Context 164 - May 2020

C O N T E X T 1 6 4 : M A Y 2 0 2 0 13 The war on damp good indicator of moisture content,provided that conditions are not saturated. The method is used to monitor relative changes in a wall,and can be repeated at intervals. The underly- ing principle is that equilibrium is achieved between the moisture content in the masonry and the timber dowel. However, there are limi- tations: the dowel and the surrounding fabric require good contact but often, to enable the dowel’s easy retrieval, there is a small air gap that can affect the readings. Second, the dowel requires time to equilibrate to the environment, which may cause a considerable lag in the values obtained, and once saturated can give ‘wet’ readings for a long time. 5 Nonetheless, in laboratory tests the timber dowel gave a good range of readings, and expe- rience in the field suggests that this method can be a reliable indicator of moisture content, provided that conditions are not saturated. The method is used to monitor relative changes in a wall, and can be repeated at intervals. The underlying principle is that to 30 cm, with the Endo probe inserted into a narrow borehole. Using the non-destructive sensor heads, sub- surface information can reportedly be revealed to a depth up to 80 cm using four different sensing heads (3 cm, 11 cm, 30 cm and 80 cm). Depth penetration tests were carried out on the 3 cm and 11 cm heads.The results for the 3 cm sensing head were reliable but the results for the 11cm heads were ambiguous. The microwave moisture meter provides a spatial distribution of moisture and is not affected by salts. However, materials such as metals, voids and rough surface textures can interfere with the readings. If calibrated, the meter can give quantitative readings, but it is most reliable as a comparative technique. More work is needed to be able to correlate the values of a range of materials to known moisture scales such as relative humidity. This will help in the interpretation of the readings. So far when used in situ the measurements have been carried out with other techniques for this reason. Both the microwave moisture meter and infrared thermography, another non-destructive scanning technique, can analyse large areas at a glance and provide moisture maps that can be overlaid with detailed information of condition. However, infrared thermography does not measure moisture, but rather the temperature of a surface. There are two forms of thermography: pas- sive and active. Passive thermography captures the natural thermal patterns of a surface, where blue/purple areas are ‘cold’ and red/yellow areas are ‘warm’. Active thermography requires energy (heating or cooling) to create tempera- ture differentials. This method can sometimes Regimes of moisture storage in hygroscopic porous building materials Common methods of moisture assessments categorised by destructive (direct and indirect) and non-destructive techniques Techniques categorised by quantitative, semi- quantitative and qualitative methods, and different strategies of moisture assessment be used to locate sub-surface moisture, to diagnose roof defects after a warm sunny day, for example. It relies on the fact that water has a higher thermal capacity than most building materials and retains heat for longer, so in this instance wet areas will appear warm, and dry areas will appear cold. In most situations, a qualitative analysis is sufficient. If quantitative measurements are required, the emissivity of the material and the reflected apparent temperature need to be determined. In general it is good practice to input these values whenever this method is employed. Thermal imaging has many limitations: the most critical is to carry out the survey during appropriate weather conditions as otherwise the results can be misleading. Further, background knowledge of the site is necessary as what may appear to be moisture may actually be missing insulation, air leakage or other deficiencies unrelated to moisture. Overall, the results from the research sug- gests that most of the techniques tested in the 5 According to Paul Baker, Glasgow Caledonian University, it takes nine days for a wooden dowel to equilibrate to the surrounding masonry. See Baker, P, Galbraith, G, McLean, RC and Nicol, D (2007), Evaluation of the use of wooden dowels as a technique for the measurement of the moisture content in masonry , Engineering Historic Futures Research Report.