1 Introduction. Trondheim, October 26-28, of the moisture interactions between

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1 Trondheim, October 6-8, 005 Effect of airflow elocity on moisture exchange at surfaces PhD student Lone Hedegaard Mortensen Associate professor Carsten Rode Assistant professor Ruut Peuhkuri Abstract The moisture transfer between air and construction are affected of the boundary layer conditions close to the surface, which is influenced by the airflow patterns in the room. Therefore an inestigation of the relation between the surface resistance and the airflow elocity aboe a material sample has been performed. The experiments are performed by use of the ordinary cup method for permeability tests. A total of 3 different types of experiments hae been performed and for each type different airflow elocities hae been tested and compared. As expected the results show that the surface resistances decreases for increasing airflow elocity aboe the boundary layer of the material surface. The measured resistances are somewhat higher than the ones estimated by use of Lewis relation. 1 Introduction 1.1 Bacround Moisture interactions between room air, the surrounding constructions and furniture hae a great influence on the indoor enironment. The moisture transfer between air and construction depends strongly upon the boundary layer conditions close to the surface, which is influenced by the airflow patterns in the room. Thus, it is important to inestigate the airflows carefully and to estimate their effect on the moisture transport. For instance, small elocities will lead to an increased air layer resistance. Bednar & Dreyer (003) showed that it is possible to measure the surface transfer coefficients by use of small specimens with high liquid moisture conductiity. The surface transfers are a central issue for better understanding of the moisture interactions between the room air and the constructions since the actual airflow elocities found in dwellings are relatiely small. The rapid modelling deelopment in hygrothermal building performance urges measurements of moisture parameters of building materials. A widely used method is the cup test but other methods hae also been deeloped. Sennberg & Wadsö (003) inented a modified cup method that is more applicable for ery permeable materials such as textiles and the method only requires ery small sample sizes. Also Galbraith et al. (003) has deeloped a method of low pressure permeability tests where the results can be obtained much faster. In addition Scheffler & Plagge (005) has designed a new type of equipment especially for drying measurements, which has the adantage that it can also produce data for the surface boundary of the material sample. The effect of airflow elocities on material surfaces and how this boundary layer relates to the moisture transfer rate is highly important in order to hae better understanding of the moisture interactions between the room air and the constructions. The actual airflow elocities found in dwellings are relatiely small, which increases the releance. Therefore, the experiments in this study focus on determination of the surface transfer coefficients for moisture as a function of the airflow elocity aboe the boundary layer. The surface resistances can be determined by measurements on samples with different thicknesses as shown by Worch (004). This corresponds well with the Japanese standard JIS A :00. In the standard it is proposed that the surface resis-

