Staf Roels

Interlaboratory comparison of hygric properties of porous building materials

The precision of methods used for the determination of hygric properties of porous building materials was investigated. The study was performed in the framework of the EU-initiated HAMSTAD-project. Six laboratories measured the selected hygric properties of three porous building materials. While the most measured properties show acceptable agreement, yet, it was found that some of the existing standards or commonly accepted measurement methods need improvement. Most striking were large variations in the results of the vapour transmission tests performed in accordance to the existing European Standard.

Assessment method of numerical prediction models for combined heat, air and moisture transfer in building components: benchmarks for one-dimensional cases

The standardised Glaser method for calculation, prediction and evaluation of moisture performance is considered as rarely applicable. The present state of knowledge, analytical as well as experimental, concerning heat, air and moisture demands updating of standards. This paper presents five numerical benchmark cases for the quality assessment of simulation models for one-dimensional heat, air and moisture (HAM) transfer. In one case, the analytical solution is known and excellent agreement between several solutions from different universities and institutes is obtained. In the remaining four cases, consensus solutions have been found, with good agreement between different HAM models. The work presented here is an outcome of the EU-initiated project for standardisation of HAM calculation methods (HAMSTAD WP2).

A Comparison of Different Techniques to Quantify Moisture Content Profiles in Porous Building Materials

Several advanced non-destructive techniques are available to measure the evolution of content profiles with time, allowing the analysis of unsaturated flow and the determination of the moisture diffusivity of porous building materials. The reliability of six different techniques is investigated: the NMR-technique, the MRItechnique, the γ-ray attenuation technique, the capacitance method, the X-ray projection method and the TDR-technique. All of them were applied to measure the moisture content evolution during free uptake experiments on two building materials. Considering the limitations of some of the techniques, a good overall agreement is obtained. The work presented is an outcome of the EU-initiated HAMSTAD-project.

Determination of the liquid water diffusivity from transient moisture transfer experiments

The Boltzmann transformation method is used to determine the liquid water diffusivity from moisture content profiles as measured in a capillary water absorption experiment. An inter-laboratory comparison for analyzing the reliability of the determination method showed that the inaccuracy in the liquid water diffusivity is caused by scatter in the transformed data and by uncertainty in the boundary conditions at the intake surface and ahead of the steep moisture front. A methodology is proposed based on (1) the evaluation of the validity of the diffusion approach, (2) a simplified handling of the boundary conditions, (3) smoothing of the scattered data and (4) the evaluation of the quality of the determined liquid water diffusivity. For HAM (Heat-Air-Moisture transport) calculations values of the liquid water diffusivity for moisture contents higher than the capillary moisture content are disregarded. The liquid water diffusivity can be described by an exponential function limited at a lower moisture content bound. To describe the moisture diffusivity including liquid water and water vapour transports, a new parametric description of the moisture diffusivity is presented, which shows sufficient flexibility both in the hygroscopic and overhygroscopic ranges. When permeability is calculated from diffusivity, the permeability should monotonically increase with decreasing capillary pressure. In the hygroscopic region it should coincide with the measured water vapour permeabilities.

Measuring and simulating moisture uptake in a fractured porous medium

Water uptake in fractured brick samples is monitored with X-ray radiography. This technique can measure both the moisture profiles in the matrix and the height of the waterfront in the fracture during the experiment. It was found that the waterfront in the fracture could quickly reach the opposite side of the specimen, resulting in an extra water source for the surrounding matrix over the total height of the sample. For smaller fracture apertures, however, the waterfront in the fracture stagnates and moisture profiles in the matrix run ahead of the waterfront in the fracture. To simulate the experiments, a numerical model for unsaturated moisture transport in fractured porous media is developed. The model combines a discrete fracture model for moisture flow in a variable aperture fracture with a finite element model for unsaturated flow in the porous matrix. To achieve a stable numerical solution, movable nodes, following the moisture front in the fracture, are introduced on the boundary of the finite element mesh. The numerical results of height of rise in the fracture as well as of moisture profiles in the matrix show to be in good agreement with the measurements. The paper ends with a parameter analysis, investigating the effects of fracture aperture and matrix properties on the unsaturated moisture flow in fractured porous media. 2002 Elsevier Science Ltd. All rights reserved.

