Mould growth is the result of temperatures and humidities being maintained at levels that suit fungal growth. All moulds are natural and have a beneficial effect in breaking down organic materials. Dry-rot, which is a particularly destructive form of fungal decay in timber structures, occurs naturally on tree stumps in the foothills of the Himalayas. Mould grows in buildings when we allow the conditions to occur either on internal surfaces, or within the construction itself.
Moisture is always necessary for mould growth. It occurs often through rising damp or through blocked downpipes and gutters. It also occurs through Condensation - both surface and interstitial. Surface mould growth is commonly found in poor quality housing where there is insufficient insulation, lack of heating and inadequate ventilation combined with intensive moisture-generating activities such as drying clothes. The use of unflued gas and paraffin heaters also produce huge quantities of moisture which contribute to the problem.
Ventilation reduces the Relative Humidity of the air, by introducing air with lower quantities of moisture. Ventilation in cold, moist climates such as the UK is only beneficial up to circa 1 air change per hour. Trickle ventilation in windows required under regulations to provide constant background ventilation without creating draughts.
Heating raises the temperature of the internal air, thus lowering its relative humidity. It also raises the surface temperature of the internal surfaces preventing surface condensation. Mould growth is often a problem in upstairs bedrooms where heating is turned off and the warm moist air rising from below finds cold spots on which to be deposited. Again constant background heating and venilation is the key to solving this problem.
Insulation keeps wall surfaces warm and prevent cold-bridging. Insulation alone without heating or ventilation will not cure mould growth.
This is often the result of interstial condensation. However, softwood is prone to rot when the equivalent moisture content (emc) exceeds 20%. This can happen even without actual moisture deposition occuring. The emc of timber is a function of a number of factors - one of which is the proximity of the water vapour in the air becoming saturated. At very high relative humidities moisture is absorbed directly from the air into the wood.
Where there is a risk of timber rotting, some form of timber preservation treatment is required. Timber treatment tends to use poisonous chemicals which are not necessarily a long-term solution.
Long-lived timber constructions rely on timber being well ventilated. However, there can be a conflict of interest between the need to insulate and weather-protect and the need to allow timber to breathe (see Breathing Walls).
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