Black Mould on the Window Sills Again? What Actually Works

You wipe it off in November and it’s back by January. That isn’t bad cleaning — it’s the water, and it’s coming from inside the house.

Why it keeps coming back

Donegal is mild and damp for most of the year. Inside, we add to it: cooking, showers, kettles, and clothes drying on the radiator because there’s no chance of getting them out on the line.

All that moisture hangs in the air until it meets the coldest surface in the room — usually the window glass, the frame and the sill. It turns back into water, sits there, and within a couple of weeks the black spots start in the corners. Bathroom silicone gets it for the same reason.

So cleaning it off is only half the job. If the moisture stays, the mould comes back.

Cleaning it off safely

• Hard surfaces — glass, uPVC frames, tiles — can be scrubbed with detergent and water and then dried completely. Drying is the part people skip. [US EPA]
• Never mix bleach with other cleaners, especially anything containing ammonia. The fumes are toxic. [US EPA] Open a window while you work.
• Silicone is the honest exception. If the black has gone into it rather than sitting on it, cleaning will lighten it but won’t make it white. That silicone needs cutting out and redoing.
• Soft things — curtains, blinds, the back of a wardrobe, carpet near an outside wall — hold moisture. Porous materials that go mouldy sometimes can’t be saved.

If a patch is bigger than roughly a metre square, or it’s coming through the wall itself rather than sitting on a cold surface, that’s past cleaning. Get the cause looked at: a leak, a gutter overflowing down the wall, or missing insulation.

Stopping it coming back

• Open the window or run the extractor while you shower and cook, and for a while after.
• Keep trickle vents on the windows open. They’re there for this exact reason.
• Wipe the condensation off the glass in the morning with a cloth or a squeegee before it soaks into the sill.
• If you dry clothes indoors, do it in one room with the door shut and the window open a crack, not on every radiator in the house.
• Keep some heat on steadily in cold weather — a house that swings between cold and warm makes more condensation.
• Check the gutters. Overflowing gutters run down the outside wall, and the damp patch inside often starts there.

When a proper clean helps

If it’s been building up for a couple of winters, a deep clean or a bathroom deep clean takes it back to a clean starting point — frames, sills, tracks, grout and tiles — so your weekly wipe has a chance of keeping up. Window cleaning covers the glass, frames and sills inside and out.

I use products that are safe for kids and pets, and I’ll tell you straight if what I’m looking at needs a builder rather than a cleaner.