BCD 2018

144 C AT H E D R A L COMMU N C I AT I O N S C E L E B R AT I N G T W E N T Y F I V E Y E A R S O F T H E B U I L D I N G CO N S E R VAT I O N D I R E C TO R Y 1 9 9 3 – 2 0 1 8 Figure 2. Absolute humidity in the roof space immediately above the bathroom ceiling and below the loft insulation (red) follows closely that of the roof space immediately above a bedroom (green), irrespective of the wild fluctuations in the bathroom below (blue). This suggests that plasterwork alone provides an effective vapour barrier. Figure 1. Absolute humidity levels inside the roof space (blue) generally follow the rise and fall of those outside (green), with divergences coinciding with peaks in solar gain (red). Figure 2 shows data from a bathroom, bedroom and air beneath the insulation at ceiling joist level. The plot from the bathroom shows numerous spikes of high air moisture content caused by usage. But the curve obtained beneath the insulation remains similar to the curve obtained beneath the insulation above the bedroom. Most of the additional water vapour will have condensed in the bathroom and been dispersed within the building’s general air moisture loading. MOISTURE BUFFERING BY CONSTRUCTION TIMBERS As air in the loft cools, its relative humidity will increase. However, this does not necessarily alter the roof environment significantly because the timber structure acts as a buffer to humidity change. Wood is hygroscopic and porous, and acquires a moisture content dependent on the relative humidity of the air surrounding it (temperature does not make much difference except to the rate of change). The surface moisture content in the first outer few millimetres of the wood responds dynamically to short-term humidity fluctuations, while the moisture content at greater depth will be more in equilibrium with seasonal fluctuations. Several Scandinavian studies have demonstrated that timber can modulate a room environment and the same effect will occur in a roof space. Because only the surface few millimetres of the components respond dynamically, it is the surface area of the timbers that is important and historic roofs generally have a far greater surface area of wood than modern light-weight constructions. Ancient oak is even more effective in buffering moisture because the surface often has an increased porosity. The risk that moisture rising from a normally dry building will cause condensation under normal usage should therefore be small, even after insulation has been added. This certainly suggests that excess condensation might be more likely in modern light-weight roofs, but does not explain why reducing the transmission of heat from the building might suddenly trigger moisture destabilisation. WHY MIGHT A ROOF ENVIRONMENT BECOME UNSTABLE? The first suggestion might be that the excessive condensation occurs where usage is not normal and is caused by excess moisture from patterns of occupancy. If excess moisture entered the roof space then dew point would be raised and more easily reached by restricting heat movement from below. However, this idea is not supported by observation. We investigated three out of a row of four houses in the Midlands. One was tenanted by a family at home all day with two children and had clothes drying on the landing. One was occupied by a childless couple at work each day, and the third had a tenant who was only in residence at weekends. The only common factor was that the heating system was in use. The roof space above each occupancy was running with condensation. Another suggestion is that heat rising from the house, before the extra insulation was added, maintained the surface temperature of the roofing felt above dew point. But it seems highly unlikely that the relatively small amount of heat rising through the house would be enough to overcome the cooling effect of the roof surface on cold winter nights. A third suggestion is that the extra insulation, even without extra moisture, somehow raised the dew point, but this is not possible. If 100mm of insulation is added between the ceiling joists then the roof temperature drops and the relative humidity rises, but dew point does not change because the absolute moisture content of the air in the loft does not change. Extra insulation might lower the temperature further but this still cannot affect the dew point. All it will do is elevate the relative humidity of the air in the loft still further. But what effect on the performance of a roof might this volume of air in the loft have if it has a persistently high relative humidity? When relative humidity remains very high, the way in which timber absorbs moisture changes. Figure 3 shows a graph of wood moisture content at different relative humidities. Over most of its sorption curve (see Figure 3) wood adsorbs water in the vapour phase and its surface moisture content therefore fluctuates with relative humidity. At humidities near dew point the end section of the sorption curve becomes steep because the wood is now absorbing water in the liquid phase. Condensation may commence in the wood capillaries at less than 100 per cent humidity and so the timber becomes self- wetting at high humidities. Water slowly penetrates deep into the wood and it is no longer a surface effect. This, together with any water forming on impervious surfaces and coalescing into rivulets, saturates the timber so that a rise in temperature has only a slow drying effect. In a dry roof, condensation may occur on cold nights but the extra moisture is taken back by the air when the temperature rises again. If excess condensation occurs and the timbers become wet, raising the surface temperature will just pull more water from the timber and the very high humidities will not reduce substantially. INFESTATION AND DECAY If relative humidity rises then so does wood surface moisture content, but this does not present a risk to the timber over the normal range of roof humidities in a dry roof when there is moisture adsorption in the vapour phase. If the roof remains humid (above about 80% humidity) for an extended period of time then there might be some mould growth and g/m3 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.5 W/m3 250 200 150 100 50 0 9.1.16 10.1.16 11.1.16 12.1.16 AH in AH out Solar 15 14 13 12 11 10 9 8 23.9.16 24.9.16 25.9.16 26.9.16 27.9.16 28.9.16 29.9.16 30.9.16 In bathroom Above bathroom Above bedroom g/m3