Hygroscopic Building Research Articles

Development of Hygrothermal Reference Year for Hygrothermal Simulation of Hygroscopic Building Construction for Guangzhou

For developing a hygrothermal reference year (HRY) to support the hygrothermal simulation of hygroscopic building construction in Guangzhou, a typical hot-humid city located in south China, the study first clarifies the correlation between the hygrothermal simulation results and the meteorological elements based on a 10-year baseline simulation with the hourly meteorological source data series obtained from the city weather station (CWS, from 2010 to 2019), then develops the HRY with consideration to typical construction conditions, and finally evaluates the representativeness of HRY. It shows that the simulation results of HVAC demand, indoor hygrothermal environment and exterior walls moisture content are significantly correlated with the air temperature Te, relative humidity RHe, and the normal rain RN. Based on this correlation, Te.mean, RHe.mean and RNsum are used as the indexes for selecting typical months from the source data series, which are then spliced to generate the HRY. A parallel simulation comparison among four model groups accordingly with HRY, CWS as well as two commonly used meteorological data, Meteo and epw, as outdoor climate conditions, shows that the curve similarity of monthly simulation results between the HRY model group and the CWS group has been significantly improved, and the annual simulation results of the HRY group are also in good agreement with the CWS group.

Numerical and experimental estimation of building wall heat flux in presence of moisture transfer

Insulating building envelope is an efficient way to increase building energy efficiency and minimize greenhouse gas emissions related to heating. After building refurbishment, on-site measurements are suitable for verifying the actual thermal transmission properties of plane building components. For instance, the standard ISO 9869-1 describes a HFM method based on the measurement of surface heat flux with heat flow sensor (HFS). This method has been extensively investigated in the literature and successfully applied on vapor tight building walls. Nevertheless, hygroscopic building insulation materials (like biobased materials) are increasingly used, and heat transfer are coupled to moisture transfer within the wall. In this case, the HFS acts as a vapor barrier: it modifies the local moisture transfer and the associated latent heat flux. In this view, this study aims to clarify what it is measured with HFS in presence of moisture transfer. The question is first treated by numerical simulation of heat and moisture transfer within hygroscopic building wall. Then, experiments are carried out on hygroscopic building insulation where the moisture effects are exaggerated.

Moisture buffer value for hygroscopic materials with different thickness

Moisture buffering is the capacity of hygroscopic building materials or adsorbents to mitigate the humidity fluctuations in the indoor environment through the adsorption/desorption phenomenon. NORDTEST proposed the moisture buffer value to quantitively characterize the moisture buffering capacity of porous materials. Materials with high moisture buffer values have a higher ability to passively control indoor humidity conditions and thus reduce building energy consumption and improve indoor thermal comfort. However, the ideal moisture buffer value theory is based on the assumption that the thickness of the studied materials should exceed the penetration depth of that material. This assumption significantly limits the application of moisture buffer value theory to thin-layer materials, such as textiles, desiccant coating, wallpaper, wood wall panels, etc, which represent a large proportion of moisture buffering in the built environment. In this study, we developed a new numerical model to calculate moisture buffer values for hygroscopic materials with different thicknesses. Experimental measurements were also carried out to validate the solutions. The moisture buffer value of a new kind of desiccant (metal-organic frameworks) under different thicknesses were measured by climatic chamber tests. Simulated results showed agreement with both the measurements and theory. The results indicate that the new moisture buffer value model can be used to find the optimal thickness of desiccant coating or hygroscopic materials for autonomous indoor humidity control.

Impact of humidity on formaldehyde and moisture buffering capacity of porous building material

The aim of this article is to present a validated model to study the influence of humidity on formaldehyde sorption of building material and the effect of formaldehyde and moisture buffering capacity of hygroscopic porous material on indoor air environment. New empirical formulas proposed to describe the diffusion and partition coefficients as functions of humidity based on previous experimental data are incorporated in a coupled moisture and pollutants transport simulation model. The numerical model that takes into account the effect of RH (or moisture) in building materials on VOC diffusion can be used to simulate VOC emissions/sorption from building materials. The developed model is implemented in the environment SPARK (Simulation Problem Analysis and Research Kernel) using finite difference technique. The model is then applied to study the hygric and formaldehyde (FOR, a water soluble VOC) behavior of calcium silicate (CS) subjected to different dynamic conditions of RH and formaldehyde. The results obtained in this paper help to emphasize the importance of moisture and formaldehyde buffering capacity as a new key parameter when selecting clean and hygroscopic building materials in building design because they can contribute to dampen indoor RH and formaldehyde variations. In addition, the impact of RH variation is significant and needed to be taken into account in the simulation to correctly predict the indoor formaldehyde concentration. The model developed in this paper can also be used to optimize the hygric performance and IAQ in building.

