Black spots creeping across the corner of a bedroom and water streaming down the windows each morning are winter's most common complaints. The cause is nearly always condensation, and the cure is mostly about habits rather than builders.
Where the moisture comes from
A household puts litres of water into the air every day through cooking, washing, drying clothes and simply breathing. When that warm, damp air meets a cold surface such as a window or an uninsulated wall, it condenses, and mould feeds on the dampness.
Cut the moisture at source
Small changes make a real difference here, and they cost nothing.
- Put lids on pans and run the extractor while cooking.
- Shut the bathroom door and ventilate after a shower.
- Dry washing outdoors or in a vented space, never on radiators.
Ventilate and heat steadily
A house kept at an even, gentle warmth stays drier than one blasted hot then left to go cold, because warm air holds moisture and cold surfaces collect it. Crack a window or open the trickle vents for a while each day to let the damp air out.
Dealing with the mould you have
Wipe existing mould off with a proper mould remover rather than just bleach on the surface, and keep furniture a few centimetres off external walls so air can circulate behind it. Treat the cause and the black spots stay away; treat only the symptom and they come back every January.