Mould on Your Windows? Here Are the Probable Causes and How to Fix Them

Mould around your windows is easy to miss at first. It often starts small, like a dark patch in the corner of the frame or a faint musty smell. But before you know it, it becomes an increasingly visible problem that is hard to ignore.

This issue is fairly common in London homes because of temperature changes and frequently damp weather. The key isn’t just cleaning the mould away. It’s understanding why it’s there in the first place.

If you have been dealing with mould growth on your windows for the longest time, here are some possible reasons why it still comes back and how you can get rid of it.

Condensation That Doesn’t Fully Dry

Condensation is the most common cause of mould on windows. When warm indoor air hits a cold pane of glass, moisture forms. If that moisture sits for hours or happens daily, it creates the perfect conditions for mould to grow.

You’ll usually see this in bedrooms during winter mornings or in kitchens and bathrooms where humidity levels are higher. The glass itself may dry quickly, but the corners, rubber seals, and frames tend to stay damp longer.

Over time, that repeated dampness becomes a breeding ground for mould spores.

Limited Airflow Around Windows

Even well-insulated homes can struggle with airflow. If windows stay closed most of the time and extractor fans aren’t used consistently, humidity builds up indoors.

Rooms that are tightly sealed to keep heat in during colder months are often the ones where mould appears first. Without airflow, moisture has nowhere to escape.

Simple adjustments can help, such as:

• Opening windows briefly each day

• Using extractor fans while cooking or showering

• Avoiding drying clothes indoors without ventilation

These small habits should help reduce the moisture that feeds mould.

Dirt and Residue Along Frames and Seals

Mould doesn’t grow on clean, dry surfaces. It thrives where moisture and organic residue meet.

Window frames collect dust, pollution particles, and general grime over time. In areas near busy roads or construction, this build-up can be heavier than expected. When condensation mixes with that residue, it creates the ideal environment for mould growth.

This is why mould often appears around the frame first rather than in the middle of the glass.

Minor Leaks or Seal Failures

Sometimes mould is a warning sign of something more structural. Small leaks, cracked sealant, or failing window seals allow water to enter areas that don’t dry properly.

If you notice mould returning quickly after cleaning, or damp patches spreading beyond the window frame, it’s worth checking whether water is getting in from outside.

While cleaning can improve the appearance, it won’t fix trapped moisture behind the surface.

How to Remove Mould Properly

For light mould growth, careful cleaning can make a big difference. A mild cleaning solution and a soft cloth are usually enough for surface-level mould on frames and seals.

What matters most is drying the area thoroughly afterwards. Even after cleaning your windows, if you leave the surfaces damp, it can cause mould to return quickly.

Avoid abrasive tools or harsh chemicals that may damage finishes or push moisture deeper into porous materials.

If the mould covers a large area or keeps coming back, it may require more thorough treatment.

How to Reduce the Risk Long-Term

Preventing mould on windows is mostly about moisture control and routine care. These can be as simple as:

• Wiping condensation when you see it

• Keeping frames clean

• Improving airflow

It also helps to schedule periodic deep cleaning of your windows, including the frames and sills. When dirt and residue are removed properly, there’s less for mould to feed on.

Book a Professional Window Cleaning in London

Mould on windows before and after window cleaning by The Riviera Group.

While mould is primarily caused by moisture, professional window cleaning plays a supporting role in prevention. Thorough cleaning removes grime and residue that trap moisture and encourage mould growth.

At The Riviera Group, we provide thorough internal and external window cleaning across London. We focus not just on the glass, but also on frames, sills, and edges where dirt tends to build up. Our combination of traditional hand-cleaning and purified reach and wash systems ensures a clean that supports both appearance and long-term maintenance.

If mould around your windows keeps returning despite your efforts, it may be time to take a closer look at both ventilation and professional cleaning. Fill out our form today, and get a free window cleaning quote!