2 tance can be found by using cups with desiccant coered by 1 and pieces of paper, respectiely. Then the measured resistances from the two samples can be subtracted and reeal the surface resistance. The found resistances will be compared to the EN ISO 157:001 standard for determination of the water apour permeability, where the air must be mixed to assure uniform conditions and the airflow elocity should be within the range m/s aboe the sample. Howeer, this relatiely large span of airflow elocities could influence the surface transport coefficients. Therefore, this paper presents permeability experiments using both wet and dry cup methods on different samples to reeal differences in surface resistances due to changes in airflow elocities. 1. Theory The cup method for determining the water apour permeability is widely used. The principle is to hae a cup with desiccant or a salt solution, to ensure a well known relatie humidity (RH), that is coered by a sample and placed in a controlled enironment. Then the cup is weighed regularly and the steady rate of weight gain or loss expresses the apour transfer transmission. When the water apour resistance, Z t (total), for an entire sample is found by the cup test method the primary resistance is within the material itself, Z m. Howeer, there are also somewhat smaller resistances caused by surface resistances of the material sample one on each side, Z s1 and Z s. In addition, there can be a resistance of the air layer, Z a, between the surfaces of the desiccant or the salt solution and the sample. In case of using the inerted dry cup method only Z s1, the resistance on the exterior side of the sample of the cup exists. With these resistances and use of Fick s law the water apour flux density, g can be described as in Equation 1 (Hansen & Lund, 1990). g G A l Z Z + Z + Z + Z t m s1 s a δ (1) where G stationary weight change per time (/s); A exposed surface area (m ); δ apour permeability ( ) ; Pa m apour pressure difference oer the sample (Pa); and l sample thickness (m). Equation 1 can be simplified to show the transport in case of inerted cup measurements and this is shown in Equation. l Z Z + Z t m s1 g G A δ () Alternatiely the surface resistances Z s1 and can be found by use of Lewis Z s ( ) formula, which is gien in Equation 3. ρ c R T p Z 1 (3) β α p c where β p surface coefficient of apour water transfer ( ); ρ air density ( 3 ) c p heat capacity of air at constant pressure J J ( ); R gas constant for apour ( ) K m ; ; K and T temperature (K); α c conectie W surface heat transfer coefficient ( ). m K In Equation 3 the conectie surface heat transfer depends on the airflow elocity, which is expressed in Equation 4 (Hagentoft, 001). α α c c u 7.41 u 0.78 u 5m / s u 5m / s (4) where u airflow elocity aboe the sample (m/s). In Table 1 an estimate of surface resistances based on Equation 3 and 4 are shown. Table 1. Estimated surface resistances. Airflow elocity (m/s) β p ( ) Z ( ) Outline of current experiments This paper presents permeability experiments using the ordinary cup method that can reeal differences in surface resistances due to changes in airflow elocities. The inestigated airflow elocities are within

3 the range gien in the EN ISO 157:001 standard as well as higher elocities. The expected results are interesting in relation to how moisture transfers in microclimates influences both materials and indoor enironment. All the experiments are performed at isothermal conditions. Equipment.1 Description of the cup facility To perform the cup method measurements a special deeloped equipment has been used. The cup test facility consists of a closed entilation system where both temperature and RH can be controlled. A diagram of the equipment can be seen in Figure 1. Figure 1. Diagram of the equipment for cup measurements. The air is circulated and conditioned within the equipment. The cup facility is combined with a computer where information about temperature, RH, airflow elocity and pressure is recorded automatically and the weight of the cups is entered at each weighing. It is possible to test 1 ordinary cups and 1 inerted cups at the same time. In Figure a picture of the test chamber is shown. In the picture two holes can be seen in the front plate and by use of these the samples are weighed on the balance seen in the bottom. A more detailed description of the used equipment is gien by Hansen (1989). balance Figure. Picture showing the actual test chamber. The inerted cup samples are standing on a Perspex plate. For non-inerted cups the samples hang in a similar plate in the lower part of the duct. The air flows by the cups within the 5 cm high duct. The air is circulated within the equipment in a squared duct, which is stretched out to a flat 60 cm wide duct of 5 cm height in the lower part of the system. In this flat part of the duct the cup samples are in contact with the chamber air. The flat duct can be seen in Figure. The circulation ensures that the airflow elocity on the exterior side of the cups can be controlled. Within the equipment for the cup tests it is possible to set different airflow rates. The airflow is created by a entilator inside the air circulation duct.. Description of cups The cups are resistant to corrosion and the tests described in this paper are performed on circular samples. The cup consists of two parts. The upper part has a diameter of 108 mm and a lower part diameter is 80 mm with a height of 50 mm. The two parts are screwed together with a rubber sealing ring on both sides of the sample. On the upper side of the rubber sealing a Teflon ring is added that ensures smoother closing of the cup. In Figure 3 a picture of a cup is shown. 3

