The Building Conservation Directory 2020

PROFESSIONAL SERVICES 1 37 C AT H E D R A L C O MM U N I C AT I O N S 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 Most surveyors who specialise in conservation will have been trained to be sceptical about hand-held moisture measures and will either rely heavily on observations (including visible signs, touch and smells) within an essentially hypothesis-led approach. The 2015 BRE paper Diagnosis of Damp tends to be heavily hypothesis-led, looking at construction and investigating details, and this approach to pathology has become ingrained largely because conventional moisture meters are so unreliable. Building practices too have been based on hypotheses that are now considered crude and simplistic, including invasive damp-proofing practices and, more recently, retrofitting measures designed to achieve high levels of insulation. Emerging technologies are allowing building pathologists to shift from hypothesis to evidence-based diagnostics, and to specify interventions that are better-informed. EMERGING TECHNOLOGIES Microwave moisture meters are bringing entirely new data to bear on diagnosis. These work on the dielectric principle; water molecules have polarity because the ions tend to arrange asymmetrically and that polarity is excited by the microwave radiation at a dielectric constant that is measurably different from masonry. Because microwave radiation can penetrate to a depth of up to 800mm in solid masonry, and is insensitive to salt content, building pathologists can now gather reliable data about the moisture content of masonry at depth using non-destructive techniques. The results, like capacitance meters, are relative but can be calibrated if the type of stone and construction are known. Thermal imaging cameras , which also operate in the electromagnetic range, are proving increasingly useful. Pathologists have always relied heavily on inspection using visible light and are likely to continue to do so - the changes they observe, whether dimensional, refraction or pigment are usually the best and most leading evidence. Thermal imaging cameras enhance the visible light spectrum in two ways; firstly, variations in surface temperature which are visible at the infra-red end of the spectrum are recorded by the camera as a thermal image; and secondly, the camera may be calibrated to record surface temperatures in absolute terms, with an accuracy of about 2°C or 2% achievable on even the most affordable models allowing the accurate identification of dewpoint locations. Variations in surface temperature are significant because moist masonry will tend to heat up or cool down sluggishly. So, as long as the ambient temperature is changing in the area under observation, say at dawn or dusk externally, or when the heating is turned off or on internally, the thermal performance of an otherwise uniform surface will be disrupted and the variations will be visible in the thermal image. Even in reasonably static circumstances, a damp but drying wall will tend to be cooler due to the evaporation of moisture. Thermal image diagnoses are most accurate where the source of moisture is condensation. Thermal images require some skill to interpret, but the equipment and principles are relatively simple to master as surveyors are already adept at recording using visible light images, and most infra- red cameras tend to capture the visible and non-visible light side by side. The rendering or colour scale of images can be set to highlight the dewpoint temperature for the temperature relative humidity of the room. This means that condensation events, which are sometimes very short-lived or marginal effects, can be caught by prolonged and repeated observations as the conditions change. Remote detection of temperature and humidity is the established method for these longitudinal studies, which are enhanced by sensitive thermal imaging cameras. When creating an evidence base, building pathologists can introduce a set of forced conditions to the temperature and humidity, emulating the seasonal and diurnal ranges to help identify the circumstances in which the condensation risk occurs. DATA MANAGEMENT AND MODELLING Recording exactly where images and moisture measurements have been taken has, in the past, been difficult or required identifying markers to be attached to buildings. This is undesirable where the surface finishes themselves are important and the study is examining moisture variations in the substrate. Accurately recording the position of all the data we collect is essential not just to eradicate bias in the data sets, it is essential to enable reliable storage, management and presentation. Improved setting out systems mean the monitor locations can be fixed in truly non-destructive ways and these rely heavily on advances in building modelling. The relatively low cost of large-sensor digital cameras coupled with image recognition software innovations and the sheer number-crunching capability of even an average processor, has brought photogrammetry back to the fore for 3D modelling. Laser scanners had for some time provided the best way of measuring a building in 3D, because early photogrammetry techniques tended to be time-consuming and cumbersome to use. Scanning is still preferable for many applications, and is merging with photogrammetry. The leading trend in laser scanner development has been to add better cameras that record images through 360° and capture HDR imagery to increase the quality of colourised point clouds. Thermal imaging capabilities have also been added to some laser scanners further converging the reflected radiation technologies. Commercial grade drones equipped with high-end photographic cameras, thermal imaging cameras, or even probes equipped with measuring sensors, allow these virtual models to include areas that were, until recently, extremely difficult to reach and next to impossible to accurately gather data from. The areas that have been monitored have tended to be the most accessible, but many of the most serious problems occur out of sight such as on rooftops, at the top of high walls, in floor voids or in other confined spaces. A thermal image taken at Lytham Hall, Lancashire investigating variations in the surface temperature of the Guiseppe Cortese plaster ceiling while planning repairs to the roof and internal gutter above (Both photos: Jubb Clews Ltd)