1
Why Mould Grows on Window Frames
Black mould (usually Aspergillus or Cladosporium) needs three things to grow: a food source (dust, plaster, wood, or the plasticiser in old sealant), a temperature above roughly 5°C, and sustained moisture. Window reveals and frames are common hotspots because they are the coldest surfaces in a room, so warm moist air condenses there first and stays damp for hours after a shower or cooking. If mould keeps returning within days of cleaning, the trigger is ongoing condensation, not a one-off spill or leak.
2
Old Single Glazing and Poor Frames Make It Worse
Single glazing and older aluminium or steel-framed windows have very low surface temperatures, so they trigger condensation at far higher room humidity levels than modern double glazing with a warm-edge spacer bar. If mould is concentrated around one or two older windows while the rest of the house is dry, the frames themselves — not general house ventilation — are usually the root cause, and replacement is often more effective than any amount of extractor fan use.
3
Ventilation Changes That Actually Help
Trickle vents in modern uPVC windows should be left open, not taped shut, even in winter — closing them traps humid air with nowhere to go. Extractor fans in kitchens and bathrooms need to run for at least 15-20 minutes after use, and tumble dryers without an external vent should be avoided or fitted with a condensing unit. These changes reduce the moisture load reaching cold glass and frames in the first place.
4
Cleaning Mould Safely Without Spreading Spores
Wipe mould with a dilute bleach solution or a proprietary mould remover, working from the top down and using a damp cloth rather than dry wiping, which can release spores into the air. Wear gloves and ventilate the room during and after cleaning. Painting over mould without removing it first will not work — the spores survive underneath and reappear through the new paint within weeks.
5
When Replacement Windows Solve the Problem for Good
If mould keeps returning despite good ventilation habits, the window itself is too cold to cope with normal household humidity. Replacing old frames with modern uPVC or aluminium units fitted with warm-edge spacer bars and correctly specified low-E glass raises the internal glass and frame temperature significantly, which is often the single most effective fix — no amount of cleaning changes the physics of a cold surface.
