G.H. Galbraith

Moisture measurement in building materials: an overview of current methods and new approaches

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.

Suitability of time domain reflectometry for monitoring moisture in building materials

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.

Moisture permeability data: Mathematical presentation

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.

Temperature gradient effects on moisture transport in porous building materials

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.

Nonisothermal moisture diffusion in porous building materials

The hygrothermal performance of building envelopes has been the subject of intensive research over the past decade, culminating in the development of a series of advanced computer-based simulation models. However, in spite of the considerable progress that has been made, a question which is not, as yet, fully resolved is the coupled effect of temperature gradient on moisture diffusion rates. As a result, the material transport data used as input for these models is generally determined from isothermal permeability measurements. This paper describes two investigations in which moisture flux experiments were carried out on small-scale material samples subjected to gradients in both temperature and humidity. In each case a different experimental technique was used and different materials were tested. An analysis of the results was undertaken in an attempt to identify the significance of any temperature-driven transport compared to the concentration-driven component. Le comportement hygrothermique des envelop...

Moisture permeability measurements under varying barometric pressure

The development of theories to describe combined heat and moisture transport through porous media has been the subject of a large volume of research over recent years. These theories are important for many areas of study including the investigation of the hygrothermal performance of building envelopes. This paper highlights the difficulties of obtaining separate values for the material vapour and liquid transfer coefficients, a knowledge of which is essential if combined heat and mass transport models are to be accurately solved. The determination of these individual coefficients is not possible using existing measurement methods which enable only a total moisture permeability to be measured with the liquid and vapour flux components combined. The possibility of carrying out measurements under varying barometric pressures to obtain this data is investigated and experimental results using particle board as a test material are reported and compared with results from a previously developed analytical approac...

A System of Permeability Specification for Use in Moisture Simulation Models

The successful application of moisture simulation models to building enve lopes requires accurate values of material permeability. Unfortunately, al though the presently available database is reasonably voluminous, much of the information contained therein is of limited use in such applications. This paper describes the specification of permeability data in terms of the differ ential permeability function. Several alternative mathematical forms are com pared using experimental results from a representative range of building mate rials and the most appropriate form is identified. The use of this function allows the accurate evaluation of permeability appropriate to the humidity conditions to which a material is subjected in practice. On the basis of theoret ical analysis, it is also suggested that this function allows the separation of the total moisture flux into its vapour and liquid components. This hypothesis has been tested for two materials using a prototype experimental procedure in which permeability measurements are made under different barometric pres sures.

Non-contact methods of measuring moisture concentration in external layers of building partitions. I—The influence of geometrical microstructure on the kinetics of moisture condensation on glass surfaces

Abstract This paper presents the results of the larger study on the potential application of laser radiation for the detection of phase transition processes. The proposed experimental method allows a dynamic study of the phenomena without mechanical intervention. This seems to be the best practical solution particularly when the access for the other methods is very restricted. The investigations give an evidence of the strong relationships between the rapid growth of water vapour condensate and the roughness of the surface. Condensation process therefore can be modified by a careful design of the geometrical structure of surface.

Evaluation of Discretized Transport Properties for Numerical Modelling of Heat and Moisture Transfer in Building Structures

Over the past decade, a large number of numerical models have been developed to predict heat and moisture transfer within building envelopes. In these models, the moisture transfer mechanism has been described and correlated by reference to the various transport phenomena and corresponding theories, viz. heat transfer and fluid flow. However, predicting the coupled heat and moisture performance of a building construction has never been a straightforward task, since a steady state situation hardly ever occurs and the transport properties (heat and moisture) of a material vary with moisture content and temperature. This paper discusses the transport phenomenon and the various numerical algorithms used in the discretization equations and how different algorithms affect the modelled results. Computer simulations have been conducted for different building materials and material combinations and comparisons have been made to evaluate the selection of discretized transport properties. Discrepancies in results are demonstrated between different mathematical interpolations, namely the Resistance(R) type formula and Linear(L)interpolation. Recommen-dations are given as guidance towards applying the most appropriate formulations for a given modelling scenario.

The Influence of Space Discretization on the Accuracy of Numerical Simulation of Heat and Moisture Transport in Porous Building Materials

It is well recognised that the accuracy of numerical solutions for coupled heat and moisture transport problems are highly dependent upon the space discretization regime adopted. While a range of possible space discretization methods are outlined in the literature, most of the commonly available simulation models for heat and moisture transfer through building constructions adopt the oneway expansion method. As part of the process of developing a new simulation model based on the Control Volume technique, the Authors have conducted a range of computer simulations to study the influence of space discretization on modelling results. Comparisons between different space discretization methods are presented which show that the two-way expansion method generally produces the best solution. This method has now been adopted by the Authors and has the advantage of avoiding the need to use extremely fine grids even when modelling complex building structures.