Dealing With Condensation and Mould in Winter
Black mould in corners, behind furniture and around windows is almost always condensation rather than a building defect. It is treatable, and treating the symptom alone never works.
Why it happens
Air holds moisture, and the warmer it is the more it holds. When warm moist air meets a cold surface, it cools and releases water onto that surface.
Ordinary living generates a substantial quantity of moisture daily — cooking, washing, drying clothes, breathing.
Cold surfaces occur where insulation is poorest: external corners, window reveals, behind furniture against external walls, and cold bridges around lintels.
The combination of high moisture and cold surfaces produces condensation, and mould follows.
The three levers
Reduce moisture at source. Lids on pans, extractor fans used during and after cooking and bathing, and drying laundry outside or in a vented tumble dryer rather than on radiators.
Ventilate. Trickle vents open, brief purge ventilation by opening windows fully for a few minutes, and extractor fans that actually vent outside rather than into a loft.
Keep surfaces warmer. Steady background heating rather than short bursts, and improved insulation where feasible.
All three together work. Any one alone usually does not.
Specific fixes
Move furniture a few centimetres off external walls to allow air movement behind it.
Fit or upgrade extractor fans with humidity sensing and overrun timers, which is inexpensive and among the most effective single measures.
Insulate cold spots where possible, particularly window reveals and behind boxing.
Consider a dehumidifier in a persistently damp room, which is cheaper to run than heating the space enough to prevent condensation.
Cleaning existing mould
Use a proprietary fungicidal wash rather than bleach, which lightens the mould without killing the roots on porous surfaces.
Wear gloves and a mask, ventilate while cleaning, and dispose of cloths rather than reusing them.
Repaint with a fungicidal paint after treatment, which delays recurrence but does not prevent it if the cause remains.
When it is not condensation
Damp confined to one patch that worsens after rain suggests water entering from outside — check gutters, pointing, flashing and window seals.
A tide mark at consistent height above the floor with salt deposits suggests rising damp, which is genuine but far rarer than commonly diagnosed.
If treatment of the ventilation and heating makes no difference over a winter, investigate the building fabric rather than continuing to clean.
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