Laboratory study on hygrothermal behavior of three external thermal insulation systems

This work focuses on the hygrothermal behavior of three external thermal insulation systems: one ETICS including EPS insulation and two ventilated cladding systems including either mineral or biobased insulation. These systems were applied on a rendered hollow concrete wall and tested simultaneously between two climatic chambers. Thermocouples, humidity sensors and heat flux sensors allow investigating the hygrothermal behavior of the retrofitted wall at different stages: just after the application of insulations systems, during safe and critical use. The measured data are compared to numerical results obtained by solving a heat and moisture transfer model.Results highlight the key role of adhesive on ETICS, which provides a significant moisture source during the application and forms an additional moisture transfer resistance within the wall. Results on ventilated cladding systems show that using biobased insulation may delay and even prevent the risk of interstitial condensation. The comparison between numerical and experimental results is satisfying for temperature and heat flow density, but underline the sensitivity of relative humidity to the sorption capacity of hygroscopic building materials. In addition, the systems design has a great influence on the condensation risks.

Humidity control effect of vapor-permeable walls employing hygroscopic insulation material.

Airtight construction and high-performance thermal insulation materials are commonly considered important building features to enhance indoor thermal comfort while reducing thermal load. However, when water vapor is generated in such airtight indoor spaces, it cannot be discharged to the outside, causing interstitial condensation and subsequent intrusion of moisture into the walls. Hygroscopic building materials such as cellulose fiber insulation (CFI), characterized by high water capacity, are a potential countermeasure against such condensation. In this study, the humidity control performance of external walls containing CFI was evaluated using data measured inside a demonstration house and calculated by numerical simulations based on thermodynamic chemical potential theory. The changes in moisture adsorption and desorption were then evaluated for different wall constructions and different climate conditions using a parameter sensitivity analysis. Finally, the effective application of CFI to prevent interstitial condensation was confirmed by comparing different wall compositions.

Building incorporated bio-based materials: Experimental and numerical study

In building environments, several research on dynamic moisture storage in hygroscopic building materials has improved interest in moisture buffering capacity of bio-based materials and demonstrated the potential of these materials to improve thermal comfort and buildings energy consumption. This paper presents an investigation, which aims to compare the hygroscopic behavior of different bio-based materials to identify their hygric proprieties. For this, an experimental facility is used to measure the moisture buffer value (MBV) for different categories of building materials. Thereafter, a dynamic simulation has been carried out by using TRNSYS software in order to evaluate heating and cooling loads. The results showed that the wood-cement composite still has excellent moisture regulator when compared to wood panels or fibrous materials. It has a buffer capacity superior to 3 [g/m2.% RH] very close to that of wood fiber panels. The simulation results of the different cases showed that the insulation reduces significantly total energy demand, but it does not lead heat evacuation in the summer season. Thus, from this study, it can be concluded that a natural ventilation mechanism is recommended to improve energy efficiency in the building.

A PC-tool to calculate the Moisture Buffer Value

Hygroscopic building materials have the ability to moderate the relative humidity variation without the need for active systems. The moisture buffer phenomenon can be assessed by way of the Moisture Buffer Value (MBV). Some authors have pointed out that the MBV is sensitive to several parameters, however, there is no model that involves all them.The aim of the developed PC-tool is to take into account all these variables to calculate the MBV of hygroscopic building materials. The material hygroscopic properties will be needed to solve the moisture storage and transport inside the porous materials and consequently to predict its MBV.

Material moisture content of wood and cement mortars – Electrical resistance-based measurements in the high ohmic range

The performance of porous and hygroscopic building materials is closely connected to its sorption properties and ability to get wet. Weight, mechanical, acoustic, and thermal properties as well as the resistance to discoloring and decay causing organisms are affected by moisture. For various reasons and purposes it is therefore recommendable to monitor the moisture content of building components continuously. The most common and most easily applicable methods are electrical resistance measurements. They can be applied to wood and wood-based materials as well as to mineral products such as mortar and brick. However, resistivity measurements require material specific characteristics and a temperature compensation since both parameters have a significant effect on electrical conductivity.This study aimed on developing a model to determine the moisture content at any temperature for different building materials such as native and modified wood as well as untreated and hydrophobized mortar. Therefore, the electrical resistance was measured with a data logging device in the giga ohm range to obtain values at low moisture contents. The model enables measurements at a wide range of moisture contents and suited with an acceptably high accuracy by using the appropriate resistance characteristic for each building material.

