Black Mould in Bedroom Corners: Find the Cause

Rung 1: Clean it and check it is not a leak

Before anything else, work out whether the patch is wet because of condensation or water coming in. Condensation mould is often on external walls and ceilings in winter, in the corner behind a wardrobe or above a window. A stain that grows after rain, or that sits under a bay or a downpipe, points to a leak.

• Wipe small areas with a damp cloth and a mould and mildew remover, following the label.
• Do not brush or dry-scrub dry mould; that throws spores into the air.
• Do not paint over mould; it comes back through the paint.
• Dispose of the cloths in a sealed bag.

For health guidance on damp and mould, see the GOV.UK guidance linked at the foot of this page.

Rung 2: Give the corner air and warmth

• Pull wardrobes and beds a few centimetres off external walls so air can circulate.
• Do not dry clothes in the bedroom, and leave the door open in the morning to let moisture out.
• Keep the room warm enough: a sustained low heat is better than a short burst.
• Open a window for a few minutes after waking, and keep trickle vents open.

Rung 3: Remove the cold spot

If the corner is on an external wall, it is probably a cold bridge: the wall is colder there than the rest of the room. Solid brick walls, uninsulated lintels, and the junction between a wall and a ceiling are common culprits. A surveyor can tell you whether insulation is missing, wet, or has gaps.

Internal wall insulation can cure a cold bridge, but it must be done right in older homes, because it changes how the wall dries. Get advice that matches your wall type before you buy boards.

Rung 4: When humidity control helps

If you have cleaned, ventilated, heated steadily and checked for leaks and the mould keeps coming back, whole-home humidity may be the remaining factor. A loft-mounted dehumidifier is one option for owner-occupied homes with a loft. It is not a fix for leaks, rising damp or cold bridges, and it is not suitable for rentals unless the landlord authorises the changes.

Mistakes that make bedroom mould worse

• Using bleach on porous plaster and wallpaper. It can lighten the stain without dealing with the cause, and it adds moisture.
• Painting over it without cleaning first, or using a stain-blocker as the only fix.
• Sleeping with the door and window shut and a wet towel on the radiator.
• Ignoring a patch that is growing after rain. That is a different problem from condensation and will not respond to ventilation alone.
• Waiting until the wallpaper peels. Early cleaning and fixing the cause is cheaper than replastering.

In Chatham and the Medway Towns

In Medway's solid-wall terraces, the cold corner is often on a gable or rear wall. Medway Council's conservation area appraisal for Brompton notes terraces of narrow, two-bay houses, which gives small rooms and little space for moisture to disperse; see our Brompton page. On higher estates such as Lordswood, exposed walls can be both colder and rain-battered, so check pointing as well as ventilation.

Questions we are asked

Related guides

• Damp Smell in Cupboards and Wardrobes: Causes and Fixes
• Condensation on Windows: What It Is Telling You
• Rising, Penetrating or Condensation Damp? How to Tell
• Victorian terraces vs new builds

Local help

• Damp, mould and condensation help in Brompton
• Damp, mould and condensation help in Lordswood
• Damp, mould and condensation help in Strood

Sources: GOV.UK: Understanding and addressing the health risks of damp and mould in the home; Property Care Association: Condensation in homes; Medway Council: Brompton Lines Conservation Area Appraisal.