old houses. New builds seem to have their fair share of problems too.
Last year, we did a survey and found that a shocking 62.38% of homeowners are dealing with mould.
People are starting to realise how bad it can be for their health and the damage it can do to their homes.
But why is it happening so often?
1. New Builds Are Too Airtight for Their Own Good
Older houses breathe. They have chimneys, slightly dodgy windows, and natural gaps that let air flow in and out.
New builds? Not so much.
They are designed to be super airtight and energy-efficient, which is great for keeping the cold out but not so great when it comes to moisture escaping.
All that steam from showers, cooking, and even just breathing has nowhere to go.
So it sticks around, looking for a cold surface to settle on, like your walls, ceilings, and windows.
And that’s when the mould party starts.
2. Weak Extractor Fans Don’t Help
Yes, new builds do have extractor fans.
No, they’re not always powerful enough.
Most standard fans move about 15 to 30 litres of air per second, which sounds decent… until you realise it’s nowhere near enough to clear all the moisture from your bathroom or kitchen.
A proper high-powered fan that moves 60 litres per second or more will do a much better job.
Even better, get one that automatically switches on when humidity gets too high, so you don’t even have to think about it.
3. Trickle Vents Exist, but People Forget to Use Them
Trickle vents are those little openings at the top of your windows that are supposed to let fresh air in without you having to open a window.
The problem?
Most people keep them shut because they feel a draught.
If your home is feeling humid, check if your trickle vents are actually open.
Also, make sure air can move between rooms. If doors are constantly shut, the moisture just stays trapped in certain areas.
4. Mould Can Start Growing at Lower Humidity Than You Think
You might have heard that mould needs 66% humidity to grow. That’s not exactly true.
In reality, mould can start growing at just 60% humidity, especially in colder spots where moisture condenses, like windows, ceilings, and external walls.
It’s not just about the air being humid. If your walls are cold, even normal levels of moisture can cause problems.
5. Moisture Might Have Been Trapped When Your Home Was Built
One of the biggest problems we see with new builds is that moisture gets into the structure before the roof is even on.
Concrete, plaster, and timber all contain moisture and can take months to fully dry out.
If the house is sealed up too soon and doesn’t get enough ventilation during construction, the walls and floors can stay damp.
Then, as soon as people move in and start heating the place, that hidden moisture starts creeping out.
6. Cold Spots Attract Mould
Moisture loves a cold surface.
I know someone with a three-storey new build. The top floor rarely gets heated, so all the warm, humid air from the lower floors rises and hits that cold third-floor ceiling.
The result? Mould everywhere around the windows.
The simple fix is to keep the heating at a minimum of 16 degrees throughout the house.
That stops cold spots from forming, but heating alone won’t solve everything.
7. So, How Do You Prevent Mould in a New Build?
If you’re struggling with mould, it’s probably down to one of these things:
- Your extractor fans aren’t strong enough, or you’re not using them properly.
- Your trickle vents are closed.
- Your heating is too low, which is creating cold spots.
A few other things that can help:
- Use a dehumidifier to keep humidity below 60%.
- Check for cold spots on walls and ceilings, which could mean poor insulation.
- If you want the ultimate fix, consider an MVHR system. It brings in fresh air while keeping warmth inside, solving both ventilation and heat loss problems at the same time.
If you’ve tried everything and the mould is still coming back, you probably need a survey. There could be a bigger underlying issue.
I hope this helps.
We’ll be posting more tips each month, so stay tuned.
