J. Cent. South Univ. (2012) 19: 669-674

DOI: 10.1007/s11771-012-1055-9

Development of experimental study on coupled heat and

moisture transfer in porous building envelope

CHEN Guo-jie(陈国杰)^1,2, LIU Xiang-wei(刘向伟)¹, CHEN You-ming(陈友明)¹,

GUO Xing-guo(郭兴国)³, DENG Yong-qiang(邓永强)¹

1. College of Civil Engineering, Hunan University, Changsha 410082, China;

2. College of Mechanical Engineering, University of South China, Hengyang 421001, China;

3. School of Civil Engineering and Architecture, Nanchang University, Nanchang 330031, China

? Central South University Press and Springer-Verlag Berlin Heidelberg 2012

Abstract: A new facility was presented which can expediently and cheaply measure the transient moisture content profile in multi-layer porous building envelope. Then, a common multi-layer porous building envelope was provided, which was constructed by cement mortar-red brick-cement plaster. With this kind of building envelope installed in the south wall, a well-controlled air-conditioning room was set up in Changsha, which is one of typical zones of hot and humid climate in China. And experiments were carried out to investigate the temperature and moisture distribution in multi-layer building envelope in summer, both in sunny day and rainy day. The results show that, the temperature and humidity at the interface between the brick and cement mortar are seriously affected by the changes of outdoor temperature and humidity, and the relative humidity at this interface keeps more than 80% for a long-term, which can easily trigger the growth of mould. The temperature and humidity at the interface between the brick and cement plaster change a little, and they are affected by the changes of indoor temperature and humidity. The temperature and humidity at the interface of the wall whose interior surface is affixed with a foam plastic wallpaper are generally higher than those of the wall without wallpaper. The heat transfer and moisture transfer in the envelope are coupled strongly.

Key words: coupled heat and moisture transfer; transient moisture content; multi-layer porous building envelope

1 Introduction

Moisture damage is one of the most important factors limiting the service life of a building. High moisture level not only can cause metal corrosion, wood decay and structure degradation, but also may have a significant influence on indoor air humidity and air-conditioning loads [1-2]. Consequently, studying the coupled heat and moisture behavior of porous building materials is essential to improving their performances.

Coupled heat and moisture transfer through porous material is complex and random for the non-uniformity of porous media and the interaction of heat and moisture transfer process. Numerical models of heat and moisture transport in building components continue to advance, but the reliability of the measured material properties, which serve as input for the models, remains very uncertain. And often, the results of numerical models need to be validated for their reliability [3].

Moisture content profile is one of the most important moisture transport properties frequently used in hydrothermal analysis. How to experimentally determinate it in multi-layer porous building envelope is critical to study the coupled flow of heat and moisture. There are several experimental methods and apparatus that can be used to achieve this purpose [4]. But how to enforce measurement expediently and cheaply is still a problem.

TENWOLDE [5] reported the experimental results for moisture movement in wood construction walls in a warm humid climate. Nine different test walls were constructed on the campus of Lamar University in Beaumont, Texas, USA. SHAKUN [6] studied the causes and control of mold and mildew in Florida hotel-motel sites. The primary cause of the interior moisture damage was a direct result of the infiltration of warm moist outdoor air into the wall spaces. Moisture from this warm moist air would condense due to the air-conditioning having cooled the building wall surfaces below the dew point temperature. He also concluded that the use of vinyl wall-coverings traps moisture inside the wall cavities, which results in high moisture levels in the building materials. HOSNI et al [7] carried out a series of experiments about the building heat and moisture transfer on the concrete block. The experiment was divided into two phases. The main objective of Phase-1 was to evaluate heat and moisture transfer by diffusion. In Phase-2, in addition to the diffusion through the test walls, air infiltration was added by maintaining a pressure gradient of 12.4 Pa during the test period. Data were collected at the end of the Phase-2 testing to determine the rate at which the test walls would dry out without infiltration present. Thus, he concluded that the external surface vapor retarder will reduce the moisture transferred into the wall, however, it is still less than that reduced by interior wall protective layer. SU [8] tested the temperature and relative humidity at the interface between block and gypsum board under different environmental conditions for different time. She found that in hot and humidity conditions, when the relative humidity of the outside environment of experimental wall increases, the temperature and relative humidity at the interface between gypsum board and brick close to indoor have almost no change, the temperature at the interface between gypsum board and brick close to outdoor decreases and the relative humidity increases at this place with the outdoor temperature decreasing. ROELS et al [9] performed a round robin experiment on the determination of the hydrothermal properties of porous building materials to generate a dataset for benchmarking numerical models.

