The measurement of moisture in building materials has been of importance to building professionals for many years to aid them in diagnosing the nature and cause of building defects. Likewise building researchers have developed an extensive selection of measurement approaches to aid them in more detailed studies of the fundamental physical processes that underlie moisture transport. This paper reviews current moisture measurement practices within the UK, and examines the more sophisticated techniques being utilised, including three particular techniques: dual probe heat pulse method; time domain reflectometry; and more sophisticated electrical approaches. These three are currently the focus of a study to evaluate how advanced research techniques can be applied to the in-situ measurement of moisture contents in practice. Practical application: This paper reviews the current UK application of measurement techniques to assessing the moisture content of building materials. It discusses the differences between the techniques used by building professionals and building researchers, and highlights potential areas currently in development which may yield more advanced site measurement options in the future.

Whilst considerable research has been carried out on the process of moisture transmission through porous building materials under a concentration gradient with isothermal conditions, limited experimental data are available on the influence of temperature gradients on moisture transfer rates. Such thermodiffusion can be predicted from irreversible thermodynamics, however, its significance to concentration-driven transfer in materials has not been definitively established. Models for the prediction of moisture movement in building structures generally neglect such effects, and rely on moisture transport properties based on isothermal measurements. This paper describes an investigation to determine the significance of non-isothermal effects on the total moisture transfer through building materials. The investigation concluded that the vapour pressure gradient is the critical driving potential for moisture transfer, whilst thermodiffusion is not significant. Practical application: Building professionals can be confident that the use of vapour permeabilities of building materials measured under isothermal conditions are satisfactory for the prediction of moisture transport through building envelopes under temperature gradients: no correction for thermodiffusion effects is necessary

Time Domain Reflectometry (TDR) has been used since the 1980s for measuring the moisture content of soils. The principle of TDR is based on measuring the reflection time of an electromagnetic signal sent down a set of waveguides that are inserted into the material. This paper reports research work examining the suitability of TDR for measuring the moisture content of building materials. Results suggest that TDR is an effective way of monitoring relative changes in the moisture content. Measurement of absolute moisture content is more complicated as it requires a calibration function, and the existing empirical relationships used in the soil sciences consistently overestimate the moisture content. A semi-empirical calibration relationship has been found to offer a better approach to absolute moisture content evaluation with TDR. Practical application: This paper reports research work examining the suitability of TDR for application to the measurement of the moisture content of building materials. It finds that TDR can rapidly identify changes in the relative moisture content and may therefore be suited to monitoring the long-term moisture behaviour of a building material in situ. Application of the technique to the measurement of the absolute moisture content is more difficult; further work is needed to address the accuracy of absolute measurements

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The successful application of moisture transport models to building envelopes requires accurate values of moisture permeability. Unfortunately, although a considerable amount of information is available, much of this is in a form which is of limited use to a designer. This paper examines the presentation of permeability data in terms of the concept of differential permeability. This allows permeability values to be predicted accurately for the humidity conditions to which materials are exposed in practice. Several suggested mathematical forms of the permeability-humidity functions have been compared by applying them to experimentally determined permeabilities for four representative building materials. From this comparison the functional form which would provide the most appropriate basis for a standardised system of data presentation has been identified.