37 CATHEDRAL COMMUNICATIONS THE BUILDING CONSERVATION DIRECTORY 2024 PROFESSIONAL SERVICES 1 MEASURING MOISTURE IN MASONRY MATTHEW WELLESLEY-SMITH ISSUES CAUSED by excessive moisture in masonry buildings range from minor blemishes on wall finishes to reduced thermal comfort and occupant health, and ultimately to severe deterioration of wall finishes, facing stone or brick and even building structure. For this reason, it is often assumed that accurate measurement of moisture in masonry is necessary to mitigate these risks. However, measurement in isolation is not a useful exercise. Simplistic concepts of ‘damp’ and ‘dry’ make little sense when you consider that masonry will always incorporate some water. Even in a wall with no moisture problem, there will be water molecules bound to pore walls, capillary condensation within pores and unbound water moving through the pore system via surface diffusion or liquid movement via capillary pressure. Any attempt to assess moisture levels in masonry must be framed within the context of the symptoms of damp and their impact, which will depend on how the space is used and the perception of the occupants. For example, high moisture content in the walls of a stone vaulted wine cellar or ground floor hearth might be acceptable if the only consequence is minor salt efflorescence, but where plaster is blowing or paint delaminating it may well be unacceptable even if the moisture content of the stone is in fact lower. In other words, moisture content is not the principal determinant of whether a masonry wall is suffering from ‘damp’. Good assessments of damp masonry thus combine measurement with subjective appraisal of the symptoms. By contrast, if moisture measurements are allowed to define the problem, flashing red LEDs or a bleeping measurement device can easily lead to reflexive (and possibly unnecessary) remedial work rather than considered, targeted solutions. If a surveyor has good reasons for needing to measure moisture as part of an assessment, the techniques used and the granularity of measurements need tailoring according to the context and the aims of the assessment. Various methods of measuring moisture content in masonry are available. Moisture content can be measured indirectly by relative measurements giving qualitative or quasi-quantitative results, or directly giving quantitative results of absolute moisture content of a wall sample (not necessarily the wall). Choice of the most appropriate method depends on the purpose of the investigation. If the aim is preventative maintenance, identifying building elements vulnerable to damage and assessing likely sources of moisture, then qualitative and quasi-quantitative measurement may be sufficient. But for defining the scope for remedial work to address an obvious issue such as an overflowing hopper, quantifying the level of moisture in a specific volume of masonry might be required to determine the potential drying time required. In masonry, quantification requires invasive testing, such as taking drilled samples. The use of different techniques allows these invasive tests to be minimised, the goal in most contexts being to gather sufficient information to determine an appropriate solution, no more. It is therefore sensible to begin with noninvasive techniques, introducing invasive measurements only if they are needed. Measuring moisture in masonry is an imprecise science even when using invasive methods that provide the absolute moisture content for a particular sample. Whether an elevated result is important depends on when and where the sample was taken, from what material, at what depth and height in the wall and on how it compares with results from samples or measurements taken from elsewhere. It is vital that surveyors have a clear understanding of the strengths and limitations of each technique at their disposal and how the measurements will contribute to their requirements. In this article, the techniques available to building surveyors are introduced, together with their intrinsic benefits and limitations and suggestions for how they can be employed to provide useful information. A further article to be published later will examine more innovative measurement techniques such as TDR that can be used for monitoring masonry moisture content over time. QUALITATIVE TECHNIQUES The fundamental technique for investigating damp is always a thorough visual assessment of the condition of the building, both inside and out, which can then be supplemented by qualitative and quantitative investigation if necessary. The most effective assessment of damp involves a hierarchy of complementary approaches. It begins with the surveyor’s senses and their experience of possible causative defects; in this, measurement plays no more than a supporting role. In many cases, it simply provides additional evidence of a trend or spatial distribution of moisture already obvious to the trained eye, or at least deducible from experience. Surveyors may find that some Solid masonry walls absorb and release moisture naturally, and even relatively high levels of moisture may have little effect on the interior fabric and comfort, so measurement should always be combined with a subjective appraisal of the symptoms: is the level of moisture causing harm?
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