Many experiments often regarded temperature and humidity of indoor and outdoor as a constant value. Most of these results can only be used under specific conditions [10-11]. To reflect the migration process of temperature and relative humidity inside the exterior building envelope, in this work, a new method is put forward at first, which can expediently and cheaply measure the moisture profile in multi-layer porous building envelope. Then, the temperature and relative humidity distribution are experimentally analysed within porous building envelope exposed to actual situation in the summer of July in Changsha, Hunan Province, China.

2 Measurement of transient moisture content profile

The measurement of moisture content profile in building envelope has been of interest for many years. There are several methods and apparatus that can be used for measuring the transient moisture content profile of building materials, and each one has its own pros and cons [12]. In this work, a new facility was presented, which can measure the transient moisture content profile expediently and cheaply in multi-layer porous building envelope.

2.1 Slice-dry-weigh method

Slice-dry-weigh method is a traditional destructive method. Whenever the moisture content profile is to be measured, the specimen is quickly sliced into thin pieces that are weighed, dried, and then weighed again. By this means, the moisture content of each slice of the specimen can be determined. This method is the most accurate when the amount of water in the material is precisely measured. But the specimen is destroyed, and for each measurement, a new specimen is used.

2.2 Gamma-ray attenuation

The gamma-ray attenuation has been one of the most commonly used methods to determine the moisture content profile of building materials. In 2008, NIZOVTSEV et al [13] used gamma-ray attenuation to determinate moisture diffusivity in porous materials. In a gamma-ray test, the test specimen is placed between the source and detector of the equipment. The source can consist of Am241, Cs137 or Co60. The detector is usually a scintillation crystal, e.g. a NaI crystal. A grid of vertical and horizontal co-ordinates defines the area that faces the gamma-ray source. At each segment of the specimen, the gamma-rays are emitted from the source and captured by the detector on the other side. The test is first done on a dry material then for the wet state. By scanning each segment in the wet state and comparing with the scan in the dry state, an average value for moisture content at a given interval can be obtained. When they are all put together, it results in the moisture distribution in the specimen as a function of time. The disadvantages of this technology are the high cost of the equipment and its radioactivity for which special safety arrangements must be made.

2.3 Nuclear magnetic resonance (NMR) technology

Nuclear magnetic resonance (NMR) method is another well-known technology applied in building material measurements. In an NMR measurement, the number of hydrogen nuclei can be counted, therefore the water content in the material can be determined. Compared with gamma-ray attenuation, an advantage of the NMR method is that no radioactivity is involved during the experiment. VANDERHEIJDEN et al [14] developed a new dedicated NMR setup which is capable of measuring one-dimensional moisture profile in heated porous materials. The cost of the equipment is also a drawback.

2.4 X-ray projection method

The X-ray projection method allows the visualization of the interior of non-transparent objects in a non-destructive way when an object is illuminated with X-ray and the change in transmitted X-ray intensity is measured [15]. For a water uptake experiment, an oven dried specimen is placed in the X-ray apparatus. First, the X-ray image of dry specimen is taken, then the specimen is placed in contact with water, and the images are taken at regular time steps. YE et al [16] focused on two techniques which offer the potential for some significant advantages: thermal dual-probe and time domain reflectometry, by using a laboratory-based X-ray measurement facility.

