12 C O N T E X T 1 6 4 : M A Y 2 0 2 0 SOKI RHEE-DUVERNE Assessing moisture in porous building materials Despite the plethora of techniques that are available for assessing moisture in traditional porous building materials, none is completely effective, and all have their limitations. All traditional building materials are semi- porous and contain some inherent moisture. It is important to differentiate between a building in its ‘natural’ state, that is in equilibrium with the surrounding air, and a building that is suffering from damp. Inherentmoisture is usually not problematic unless it accumulates over long periods. Damage occurs when there is an excessive amount of moisture, known as free moisture, which is caused by liquid water. 1 Different ranges of measuring moisture are suitable for differing ranges of moisture condi- tions. Some methods are appropriate in the hygroscopic range at relative humidities below 95-98 per cent, while others are more applica- ble in the capillary range at relative humidities above 98 per cent. There are many ways to measure moisture, some destructive and others not. Moisture content can be measured either directly or indirectly, based on a variety of physical phenomena. Some are quantitative, measuring absolute moisture content. The majority are semi-quantitative, meaning that they can relate readings to the relative moisture content of materials.If calibration of these techniques is possible, readings can represent actual moisture content. Both semi-quantitative and qualitative methods are reliable only when used comparatively. Differing strategies are available, depending on what is required. Despite the plethora of techniques avail- able, none is completely effective, and all have their limitations. Comparative information on performance and how they should be used in situ is lacking. There is a lack of confidence in their accuracy and their comparability. There are difficulties in interpretation as techniques are based on different scientific principles.To address this knowledge gap, Historic England commissioned Heather Viles of Oxford University to compare the performance of different methods of assessing moisture in porous building materials. 2 The research tested 12 non-invasive and five invasive moisture measurement methods on sample blocks of masonry in the laboratory under controlled conditions. They examined how the techniques performed under drying and wetting conditions by plotting gravimetrically determined mois- ture contents against individual meter readings. The gravimetric test method is the most accurate as it determines the absolute moisture content by comparing the wet weight of a sample to its oven-to-dry weight. It is often used as a benchmark for comparing all other methods or for calibration. 3 However, it can still give a wide range of moisture contents across one wallbecause of natural material variability, contaminants and the non-homogeneity of tra- ditional building materials. 4 In most instances, gravimetry is unnecessary, unfeasible and sometimes not permitted as it requires a sample of the material to be taken. It cannot be used to compare moisture contents of dissimilar masonry materials because of their differing moisture storage functions. The method should not be confused with gravimetric methods using proxy materials. One well-established example of this is to insert timber dowels into predrilled holes and leave them there until their moisture content stabilises. This proxy material can then be removed for gravimetric analysis, minimising the destruction of original fabric. Using timber dowels in this way is inexpensive and can be a The passive thermal image of a wall has detected moisture ingress from the chimney on the right- hand half of the rear east elevation of a house in Cumbria. The purple indicates damp areas. 1 Straube, John (2006) ‘Moisture and Materials’, Building Science Digests 138, October, www. buildingscience.com/ documents/digests/ bsd-138-moisture- and-materials 2 Viles, H, Zhang, H and Orr, S (2019) Unpublished report for Historic England, Damp Monitor: a comparative evaluation of methods to monitor moisture in historic porous masonry materials , School of Geography and the Environment, Oxford University 3 ASTM (2003), Standard Test Methods for Use and Calibration of Hand- held Moisture Meters , D4444-92 4 Brian Ridout has gravimetrically measured the moisture content of three courses of brick and found a range of moisture content values from less than one per cent to greater than six per cent. See Ridout, B and McCaig, I (2016), Measuring Moisture Content in Historic Building Materials , Historic England.