Condensation

Indoor air always holds water vapour, from cooking, bathing, drying clothes and simply breathing. When warm, moist air touches a cold surface, it cools and the vapour turns into droplets. That is condensation: what you see on windows on a cold morning also happens on walls.

It appears mainly in winter, on outside walls, in corners, behind wardrobes and around uninsulated columns and beams (thermal bridgesThermal bridgeA spot in a building where heat and cold pass more easily than through the rest of the wall or slab, such as columns, beams and the edges of a slab.). It leaves black spots of mouldMouldFungi that grow on damp surfaces, leaving black or greenish spots and a musty smell. It is a symptom of damp, not a problem with the paint. and spoiled paint, often high on the wall or on the ceiling, not at the bottom like rising dampRising dampMoisture from the ground that climbs up inside walls through the pores of the materials. It shows as stains, blown render and salts at the foot of walls..

Painting alone won't solve it. Regular airing, extractor fans in the kitchen and bathroom, steady heating and, more permanently, insulatingThermal insulationA layer of material that lets heat through only slowly, so a home keeps its warmth in winter and stays cool in summer. the cold surfaces, for example with external wall insulationExternal wall insulation (ETICS)An external insulation system: insulation boards are glued and fixed to the outside walls, then covered with reinforced render and a finishing coat., all help. Anti-mould paints slow the mould down but don't remove the cause.