Why Is Black Mould Growing Around My Bedroom Window?
For black or dark mould recurring on the reveal, frame, sealant or wall immediately around a bedroom window.
What this usually means
Black mould around a bedroom window is often caused by repeated condensation on a cold surface, especially where indoor moisture is high or ventilation is poor. But mould concentrated around one window can also be worsened by a window, seal or external building defect that lets water in. Cleaning the visible mould is only a short-term step: the moisture source has to be identified and reduced or repaired.
What may be causing this
Condensation on a cold window or reveal
Clues: Water droplets are common on the glass in cold weather, mould is made up of scattered black spots, and the problem is worse after sleeping, drying clothes or other moisture-producing activity.
What to do: Reduce indoor moisture, keep useful background ventilation open, and improve heating/air movement so surfaces stay drier.
Poor or blocked ventilation
Clues: Trickle vents are shut or blocked, the room feels stuffy, condensation lasts well into the day, or moisture is also collecting on other cold surfaces.
What to do: Restore intended ventilation and use available extraction elsewhere in the home when cooking, bathing or drying clothes.
Cold bridging or weak insulation around the window
Clues: The same reveal or corner stays noticeably colder than surrounding surfaces and repeatedly develops mould despite reasonable ventilation.
What to do: A property or damp professional can assess insulation, thermal bridging and the condition of the window installation.
Water entering through a window or external defect
Clues: Dampness worsens after rain, staining is localised to one side/top of the window, finishes blister or a tidemark appears rather than only surface condensation.
What to do: Inspect window seals, external pointing/render and adjacent rainwater details from a safe location; defects should be repaired rather than treating the surface mould alone.
How to check what is happening
That pattern is typical of condensation on a cold surface.
Penetrating water through the window or surrounding building fabric becomes more likely.
The issue is more likely to involve room humidity, heating and ventilation rather than only one failed window seal.
Can you fix this yourself?
A homeowner can usually document the pattern, reduce condensation and clean a very small surface area using an appropriate product. Do not assume cleaning has fixed the problem: recurring mould needs the underlying moisture source addressed.
- Phone/camera to record the pattern
- Absorbent cloth for condensation
- Appropriate mould-and-mildew cleaner
- Gloves, eye protection and suitable mask as directed for the cleaning task
What you can safely try
- 1
Photograph the mould and moisture before cleaning
Record where the mould is, how large the area is and whether the glass/reveal is wet. This helps distinguish a recurring condensation pattern from a localised leak or building defect.
- 2
Reduce moisture and keep intended ventilation working
Keep trickle vents and other designed vents open, use kitchen/bathroom extraction, avoid adding unnecessary indoor moisture and wipe persistent window condensation rather than leaving the reveal wet.
- 3
Clean only a small manageable area safely
Use an appropriate mould-and-mildew product rather than simply painting over the mould. Follow the product instructions, ventilate the room and use suitable protective equipment.
- 4
Monitor the same area after it is dry
If mould quickly returns despite reasonable heating and ventilation, or the area becomes wet after rain, move from surface treatment to investigation of the building/window defect.
Call a damp specialist if…
- The mould covers a large area, is spreading rapidly or keeps returning soon after cleaning.
- Anyone in the home is experiencing symptoms that may be affected by damp or mould, or is particularly vulnerable to its health effects.
- The wall or reveal is wet after rain, plaster is breaking down, or there is evidence of water entering through the building fabric.
- You would need to work at height outside the window or disturb building materials to investigate the source.
Get the right professional.
This problem can have more than one cause. Start with the checks above: a damp specialist is the usual first route, while a window fitter may be more appropriate when the clues point that way.
Sources used for this guide
We prefer primary UK guidance, regulators and manufacturers for technical and safety-sensitive claims.
- Understanding and addressing the health risks of damp and mould in the homeGOV.UK
- How to reduce condensation in your homeOxford City Council
Last reviewed: 31 August 2026. Home Manual updates review dates only when the substance of a guide is checked or changed.