4 Figure 3. Picture of cup with half a 1-layer paper (190 g/m ) sample. No desiccant is in the cup. The white ring aboe the sample is Teflon and the black ones aboe and below are rubber sealing rings. 3 Measurements In order to test different methods for determination of the surface resistance by use of the described cup method equipment 3 different series of tests hae been performed. 1. Wet cups with gypsum board. Wet cups with glass fibre membrane 3. Dry cups with paper (190 g/m ) The measurements were performed in the described facility for cup tests. The purpose of the measurements was to gain information about the water apour transport through a sample as a function of airflow elocity on the upper surface. During all tests the climate aboe the cups proided by the climate chamber was kept constant with temperature of.6 C ± 0.1 C and 54 %RH ± 1.5 %RH. The tests were either dry cups with desiccant (Mg(ClO 4 ), 0 %RH) or wet cups with a salt solution (KNO 3, 94 %RH). In Table information of the tested cases are gien. The used samples are circular and hae an exposed surface area of 4858±30 mm for the gypsum and 507 mm for the other tests. The small sample size means that 5 samples are required according to the EN ISO 157:001 standard. In the experiments the tested airflows were between 0.06 to.68 m/s. The airflow elocity measurements were performed with around.5 cm distance to the sample surface. The airflow measurements were performed with anemometers, a Brüel & Kjær, Indoor Climate Analyser, type 113 for airflows in the range 0-1 m/s and with a Dantec, Low Velocity Transducer, type 54R10 in the range 1-5 m/s. Table. ed cases. Gypsum board Wet cup, KNO 3, 93 %RH Airflow elocity (m/s) A1 A A3 A4 A5 Glass fibre membrane Airflow elocity (m/s) Wet cup, KNO 3, 93 %RH B1 B B3 B4 Paper 190 g/m Dry cup, 0 %RH, Airflow elocity (m/s) Mg(ClO 4 ) C1 C C3 C4 3.1 Gypsum board tests The samples were plain gypsum board samples, which had already been tested as part of a Round Robin in IEA Annex 41 (Hens, 003). It was chosen to use wet cup measurements since this RH is closest to what is usually found in dwellings. The tests were performed with 5 different elocities set by 4 different leels on the entilator and one where the cups were placed in the measuring chamber outside the flat duct. The elocities were assumed to be uniform is the entire duct and were measured as an aerage between two measuring positions. Therefore, the 5 tested elocity leels were 0.07, 1.45, 1.60, 1.73 and.68 m/s. In the experiments 5 samples were tested and repeated once for 3 of the elocities, see Table. 3. s with glass fibre membrane In the Japanese standard JIS A :00 a test method is gien and this was tested in a modified form. The tests were performed on wet cups and not on dry cup as recommended. Also, glass fibre membrane was used instead of paper since an organic material might change properties at high RH. In the tests 3 cups with 1-layer of glass fibre membrane, cups with -layers and

5 cups with 3-layers were tested. The elocities were again assumed to be uniform is the entire flat duct and were measured as an aerage between to measuring positions. The 4 tested elocity leels were 0.1, 1.45, 1.60 and 1.73 m/s. 3.3 s with paper (190 g/m ) In these tests the Japanese standard JIS A :00 test method was followed except that paper of 190 g/m instead of Kent paper of a thickness of 0.5 +/- 0. mm was used. A total of 10 cups with samples of plain drawing paper with a weight of 190 g/m are used. Half of the cups hae 1 layer of paper and the rest hae layers of paper. Howeer, in each test 10 samples are tested but the airflow elocity aboe the boundary layer was not the uniformly distributed aboe the 10 cups. The tested cases were actually measured twice, on day one it was performed with one layer of paper and secondly the following day with two layers of paper. The three tests C1-C3 are coering similar ranges of elocities but they were performed in order to assure repeatability of the measurements. In the experiments the tested airflows were between 0.06 to 1.73 m/s. 4 Results and discussion For all the tested cases the measurements were continued until stationary weight loss or gain of the samples was achieed. The measuring period typically included 7 weighings. In the tests with the gypsum board and glass fibre membrane the samples were weighed 1- times a day and for the test with paper the weighings were done eery hour. The results shown in this paper are based on at least 5 weighings. 4.1 Gypsum board tests The results of the tests A1-A5 are gien in Figure 4. The results do not clearly indicate how the airflow influences the total resistance of the material sample. As it can be seen in the figure some of the tests were repeated. The red results are from a student project made on the same samples in spring 005. Figure 4 shows ery dierse results so it was impossible to conclude anything about the influence of the airflow elocities influence on the surface resistances. Influence of airflow elocity on material apour permeability,9e-11 Vapour permeability [/Pa m s],8e-11,7e-11,6e-11,5e-11,4e-11,3e-11 wet,e ,5 1 1,5,5 3 Air elocity [m/s] Figure 4. Results of the total material water apour permeability for the tests A1-A5 with gypsum board samples. 5

