38 THE BUILDING CONSERVATION DIRECTORY 2024 CATHEDRAL COMMUNICATIONS measurements are only really useful for demonstrating the causes of moisture problems to clients. Thermography The surveyor’s eyes can be supplemented with a thermal camera which sees the building surfaces in the infra-red spectrum; in other words, as a pattern of surface temperatures. This can be useful for quickly tracking qualitative moisture profiles: surface temperature differentials may relate to elevated moisture at the surface, or to evaporation. A word of caution: it can be all too easy to misinterpret images since water is just one of several causes that result in surface temperature anomalies (others include building use - space heating or cooling, orientation, sunlight and other sources of radiant heating, airflow, and the time of day at which the measurements were taken). For masonry, thermography tends to be most effective for assessing leaks (from plumbing and other sources) or acute penetrating water, both of which can cause stark temperature differentials when the building is heated in winter. Although thermography rarely reveals information that could not equally be delivered by touch or by measurement with quasi-quantitative devices, it does allow patterns to be more easily presented to clients. Damp masonry requires a relatively high change in temperature to warm or cool it relative to dry masonry, so reservoirs of moisture can occasionally be revealed in unheated buildings or on the exterior surfaces of the building envelope. A relatively quick sweep of the walls can suggest areas meriting further investigation and indicate where water uptake is ongoing where a surface temperature differential corresponds with staining or damaged paint. Thermography can also be useful for identifying vulnerable elements of the building, such as embedded pipework or internal downpipe routes, and thus can minimise the chance of misreading the results from relative measurements or the risk of drilling into pipework during sampling. By exercising extreme caution, and in conjunction with measurement of relative humidity and air temperature (which will set out the environmental context for localised moisture content, especially where salts are present), thermography can sometimes be used quantitatively to assess the contribution of condensation or other excessive surface moisture to damp. This is done by cross-referencing the surface temperatures recorded by the camera against the ambient dewpoint temperature at the time the thermal image was made, or with surface humidity thresholds for mould growth. The camera settings must be carefully tuned to the characteristics of the surface and the environment under investigation, including the emissivity of the surfaces, and accounting for reflected temperature. Note that if thermal images are made on a single visit, the outcome will present conditions solely at that time. Since surface conditions can vary significantly, for example, during warm fronts or periods of intensive occupancy, contextualisation is vital. This means assessing ventilation and benchmarking readings against quantitative measurements of moisture content, both at the surface and deeper within the wall. QUASI-QUANTITATIVE AND RELATIVE MOISTURE METERS The quasi-quantitative techniques most widely used by surveyors are relative electrical readings, taken using handheld meters. Broadly, three types are commonly available: • resistance meters (with pins) • capacitance meters (with plates or bulb) • microwave meters (with plates). All respond to the presence of moisture in different ways and this can be turned to advantage by using two or more techniques on the same area of wall. None directly measure moisture content (MC); instead, the electrical signals they record are converted to arbitrary values. Without specific calibration to the specific materials and homogeneity of the masonry in the wall, they cannot provide absolute measurements. Instead, readings are used to reveal patterns of varying moisture levels across the wall or over time. The use of electric moisture meters for assessing masonry moisture has become contentious because they are so often used in isolation with little or no understanding of their limitations. If these limitations are accounted for as part of the assessment, electric meters can be helpful for assessing the distribution of moisture. The fundamental errors and uncertainties can be partially addressed by increasing the number of measurement locations; and, to some extent, readings can be benchmarked against the results from invasive sampling to increase information and accuracy. The principal uncertainty arises from sensitivity to electrolytic solutions: masonry walls inevitably include salts in solution in the pores, which means that readings will often be deceptive. Resistance meters and, to a slightly lesser degree, capacitance meters must be recognised as responding to both the moisture and the salt content of the masonry. Embedded metals such as cramps, fixings or hoop iron will also induce high readings. On the plus side, electrical meters do not cause significant damage to a wall. Grid measurements may be taken with a fine granularity over a wide area to create a map of moisture that can be helpful for pinpointing the sources of problems. The finer the granularity, the more effective the mapping will be at ironing out measurement anomalies and variations in results due to other characteristics of the masonry, that is, between stone, void or mortar. Furthermore, measurements can be repeated at later dates to monitor how moisture distribution might be changing. In some cases, the relative measurements can be roughly calibrated either by measuring areas of the same wall that are clearly dry, or more accurately via invasive sampling and gravimetric measurement at a statistically valid number of locations across the full range of the meter’s scale. After a baseline has been established, relative measurements may be all that is necessary to monitor trends. It can also be very useful to combine techniques for an indication of the causes behind a pattern of damage. For example, to assess the contribution of condensation and/or hygroscopic moisture, capacitance measurements (which reveal variations in salt and moisture levels near the surface) can be cross-referenced with microwave measurements (which can show moisture distribution deeper within the wall) taken at the same locations. A sufficiently fine grid of corresponding measurements can provide a threedimensional picture of the distribution of relative levels of moisture and salts between the wall surface and its core. Resistance meters The electrical resistance of a material decreases when its moisture content increases, so the resistance between the pins of a resistance meter can be used as a proxy for moisture content in some materials. While resistance meters can be calibrated to give accurate measurements of moisture content in wood according to species and temperature, they cannot measure masonry moisture content due to the strong variability in material properties such as density, porosity, and equilibrium moisture content. Of particular importance is the fact that salts in solution significantly increase electrical conductivity, giving Thermal and visible spectrum images of a cone of damp extending down from a defective parapet gutter
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