R.C. McLean

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.

Testing building materials

R.C. McLean and G.H. Galbraith of the Division of Therm‐Fluid and Environmental Engineering at the University of Strathclyde and C.H. Sanders of the Building Research Establishment, Scottish Laboratory describe an experimental and mathematical procedure for the specification of differential permeability. Functions are developed for a number of commonly used building components.

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.

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...

The comparability of water vapour permeability measurements: Investigation assessing comparability of measured vapour permeability values obtained from range of laboratories throughout European community

The authors establish the extent and causes of dispersion between results in order to provide background information for the formulation of a unified test standard. In spite of detailed instructions, several laboratories deviated from the specified test requirements and instrumentation accuracy.

Vapour permeability: Suitability and consistency of current test procedures

The avoidance of condensation problems in the future depends on designers having available accurate and reliable vapour permeability values. The emergence of free trade within the European Community requires the transferability of measurement data between countries, as well as between individual laboratories. This paper describes a recent European intercomparison carried out to assess the comparability of existing permeability measurement techniques, and to provide the information required to enable future international standards to be properly formulated. Identical samples of two materials were tested by eleven laboratories covering thirteen European countries. At the beginning of the project detailed instructions were issued on all aspects of the test procedure. Even with such an approach, many of the participants failed to comply with all of the test requirements, and the data produced exhibited unacceptably large variations between laboratories. The major contributory factors for this are discussed, and recommendations on a possible approach to the development and implementation of a future international test standard presented.

High-temperature occupational exposures: Investigation and control

Because of the dangers associated with occupational heat stress, the exposure of individuals working in high -temperature environments must be carefully controlled. This paper describes how an acceptable work pattern, which restricts exposure to a safe limit, can be established. The method adopts a rational heat transfer approach. It is based on calculating the rates of energy exchange between an individual and the surroundings. The processes involved — convection, radiation, evaporation and conduction — are examined and the defining equations explained. Thereafter, the physiological constraints appropriate to hot exposures are considered and, on this basis, a numerical assessment procedure is proposed which allows the evaluation of maximum exposure and minimum recovery periods for acclimatised workers. An example is given illustrating the application of the procedure.

Estimation of design ventilation rates for buildings

Building fabric heat loads have decreased in recent years as standards of thermal insulation have improved. This in turn has led to an increase in the relative importance of the infiltration component of the total heat loss. The quantification of this component requires the accurate assessment of air infiltration rates under relevant environmental design conditions. Unfortunately, techniques for such assessments are much less well developed than those for calculating fabric heat loss and in many cases building services designers still rely upon the use of empirical estimates which are often unrealistic. This paper presents a case study in which a combined measurement and modelling procedure is applied to provide the designer with a more realistic estimate of air infiltration rates for an existing building. The basis of this work involves the use of the fan pressurisation measurement technique to determine actual leakage characteristics for the building envelope. This information provides real input data for a nodal airflow computer model, the results of which can then be used to estimate appropriate design ventilation rates. The case study involves application of the technique to a large secondary school undergoing refurbishment, and shows clearly that empirical estimates could have led to the installation of a severely undersized heating system. The measurements, in fact, highlighted a problem of excessive infiltration rates which required remedial action. The effectiveness of this remedial action was then confirmed by further measurements and remodelling was carried out to establish the final infiltration rates to be applied by the design engineers.