C O N T E X T 1 4 9 : M A Y 2 0 1 7 15 A thermal image showing tanking applied to the wall of a Grade II listed building.The horizontal line is a join between darker, cooler tanking trapping condensation in the lower part of the wall, and lime plaster higher up. (All images: PeteWard) The room shown in the thermal image. JOHN EDWARDS and PETE WARD Understanding dampness Only when we understand how moisture moves within the fabric of a building can we predict how moisture will move after retrofitting works have been carried out. As often cited in best practice publications such as BS 7913:2013, a condition survey is essential before we contemplate any work to a building, and that must include a proper understanding of dampness. It is also essential that a proper investigation of dampness is carried out that focuses on the cause and not just dealing with the symptoms. We can measure the cause by using hygrometers, measurements of temperature, relative humidity, and the absolute humidity (total moisture content of the air). A room-by-room picture of atmospheric moisture levels can reveal previously hidden information that sheds light on the cause of problems. A particularly damp room may be above a cellar, or a suspended timber floor with poor or blocked ventilation. Often cracked or blocked drains are revealed in this way. Symptoms of dampness such as mould can be tracked to unventilated kitchens and bathrooms. Combined with thermal imaging, temperatures of the building fabric can be readily observed, and cold areas highlighted which give rise to condensation within the fabric. For example, within concealed spaces such as under floor voids which we can not fully explore due to poor access, the hygrometer can be used to collect data to enable us to observe whether condensation exists or is likely if temperatures drop to dew point (the temperature at which condensation occurs) at any time. Common terms such as ‘rising damp’, ‘breathability’, ‘penetrating damp’, and many others, all relate to moisture movement mechanisms. Hygroscopicity describes how moisture from the environment around building fabric is absorbed into it when humidity is high, as in episodes of high condensation, and is released when humidity is lowered. Commonly moisture caused initially by other moisture movement mechanisms can bring contaminated moisture to the surface of a wall and leave the hygroscopic salts behind. This frequently happens in chimney breasts, due to the high concentrations of nitrates and sulphates created by burning fossil fuels.The important issue is to be able to identify hygroscopicity and not to get it confused with other moisture movement mechanisms.This is critical as most traditional materials in older buildings are to varying degrees hygroscopic. Another moisture movement mechanism is capillarity, often termed capillary attraction, which is related to the form and shape of pore structures. Despite common reference to capillary attraction, it is rarely a significant contributor to dampness, for a number of reasons.These include vastly different pore sizes present in materials; that they rarely interconnect; that they are often clogged up with salts; and that moisture often evaporates from a material at such a rate to minimise movement through capillary attraction. Vapour permeability is often used to describe a host of moisture movement mechanisms. Some materials allow moisture movement and some do not: vapour permeability is the ability of building fabric to allow water vapour to diffuse through it.Water molecules are relatively light and in any given volume of air, gaseous water will rise naturally.When trapped in building fabric, diffusion acts outwards and upwards, and will find its way around areas where vapour barriers have been formed. Traditional buildings were designed so that moisture could move. It is when this moisture movement is restricted or stopped that one finds concentrations of moisture in other areas. Many buildings have been compromised in this way through ‘improvements’ such as the replacement of timber floors with concrete, retrofitted damp-proof courses, and the installation of impermeable, solid wall insulation, to name but a few. Moisture affects the thermal performance of building fabric. In bringing these two issues together, we call it hygrothermal performance. Critically, damp building fabric is a much better conductor of heat than dry building fabric. This increase in heat loss is cited in BS 7913: 2013 as being at least 30 per cent, and some claim it to be even greater.Therefore, managing moisture so that building fabric remains comparatively dry will save energy. Mould growth and condensation are often
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