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Anonymous user

Dampness and black mould patches on an external wall is usually a sign of high relative humidity. The humidity of the internal air increases as it passes over the cold wall surface according to the law of psychometrics (every 1C change in temperature in one direction there is a 5% change in relative humidity in the opposite direction) and people are always adding moisture to the air (4 pints per person per day) and especially in bedrooms at night from breathing when they sleep. This is why you see mould and condensation on cold external surfaces in the mornings after cold nights - the relative humidity has risen to levels where mould can grow (80% for 5 hours or more) and condensation occurs (100% or dewpoint). The solution is to provide constant 24/7 input ventilation to the internal air which has the effect of constantly diluting the moisture content of the air thus preventing the relative humidity from rising too high thus negating the effect of cold surfaces and making it impossible for condensation/mould to form. If the damp area is accompanied by a watermark or mineral tide mark it is a sign of penetrating or rising damp but there could also be a humidity issue in the same place - these types of damp create cold spots which can cause condensation/mould. In such situations it is advisable to solve the random appearance of humidity issue first which will have the effect of unmasking any other damp issues underneath which are not random in appearance. Note if you insulate the internal wall surface this may increase the surface temperature but it will not solve the problem or deal with the cause of the black mould and condensation. Firstly you dont know how cold the wall will get at night because you dont know how cold it will get outside so you wont be able to control rises in relative humidity and secondly there will still be high humidity internally due to the occupant's normal activities. If you do see any relief on this cold wall from a thermal insulation layer you apply it is likely the problem will re-appear on the next cold surface in your property. The only way to permanently solve these type of problems is constant (24/7) and effective ventilation.