The Building Conservation Directory 2020

36 T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 0 C AT H E D R A L C O MM U N I C AT I O N S BUILDING PATHOLOGY Established and emerging technologies LYNDA JUBB and STEVE BURY V ARIOUS NEW technologies are converging which provide new data for diagnosing dampness in historic buildings. These emerging technologies are allowing pathologists and other specialist surveyors to base their diagnoses on a broader range of evidence, reducing their reliance on hypothesis-based diagnostics. Long established non-destructive moisture measurement technologies such as resistivity and capacitance meters have certain weaknesses, while the analysis of core samples (using gravimetric moisture analysers for example) has the drawback that sample-taking is a destructive process. Resistivity meters are designed to measure the reduced resistance in a current between electrodes in damp timber. However, they are less useful for masonry and they tend to overestimate dampness in the presence of salts where the resistance is further reduced. Furthermore, they only measure resistivity at the surface. Capacitance meters measure variations in the electrical capacitance of the substrate caused by moisture content at a depth of up to 40mm and are suitable for masonry as well as timber. Capacitance readings are relative (uncalibrated) and are prone to false readings where conductive materials lie under the surface (such as foil- backed plasterboard) since these can also measurably disrupt the electric field in ways that resemble raised moisture content. Gravimetric moisture analysers measure the weight difference between samples when fresh and once dried. This form of destructive test is preferred in BS 5250:2011 + A1 2016 Code of practice for control of condensation in buildings , not least because samples taken from different depths and locations in a structure can provide a much better picture of moisture movement through it. These small, automated instruments can be used on site to dry and weigh samples quickly, for immediate analysis of moisture content, but the kiln-drying of samples to assess moisture content by weight is arguably simpler and safer. Both rely on removing samples from the building which are then destroyed, and the collection of a suitable number of samples can be highly invasive. An alternative approach is to insert timber dowels into bore holes as their moisture content is much more easily measured using resistivity meters, either on site or in the lab. However, the boring process is also destructive, and the technique requires an interval between embedding the dowels and measurement, which makes it unsuitable for use in single- visit inspections. The development of litmus dowels helps with fungus detection by indicating chemical changes at depth, such as the production of oxalic acid by dry rot mycelium as it probes for nutrition. The detection of fungus using visual clues and odour is another useful indicator of moisture content. For some people the odour of fungi such as dry rot (Serpula Lacrymans) is particularly noticeable, but for others some training is required, while sniffer dogs, or ‘rot hounds’ as they are fondly known, have an even greater sensory range. Visual clues backed up by hand-held moisture readings provide the basis for an hypothesis-led approach to buildings pathology A microwave moisture meter – one of the emerging technologies which is providing more reliable data on conditions at depth in masonry