Determination of the Isothermal Moisture Transport Properties of Porous Building Materials

A multiscale network approach is proposed as a practical method for estimating the isothermal permeability of porous building materials covering hygroscopic and overhygroscopic ranges. The multiscale approach is based on the concept of examining the porous space at different levels of magnification, and it allows modeling of combined liquid water and water vapor transfer over a wide saturation range. The proposed method simply requires the knowledge of common moisture properties, such as the capillary pressure curve, the capillary absorption coefficient and water vapor permeability, which can be determined from standard experiments. Hysteresis between drainage and wetting due to air entrapment can be adequately modeled by considering only those pores that become filled by water. The calculated network permeability is validated comparing experimental and simulation results of isothermal capillary absorption and drying processes in ceramic brick and calcium silicate. It is shown that transient moisture processes of capillary absorption from a dry sample and the isothermal drying from an initially capillary saturated sample can be accurately modeled by a single nonlinear permeability and capillary pressure curve. The capillary pressure curve to be used is the main wetting curve, which describes the moisture content relationship from the dry state to the capillary moisture content, correctly taking into account air entrapment phenomena. The description of the absorption or drying moisture processes by a single or a double exponential diffusivity relation was found to be less accurate.

Modelling Unsaturated Moisture Transport in Heterogeneous Limestone

In this paper we investigate the influence of microheterogeneity on the phenomenological macroscopic material properties. The material analysed is Savonnières: a French layered oolitic limestone. Due to the complex microstructure of the material the moisture permeability cannot uniquely be determined based on indirect measurement data. To overcome this problem we present an upscaling technique based on the modelling of transport phenomena on the mesoscale. The technique allows to determine an equivalent macroscopic permeability based on the geometric configuration and the permeability of the composing constituents.

A comparison of the Nordtest and Japanese test methods for the moisture buffering performance of building materials

Two test methods, one worked out in a Nordtest project and the other available as a Japanese Industrial Standard, both developed to characterize building materials with respect to moisture buffering performance, are analyzed in detail by a numerical study on four different materials. Both test methods are based on a similar kind of dynamic loading, but the specifications of each test protocol vary. Therefore, the sensitivity of the test protocols is investigated by varying different protocol parameters. Subsequently, the practical applicability of the obtained values is investigated by confronting the values obtained for the four materials with the dynamic response of a small room with each of the materials used in turns as finishing material. Finally, the results determined according to the dynamic test protocol are compared with values calculated from steady-state material data.

Determination of the Moisture Capacity of Porous Building Materials

The moisture capacity, which is required to solve the isothermal moisture transport equation, is generally expressed by parametric functions covering both the hygroscopic and over-hygroscopic regime. The modality or number of analytical functions needed to describe the corresponding pore volume distribution is introduced as an important parameter for a proper description of the moisture capacity or capillary pressure curve. We used the Markov procedure to estimate the parameters. The method allows not only to evaluate the goodness of fit and the wellposedness of the parameter identification problem, but also to determine the optimal location of the experimental data in order to minimize the effect of errors on the estimated parameters. We found that the wetting capillary pressure curve, relevant for many building physics problems of hygroscopic capillary active materials, is preferentially described by bimodal functions or unimodal functions withsufficient flexibility towards the hygroscopic zone. The use of fixed values of relative humidity for determining the limit between hygroscopic and over-hygroscopic regime cannot be recommended as useful a priori information. This limiting moisture content is rather a fitting parameter and is found not to coincide with the knick point moisture content defining the transition from vapor to liquid permeability. The optimal location of the experimental data is highly dependent on the chosen functional model and on the considered material. The goodness of fit only slightly reduces when using a minimal number of optimal data points compared to an extended data set.

Determination of Liquid Water Transfer Properties of Porous Building Materials and Development of Numerical Assessment Methods: Introduction to the EC HAMSTAD Project

Implications of moisture in building and construction are of interest to the international community because of their huge economical consequences, including effects on health, maintenance and repair, retrofitting and conservation, as well as on common welfare. The present day knowledge offers a potential to tackle such problems, both in the design process and during the service life of building. In 2001, the European Commission initiated the project ‘‘HAMSTAD’’ (Heat Air and Moisture Standards Development) to propose a better modelling methodology than the traditional Glaser method. HAMSTAD focused on the development of draft standardisation procedures on determination methods of moisture transfer properties and a draft methodology for certification of advanced moisture modelling codes. To stimulate competitiveness and progress, the project was carried out following an ‘open methodology’ instead of a system of deterministic and prescriptive (pre-) standards. This paper outlines the project and highlights the main outputs, serving as an introduction to the following more detailed research papers resulting from that work.