2.5 New facility for measuring transient moisture content profile

In this work, a new facility was presented, which can measure the transient moisture content profile in porous building enclosure expediently and cheaply. It is comprised of a recorder and some sensing detectors, which are connected with electric wires. A sensing detector is made up of a relative humidity sensor and a temperature sensor, both of which are installed in a shell. The relative humidity sensor outputs voltage signal (V_out) and the temperature sensor outputs resistance signal (R_t).

The shell is made up of plastic, which can protect the sensors from pressure. It is a hollow cylinder (its diameter is about 5 mm, and length 8 mm) with a lot of meshes on its surface to connect the sensors and the air out of the shell. This sensing detector is so small that it can be installed in the inner of the building envelope expediently. It can be used in site, or can be buried when the building envelope is built up. For the existing building, it can be emplaced in a hole drilled in the envelope. The schematic diagram of the new facility is shown in Fig. 1.

Fig. 1 Schematic diagram of new facility

It can accurately measure the moisture content profile and monitor the moisture transfer in the building envelope with less damage to the building envelope. Moreover, it can measure the temperature at the same time. Therefore, it can be applied widely.

With the measurement results, the relative humidity of air in the porous building material can be determined accurately. And then, we can use the sorption/desorption isotherm curve between the moisture content of porous building material and the relative humidity of air to determine the moisture profile.

3 Experiments

For these experiments, a multi-layer porous building envelope named as Wall 1 was built up from outside to inside of cement mortar (I) (20 mm), red brick (II)   (240 mm) and cement plaster (III) (20 mm). This type of building envelope construction has been widely used for a long time in the south of China. Another envelope named Wall 2, was built up as Wall 1 but with a foam plastic wallpaper sticking to the insider surface of cement plaster (III). The structure of the test building envelope is shown in Fig. 2. Both were installed in the south wall of a test room. In the room, it was controlled by an air-conditioning, which can control the air temperature and humidity of the room precisely. The external wall was exposed to actual environment.

Fig. 2 Schematic diagram of test building envelope

To monitor the moisture transfer, one new sensing detector was placed at the interface of I and II, and another at the interface of II and III, as shown in Fig. 2. At the same time, two temperature and humidity sensors were situated near the two sides of the envelope, respectively, which were used to monitor the temperature and humidity of the air near the envelope.

Experiments were carried out for 20 d, and the test walls were preconditioned for two weeks before the data collection. Resistance temperature sensor (Pt100) was used to measure the temperature, which can be directly shown in the data acquisition system. Thermo-set polymer capacitive sensor (HIH-4000 003) was used to measure the relative humidity, which outputted voltage signal.

4 Results and discussion

The experiments were carried on in summer of July at a test room, which is located at Hunan University (28°N, 112°E), Hunan Province, China. The test room was a 3 m×3 m×3 m rotary laboratory surrounded by open areas. The experimental results are shown as follows.

Figure 3 shows the temperature distribution in sunny day. It can be found that the temperature curves at the interface between cement mortar and red brick are coincident to the outdoor temperature curve. This results from the large thermal conductivity of the cement mortar. The temperatures at this interface are often higher than the outdoor temperatures. This is because of the solar radiation in daytime, and because of the heat of the brick spread out in nighttime. The temperature curves at interface between red brick and cement plaster are similar with indoor temperature. They are mainly affected by the indoor temperature. However, during the daytime, the temperature curves have a slight fluctuation due to heat transfers from outside to inside. And it can be found that the temperature gradient concentrates on the red brick layer and the temperature at cement mortar layer is mainly affected by the outdoor temperature and cement plaster layer by the indoor temperature.

