Mould Around Windows in London by Mould Inspection London

Mould around windows is usually one of the first visible signs of a moisture imbalance in the property. It tends to appear on the lower corners of frames, along the seals, or just above the sill where condensation collects overnight. In London, this is especially common in bedrooms and living rooms where windows stay closed for long periods and airflow is limited.

We recently dealt with a flat in Ealing where black mould kept forming along the bottom edge of a double-glazed unit. The tenant assumed the window had failed, but the issue was actually overnight condensation combined with poor background ventilation. The room stayed shut most of the time due to outside noise, so moisture built up and settled on the coldest surface — the glass and frame.

The key factor here is temperature difference. Warm indoor air holds moisture, and when it hits a colder surface like a window, that moisture turns into water. Over time, repeated condensation build-up creates the conditions for mould to grow, especially around seals where moisture lingers longer.

Window design and placement also play a role. Older frames with worn seals can hold moisture more easily, while newer, tightly sealed windows can reduce natural airflow, trapping humid air inside the room. We often see issues worsen in winter when windows remain closed and heating is used inconsistently.

Another common trigger is furniture placement. Beds, curtains, or blinds positioned close to windows can restrict air movement, allowing moisture to sit in the same area for longer. Once that happens regularly, mould starts to develop in those specific spots.

Dealing with mould around windows isn’t just about cleaning the visible marks. Unless the moisture source and airflow are addressed, it will keep returning in the same areas. The focus needs to be on how that part of the room behaves day to day, not just the surface itself.

Once mould starts forming around a window, the goal isn’t just to remove it — it’s to change the conditions that allow it to return. In most London homes, that comes down to how moisture is managed overnight and how air moves around the window area during the day.

A common pattern we see is heavy condensation build-up first thing in the morning. If that moisture is left sitting on the glass and frame, it gradually seeps into seals and surrounding materials. Over time, this creates a damp environment where mould settles in the same spots. Simply wiping it occasionally won’t be enough if the humidity levels stay high every night.

We recently worked on a property in Fulham where mould kept appearing along the silicone seal of a bedroom window. The tenant was cleaning it regularly, but the room was used for drying clothes and the heating was only on in short bursts. That combination meant the air stayed humid, while the window remained one of the coldest surfaces in the room. Adjusting how the room was used made a noticeable difference alongside treatment.

Ventilation plays a key role, but it needs to be consistent. Opening windows briefly can help, but it doesn’t always solve the problem if the moisture returns later in the day. In some properties, especially those facing busy roads, windows are kept closed most of the time. In those cases, improving background airflow through vents or adjusting how rooms are used becomes more important.

There are also situations where the issue is linked to the window itself. Poorly insulated frames or cold bridging around the reveal can make certain areas attract more moisture. While treatment can remove the mould, those colder sections will continue to collect surface condensation unless the temperature difference is reduced.

It’s also worth noting that not all solutions are immediate. Stabilising the environment can take time, particularly in winter when rooms are naturally cooler. At Mould Inspection London, we focus on practical steps that fit the property, rather than quick fixes that only deal with the visible signs.