Temperature dependence of sorption isotherm of hygroscopic building materials. Part 2: Influence on hygrothermal behavior of hemp concrete

In the part I of the paper, a temperature effect was experimentally observed for the relative humidity measurement. This effect was correlated to the temperature dependence of the sorption isotherm. In this part II of the paper, the influence of this temperature dependence is investigated on the simulated hygrothermal behavior of hemp concrete. Sorption isotherm was modeled through three approaches: a unique curve (like in WUFI), a temperature-dependent GAB model or the Clapeyron equation associated to the isosteric heat of sorption evaluated in the part I of the paper. Comparison between simulation and experiment is performed for the sample presented the part I of the paper and for a wall subjected either to a slow temperature decrease toward 0°C or to a fast temperature increase due to solar radiation. Whatever the investigated case, results underline that accounting for the temperature dependence improve the relative humidity prediction. Nevertheless, a great sensitivity to the modeling approach and to the input parameter was observed.

Temperature dependence of sorption isotherm of hygroscopic building materials. Part 1: Experimental evidence and modeling

The knowledge of sorption isotherm is of high importance when evaluating the performance and the durability of building envelope. Whereas the influence of hysteresis was often investigated, less attention has been paid to the influence of temperature. In the present paper, a specific experimental protocol is defined to investigate the influence of temperature on relative humidity variations within three building materials (Autoclaved Aerated Concrete, wood fiber insulation and hemp concrete). The measurements are analyzed in terms of a hygrometric coefficient κ [%r.h./°C], defined as the maximal relative humidity amplitude against the maximal temperature amplitude, and are compared to three modeling approaches: temperature dependent sorption model, Kelvin equation and Clausius-Clapeyron equation. Discussions show that the third approach is relevant and that it can be used to evaluate effortless the isosteric heat of sorption and the temperature dependence of the sorption isotherm.

Evaluation of Mould Growth on Wall Surface in South China

Mould contamination has negative consequences on the indoor air quality, and then on the health of the residents. It has an important significance for studying the mould contamination on building wall, especially in the south China. A dynamic mathematical model for simulating the coupled heat and moisture transfer within multilayer hygroscopic building wall was proposed. Temperature and humidity are critical for mould germinating. With the simulation results from the hydrothermal model, software WUFI-bio was used to evaluate the mould risk in the building. Once the humidity and temperature combination exceeds a limit, spore will germinate. Mould growth rate also was obtained, which was used for assessing risk level. The building with the red brick wall was studied as example, which is seated in Hengyang, Hunan province. Calculation period was a one year and time step was one hour. From the result, it can be found that there exists probability of mould growth in the red brick building in Hengyang, especially in winter and spring, and mould growth rate reaches the peak in February.

Efficiency of Different Basic Modelling Approaches to Simulate Moisture Buffering in Building Materials

The aim of this study is the numerical investigation of the capacity of porous hygroscopic building materials to damp indoor humidity variations due to external environmental loads and internal sources due to heat and moisture exchange. By means of numerical simulation, building material moisture content is computed by using a basic approach based on a diffusion model. Subsequently, a model incorporating the isothermal sorption curves of materials and complete thermal analysis is elaborated. The first modelling approach is more appropriate for material characterization even though it requires more time for modelling implementation and involves greater computational costs. The second modelling approach is useful for the assessment of hygro-thermal behaviour and energy performance of complex building components made of different materials. Moreover, this second approach can be easily applied to a 3D solid model of complex geometrical and architectural layouts. Results involve two different geometries. The first geometry belongs to a 1cm sized cube and represents the test system used in our study. The second one is representative of a usual building wall with a thermal bridge, consisting of different layers. From results analysis, it can be deduced that a more accurate numerical approach, using thermos-physical properties, porosity and hygroscopicity of materials and their corresponding sorption isotherm curves as input data, could be proposed for material characterisation and hygrothermal behaviour evaluation, in relation to the real physical indoor and outdoor transient climatic conditions. On the other hand, in many practical technical applications, our two proposed approaches can comprehensibly describe the investigated process combined with building-plant system energy performances, depending on the implementation process and computational costs we can implement.

An inverse modelling approach to estimate the hygric parameters of clay-based masonry during a Moisture Buffer Value test

This paper presents an inverse modelling approach for parameter estimation of a model dedicated to the description of moisture mass transfer in porous hygroscopic building materials. The hygric behaviour of unfired clay-based masonry samples is specifically studied here and the Moisture Buffer Value (MBV) protocol is proposed as a data source from which it is possible to estimate several parameters at once. Those include materials properties and experimental parameters. For this purpose, the mass of two clay samples with different compositions is continuously monitored during several consecutive humidity cycles in isothermal conditions. Independently of these dynamic experimental tests, their moisture storage and transport parameters are measured with standard steady-state methods.A simple moisture transfer model developed in COMSOL Multiphysics is used to predict the moisture uptake/release behaviour during the MBV tests. The set of model parameters values that minimizes the difference between simulated and experimental results is then automatically estimated using an inverse modelling algorithm based on Bayesian techniques. For materials properties, the optimized parameters values are compared to values that were experimentally measured in steady state. And because a precise understanding of parameters is needed to assess the confidence in the inverse modelling results, a sensitivity analysis of the model is also provided.