Fig. 3 Temperature distribution in sunny day

Figure 4 shows the distribution of moisture content of air in sunny day. By comparing Fig. 3 with Fig. 4, it can be found that the variation curves of moisture content in the envelope and the temperature curves are similar. It can be concluded that the moisture content of air in the envelope changes in the analogous way as the temperature. The temperature increases at the interface between cement mortar and red brick cause the saturated water vapor pressure at this interface to increase and relative humidity to decrease. Hence, the liquid water in the pore evaporates and results in the moisture content increasing with temperature. For the great vapor resistance of red brick, the moisture content of cement plaster layer is mainly affected by indoor humidity but not outdoor humidity.

The relative humidity distributions of indoor, outdoor and inside the envelope are shown in Figs. 5 and 6. It can be found that the changes of relative humidity at the interface between cement mortar and red brick have a definite time lag compared with outdoor relative humidity, but keep pace with the variation of outdoor temperature. In addition, the relative humidity at the interface between cement mortar and red brick is generally higher than 80%, where the mould is easy to emerge if this humidity level persists for an extended period. Effective measure should be taken to prevent the accumulation of moisture.

Fig. 4 Moisture content (air) distribution in sunny day

Fig. 5 Relative humidity distribution in summer sunny day (1)

Fig. 6 Relative humidity distribution in summer sunny day (2)

From Figs. 3-6, it can be found that the temperature and relative humidity at the interfaces of Wall 1 are generally lower than that of Wall 2. Wall 2 is affixed with foam plastic wallpaper, which can prevent moisture escaping from the wall and cause moisture accumulation. Wallpaper is often used in building for aesthetics, but which kind of wallpaper and how to use it are still waiting for more investigation from the point of moisture transfer. And the moisture content of materials in Wall 2 is higher than that in Wall 1, which leads to a higher thermal conductivity in Wall 2.

Figures 7 and 8 show the temperature and relative humidity distribution curves in the rainy day. It can be found that the temperature changes would dramatically lead to the changes of moisture content. By comparing Figs. 5-6 with Fig. 8, it can be found that the relative humidity at interfaces within the two walls has a significant increase in rainy day. This is mainly due to the fact that the outdoor relative humidity is higher in rainy day, and dramatic changes of outdoor temperature result in a sharp increase in relative humidity inside the walls and even lead to condensation. And the external wall surface will be inevitably wet by rain, leading to the relative humidity at interface between cement mortar and brick wall close to saturation for a long time. From Figs. 7 and 9, it can be found that in the envelope the drastic fluctuating of the temperature can give rise to the drastic fluctuation of moisture content. So, it can be concluded that the heat and moisture transfer are coupled strongly.

Fig. 7 Temperature distribution in summer rainy day

With the experimental results of the temperature and moisture distribution in the porous building envelope, it will be possible for the validation of the mathematical model in the future.

Fig. 8 Relative humidity distribution in rainy day

Fig. 9 Moisture content (air) distribution in rainy day

5 Conclusions

1) The temperature and relative humidity gradient concentrate on the red brick layer, the interior layer (cement plaster layer) is slightly affected by outdoor environment, and the variation of temperature and relative humidity inside cement mortar layer is almost synchronized to outdoor temperature and relative humidity and mainly affected by the outdoor environment.

2) The foam plastic wallpaper sticking to the insider of cement plaster dramatically affects the heat and moisture transfer through the porous building envelope. And it leads easily to condensation in the building envelope.

3) The rainy weather has a great impact on humidity ratio inside the envelope, and it may cause nearly the saturation of the relative humidity at interface close to outdoor. Effective measure should be taken to prevent the accumulation of moisture.

4) The heat and moisture transfer in the envelope are coupled strongly. And the experimental results will be used to validate the mathematical model in the future.

References

[1] QIN Meng-hao, BELARBI R, AITMOKHTAR A, SEIGNEURIN A. An analytical method to calculate the coupled heat and moisture transfer in building materials [J]. International Communications in Heat and mass Transfer, 2006, 33(1): 39-48.

