Updated
Black Mould On Window Frames And Seals
Black speckling in the rubber gasket, along the sealant line or in the corners of the frame.
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What's happening?
The black speckling in a window gasket is mould growing on the film of dust, skin cells and condensation that collects in the rubber. It appears first in the bottom corners because that is where condensation runs to and sits. Outside, the same corners grow green algae fed by rain and shade. Cleaning removes the growth, and a window cleaner will do that thoroughly, including the channels people never reach. But cleaning does not remove the reason it grew. Internal black mould on frames is a condensation symptom: warm, moist indoor air meeting the coldest surface in the room. If it comes back within weeks of a clean, the fix is ventilation and moisture control, and repeated growth around a window can indicate a failed unit seal or a genuine damp problem rather than a housekeeping one.
What usually causes it?
- Condensation collecting in the bottom gasket and sealant line where warm indoor air meets cold glass
- Blocked or painted-over trickle vents, so moist air has nowhere to go
- Drying washing indoors, or an extractor fan that runs but vents into a loft rather than outside
- Rain and shade feeding green algae on the outer frame, especially on north-facing elevations
- Old, shrunken or split silicone with a gap that holds water behind the bead
- Perished gasket rubber that no longer compresses, leaving a permanent moisture trap
- A failed double-glazed unit letting moisture sit inside the frame rebate year round
Is it dangerous?
Mould in the home is a recognised health risk, not just an eyesore. Damp and mould are associated with respiratory infections, allergy and asthma symptoms, and the risk is higher for babies, children, older people and anyone with an existing lung or immune condition. A small patch in a gasket is not an emergency; recurring mould across window reveals and walls is a housing condition worth acting on, and in a rented home it is the landlord's business as well as yours.
Can I fix it myself?
Yes, for the cleaning part on reachable windows. Wipe the growth off with a damp cloth and a household cleaner suitable for uPVC, working it into the gasket channel, and dry the area properly afterwards. Do not dry-brush or vacuum mould without a filter — that puts spores into the air. Do not use bleach on coloured or foiled frames, and never mix bleach with an ammonia-based cleaner, which produces a toxic gas. Then deal with the cause: open trickle vents, run the extractor while showering and cooking, and stop drying washing on radiators.
When should you call a window cleaner?
Call a window cleaner for a thorough frame, gasket and channel clean, including the outer frames and any window you cannot reach. Call them back if it returns within a few weeks — that result tells you it is a moisture problem, not a cleaning one, and worth raising with a damp specialist or, if you rent, your landlord. Mould inside a sealed double-glazed unit is never a cleaning job; the unit has failed and needs replacing.
What will the window cleaner actually do?
- Clean the growth from the gasket channel, sealant line, rebate and frame corners rather than just the visible face
- Wash the outer frames, drip bead and sill where algae and organic film feed the growth
- Clear the drainage slots at the bottom of the frame so trapped water can escape
- Clear and expose blocked trickle vents so the window can ventilate again
- Point out perished gaskets, split sealant or a misted unit as the cause rather than cleaning over it
- Advise on a cleaning frequency that keeps organic film off the frames through the damp months
Frequently asked questions
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