Black Mould on Window Seals & Frames

What causes black mould on window seals

Mould needs moisture to grow, and window seals and frames are one of the coldest surfaces in a room, which makes them a prime spot for condensation to form overnight, particularly in winter when indoor and outdoor temperatures differ most. That condensation sits in the seal, frame corners and any rubber gaskets, and given enough repeated dampness without drying out, mould spores that are naturally present in the air settle in and establish. North-facing rooms and those with limited direct sunlight tend to stay coldest for longest, which is why mould often shows up there first and worst.

Rooms with poor ventilation, drying laundry indoors, or limited airflow behind curtains and blinds tend to see the worst build-up, since moist air has nowhere to go. Newer, well-sealed double glazing, while good for energy efficiency, can actually make this worse by reducing the natural draught that older, leakier windows once relied on to keep moisture moving.

How to clean it safely

A mild detergent or diluted mould-specific cleaner applied with a cloth or soft brush lifts most window seal mould without needing anything harsher. Full-strength bleach is often reached for first, but repeated undiluted use can degrade rubber seals and window sealant over time, so a diluted approach or a product designed specifically for mould on these surfaces is the better long-term choice.

Wiping the area completely dry afterwards, rather than leaving it damp, helps prevent mould re-establishing within days of cleaning. An old, soft toothbrush is genuinely useful for working cleaner into the fine grooves of a rubber seal where a cloth alone can't reach.

Ventilation advice that stops it recurring

Opening trickle vents where fitted, running an extractor fan in bathrooms and kitchens, and avoiding drying laundry on radiators in poorly ventilated rooms all reduce the moisture load that leads to condensation in the first place. A simple daily habit of wiping condensation from windows first thing in the morning, especially in bedrooms, makes a noticeable difference over a winter. Leaving a small gap around furniture pushed up against external walls also helps air circulate rather than trapping damp against cold surfaces.

When it points to a bigger problem

If mould keeps returning heavily despite good ventilation habits and regular cleaning, or if it is spreading beyond the window onto surrounding wall areas, it may point to a broader damp or ventilation issue worth having properly assessed, rather than something a window clean alone will resolve.