[2] GUO Xing-guo, CHEN You-ming. Computational analysis of heat and moisture performance of multilayer wall in south China [J]. Journal of Central South University of Technology, 2008, 114(3): 106-110.

[3] DESTA T Z, LANGMANS J, ROELS S. Experimental data set for validation of heat, air and moisture transport models of building envelopes [J]. Building and Environment, 2011, 46(5): 1038-1046.

[4] WU Yang. Experimental study of hygrothermal properties for building materials [D]. Montreal: Concordia University, 2007.

[5] TENWOLDE A. Steady-state one-dimensional water vapor movement by diffusion and convection in a multilayered wall [J]. ASHRAE Transactions, 1985, 91(1A): 322-342.

[6] SHAKUN W. The causes and control of mold and mildew in hot and humid climates [J]. ASHRAE Transactions, 1996, 92: 1282-1292.

[7] HOSNI M H, SIPES J M, WALLIS M H. Experimental results for diffusion and infiltration of moisture in concrete masonry walls exposed to hot and humid climates [J]. ASHRAE Transactions, 1999, 105(2): 191-203.

[8] SU Xiang-hui. Research on coupled heat and moisture transfer in multilayer porous structure [D]. Nanjing: Nanjing University of Aeronautics and Astronautics, 2003: 69-73.

[9] ROELS S, TALUKDAR P, JAMES C. Reliability of material data measurements for hygroscopic buffering [J]. International Journal of Heat and Mass Transfer, 2010, 53: 5355-5363.

[10] JAMES C, SIMONSON C J. S, TALUKDAR P, ROELS S. Numerical and experimental data set for benchmarking hygroscopic buffering modes [J]. International Journal of Heat and Mass Transfer, 2010, 53(19/20): 3638-3654.

[11] TALUKDAR P, OLUTMAYIN S O, OSANYINTOLA O F, SIMONSON C J. An experimental data set for benchmarking 1-D, transient heat and moisture transfer models of hygroscopic buildings materials. Part I: Experimental facility and material property data [J] International Journal of Heat and Mass Transfer, 2007, 50(23/24): 4527-4539.

[12] ROELS S, CARMELIET J, HENS H, ADAN O, BROCKEN H, CERNY R, PAVLIK Z, ELLIS A T, HALL C, KUMARAN K, PEL L, PLAGGE R. A comparison of different techniques to quantify moisture content profiles in porous building materials [J]. Journal of Thermal Envelope and building Science, 2004, 27: 261-276.

[13] NIZOVTSEV M I, STANKUS S V, STERLYAGOV A N, TEREKHOV V I, KHAIRULIN R A. Determination of moisture diffusivity in porous materials using gamma-method [J]. International Journal of Heat and Mass Transfer, 2008, 51: 4161-4167.

[14] VANDERHEIJDEN G H A, HUININK H P, PEL L AND KOPINGA K. One-dimensional scanning of moisture in heated porous building with NMR [J]. Journal of Magnetic Resonance, 2011, 208(2): 235-242.

[15] BAKER P H, BAILLY D, CAMPBELL M, GALBRAITH G H, CRAIGMCLEAN R, ROFFA N, SANDERS C H. The application of X-ray absorption to building moisture transport studies [J]. Measurement, 2007, 40(9/10): 951-959.

[16] YE Z, TIROVIC M, DAVIES M, BAKER P H, DHILLIPSON M C, SANDERS C H, GALBRRAITH G H, McLEAN R C. The testing of two methods for the moisture measurement of building fabrics via comparisons with data from an X-ray system [J]. Building and Environment, 2009, 44: 1409-1217.

(Edited by YANG Bing)

Foundation item: Project(51078127) supported by the National Natural Science Foundation of China

Received date: 2011-07-26; Accepted date: 2011-11-14

Corresponding author: CHEN You-ming, Professor, PhD; Tel: +86-731-88823515; E-mail: ymchen@hnu.edu.cn