18 C O N T E X T 1 3 4 : M A Y 2 0 1 4 the structure is actually damp, the results may be equally unsatisfactory. For example, if outside air with an average humidity of 60–70 per cent infiltrates a damp sub-floor void, it will cool and its relative humidity will rise as the temperature drops.The amount of water the air can then absorb will be very small, and the result, if air moves at all, will be slowly moving damp air rather than static damp air.This will have little adverse effect on any decay organisms andmay assist themby spreading fungus spores and insect pheromones. Ventilation will only be beneficial in particular circumstances where there is excess moisture; the moisture is finite and can be removed; the method of ventilation will allow air circulation throughout the void; the air is at a relative humidity and moving at speed that allows it to absorb the excess moisture; and the damp air can be exchanged for drier air outside of the void or cavity. If roof or sub-floor timbers are sound and dry, attempting to increase ventilation is unnecessary, and may cause problems that were not there before. However, the hygrothermal equilibrium of these elements will also be affected by the addition of insulation and have unintentional consequences. Condensation in roof spaces following the addition of insulation between the joists is sometimes reported. For example, a correspondent from Cornwall recounted how he insulated his roof space with no apparent damp consequences, but severe condensation and mould growth occurred when he added more insulation some time later. What might have happened to cause this? If we consider a roof space as an enclosure bounded by inert materials, the significant accumulation of moisture should not occur. Insulation will halt the rise of heat from the rooms below so that temperatures drop and relative humidities rise but, for a more-or-less fixed air moisture content, the dew point temperature at the external surface should remain fairly constant. Condensation will occur when the temperature drops below dew point but this should be a relatively transient event. However, this theoretical behaviour does not explain the observations, so the constructionmaterials must be having a significant effect. The roof timbers have a substantial effect on the environment with the roof void; this requires us to think about how moisture enters and leaves timber. The figure below shows an equilibrium sorption curve for Scots pine. The curve is S shaped because sorption is controlled by three mechanisms with an accumulative effect. Stage 1: Wood cell walls are made up of bundles of cellulose micro fibres, the majority of which (S 2 layer) are all oriented in the same direction.These fibres have crystalline zones and amorphous zones.Water molecules (vapour phase) attach to the hydroxyls (OH bits) in the amorphous zone, forming amonomolecular layer of water. Wood will never be entirely dry under normal conditions. Stage 2: As relative humidity rises, water molecules (still vapour phase) attach to the water molecules already present, forming a polymolecular layer, which accrues more molecules as humidity rises, slowly pushing the fibres apart. Adsorption causes the wood to swell and desorption causes the wood to shrink. Because moisture is in the vapour phase, these changes only affect the surface few millimetres of the timber and the timbers are efficiently buffering changes in relative humidity. Stage 3: High relative humidities cause liquid phase moisture accumulation by capillary condensation. This occurs in the pores and spaces surrounding the fibre bundles within the cell wall because minor forces between molecules (Van der Waals forces) cause inter-molecular bonding. Capillary condensation occurs below saturated vapour pressure. Because water is now in the liquid phase, it slowly penetrates throughout the timber. Insulating a roof at ceiling level lowers the temperature within the roof space and raises the relative humidity.High relative humidities at the outer surface of the roof (when the external temperatures are low) cause the timber to accumulate moisture by Stage 3 condensation. Once the relative humidity reaches 100 per cent, the cell walls can not hold any more water, so it starts accumulating in the cell lumens (the space in the middle). Dew point on roof coverings or underlays causes moisture run-off on to the wood, but is not necessary to make the wood wet because the wood is self-wetting in these conditions. Capillary condensation acts as a reservoir, allowing moisture to penetrate deeper into the timber. It also forms a boundary layer at the surface, inhibiting evaporation. If moisture rises from the building and raises the relative humidity from 60 to 70 per cent, the timber moisture content will rise from around 11 to 13 per cent; the moisture is adsorbed from the vapour phases and is lost as easily as it is gained.This is how a normal roof functions. If, because of a temperature decrease, the relative humidity rises to 80 or 90 per cent, the timber moisture content increases from 16 to 21 per cent and capillary moisture starts to dominate.The timber now has the potential to become increasingly wet. A point may be reached where this effect becomes severe and difficult to reverse. English Heritage has embarked on a research project to monitor hygrothermal behaviour of roof and sub-floor voids in a number of domestic buildings,with andwithout insulation, continuously over four seasons. The aim is to understand how timber roofs and floors respond to internal and external environmental loads that might trigger condensation, or result in material deterioration, including the role played by ventilation. Preliminary results should be available in the spring of 2015. Brian Ridout is senior architectural conservator at English Heritage and principal of Ridout Associates, independent consultant biologists.
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