Hysteretic Behaviour and Moisture Buffering of Hemp Concrete

Moisture transfer in hygroscopic building materials affects the indoor air quality by exchanging moisture and buffering the ambient relative humidity variations. The paper deals with experimental and numerical study on hysteretic sorption behaviour of the hemp concrete sorption process. Experimental intermediate scanning curves of hemp concrete are measured and used to compare two hysteresis models, Huang’s model and Carmeliet’s model. An original method is achieved to fit the numerical results on the experimental ones leading to the identification of the main desorption curve. The most relevant model, Huang’s model, is implemented in a heat and moisture transfer model based on Kunzel formalism. The transient hydric response of hemp concrete submitted to cyclic hydric loadings is investigated and compared to experimental results issued from the literature. These investigations show the relevance to consider the hysteresis phenomenon into the model. Then, the influence of initial conditions is discussed. The results point out that transient response of hemp concrete strongly depends on the initial hydric state (initial moisture content as well as initial relative humidity).

Determination of Poroelastic Parameters of Porous Building Materials

Variations of sorption moisture in the capillary porous materials result in strong fluid-skeleton interactions due to molecular and surface forces, which produce moisture-induced deformation. The effect of moisture sorption on the non-linear elastic behavior of hygroscopic porous building materials has been experimentally investigated showing strong influence of moisture especially in the lower moisture content range. In the framework poroelasticity moisture influence on elastic behavior is described by two poroelastic coefficients, which present the fluid-skeleton coupling. This paper presents an application of the procedure for the determination of the coupling coefficients for medieval brick.

Experimental and theoretical investigation of non-isothermal transfer in hygroscopic building materials

In this paper a modified two-dimensional Luikov model for evaluating the non-isothermal moisture migration in porous building materials was proposed. The coupled heat and moisture transfer problem was modeled. Vapor content and temperature were chosen as the principal driving potentials. The coupled equations were solved by a numerical method, which consists of a finite difference technique with a fully implicit scheme in time. Two validation experiments were developed in this study. The evolution of transient moisture distributions in both one-dimensional and two-dimensional cases was measured. A comparison between experimental results and those obtained by the numerical model proves that they are fully consistent with each other. The modified model can be integrated into a whole building heat, air and moisture transfer model.

Nonisothermal moisture transport in hygroscopic building materials: modeling for the determination of moisture transport coefficients

A mathematical model for calculating the nonisothermal moisture transfer in building materials is presented in the article. The coupled heat and moisture transfer problem was modeled. Vapor content and temperature were chosen as principal driving potentials. The coupled equations were solved by an analytical method, which consists of applying the Laplace transform technique and the Transfer Function Method. A new experimental methodology for determining the temperature gradient coefficient for building materials was also proposed. Both the moisture diffusion coefficient and the temperature gradient coefficient for building material were experimentally evaluated. Using the measured moisture transport coefficients, the temperature and vapor content distribution inside building materials were predicted by the new model. The results were compared with experimental data. A good agreement was obtained.

An experimental data set for benchmarking 1-D, transient heat and moisture transfer models of hygroscopic building materials. Part I: Experimental facility and material property data

As numerical models of heat and moisture transfer in porous building materials advance and numerical investigations increase in the literature, there remains a need for simple accurate and well-documented experimental data for model validation. The aim of this two part paper is to provide such experimental data for two hygroscopic building materials (cellulose insulation and spruce plywood) exposed to 1-D and transient boundary conditions. Part I of this paper describes the transient moisture transfer (TMT) facility used to generate the experimental data as well as the uncertainty and repeatability of the measured data. The measured material properties are also presented to fully document the experimental data set and permit its use by other researchers.

An experimental data set for benchmarking 1-D, transient heat and moisture transfer models of hygroscopic building materials. Part II: Experimental, numerical and analytical data

This paper presents the experimental results on spruce plywood and cellulose insulation using the transient moisture transfer (TMT) facility presented in Part I [P. Talukdar, S.O. Olutmayin, O.F. Osanyintola, C.J. Simonson, An experimental data set for benchmarking 1-D, transient heat and moisture transfer models of hygroscopic building materials-Part-I: experimental facility and property data, Int. J. Heat Mass Transfer, in press, doi:10.1016/j.ijheatmasstransfer.2007.03.026] of this paper. The temperature, relative humidity and moisture accumulation distributions within both materials are presented following different and repeated step changes in air humidity and different airflow Reynolds numbers above the materials. The experimental data are compared with numerical data, numerical sensitivity studies and analytical solutions to increase the confidence in the experimental data set.