6 4. with glass fibre membrane The results of the tests B1-B4 with the glass fibre membranes are gien in Figure 5. The results of the test with different material thickness (here different numbers of layers) were subtracted from each other and the surface resistance found. Howeer the tests were somewhat ruined because the salt solution in the wet cup crept up at the sides of the cups and onto the material samples. In the cases with most salt on the samples the material resistance was highly reduced. Therefore, the tests of the surface resistances with wet cups were abandoned. Influence of airflow elocity on surface apour resistance Vapour resistance [Pa m s/] 5,00E+08 4,50E+08 4,00E+08 3,50E+08 3,00E+08,50E+08,00E+08 1,50E+08 1,00E+08 5,00E+07 0,00E+00 0,00 0,50 1,00 1,50,00,50 3,00 Air elocity [m/s] Series1 Series Series3 Figure 5. Surface resistances as a function of airflow elocity. The resistances are found for series 1 by subtracting the -layer results from times the 1-layer result, for series by subtracting the 3-layer results from 3 times the 1-layer result and for series 3 by subtracting times the 3-layer results from 3 times the -layer result.,5e-03 Dry cup changes -layers of paper Weight (),0E-03 1,5E-03 1,0E-03 5,0E a ,0E Time (s) Figure 6. Dry cup weight changes as a function of time in test C1 with -layers of paper, the legend numbers refers to the airflow elocity aboe the boundary layer of the material surface of the gien cup. 6

7 4.3 s with paper (190 g/m ) The results shown for the tests C1-C4 consists of a number of measurements with different airflow elocities based on the measuring results from each cup. An example of the weight uptake results for the tests with paper are gien in Figure 6 for the dry cups in test C1. In the figure the numbers in the legend refers to the airflow elocity aboe the boundary layer of the material surface of the gien cup. The slopes of the lines in combination with the exposed surface area and the apour water pressure difference oer the samples are used to calculate the total resistance of the samples. The slopes of the lines in Figure 4 are for -layers of paper. The calculated total resistances in this case are between and The lowest surface resistance is for the case with a elocity of 0.34 m/s and the highest alue corresponds to an airflow elocity of 0.06 m/s. These airflow elocities also hae the steepest and the flattest slopes respectiely in Figure 6. The measured weight uptake rates hae been post processed and the corresponding surface coefficients hae been found. The corresponding surface resistances as a function of the airflow elocity aboe the boundary layer of the four tests C1-C4 are shown in Figure 7. The results shown in the figure are based on at least 5 weighings where the weight change rate is constant within ± 5% of the mean alue, which is required by the EN ISO 157:001 standard. Howeer, in most cases the weight change rate was constant within ± % of the mean alue. In the figure a trendline calculated by the least squares fit for all measured test results by use of a power function are added. The equation for the trendline is y x. Along with the surface resistances determined from the measurements the surface resistances estimated by the Lewis relation in equation -4 are gien. Surface resistances Z-surface (Pa m s/) 7,0E+07 6,0E+07 5,0E+07 4,0E+07 3,0E+07,0E+07 Z-surf. estimated C1 C C3 C4 Power (all tests) 1,0E+07 0,0E , 0,4 0,6 0,8 1 1, 1,4 1,6 Velocity (m/s) Figure 7. Calculated corresponding surface resistances from the tests C1-C4 gien as a function of elocity. Along with the measured alues the theoretically estimated alues are gien. 7

8 The measured results in Figure 7 show that there is a tendency of higher surface resistances for lower airflow elocities. This was expected since the theoretical alues from equation -4 also show that the surface resistance declines with growing airflow elocity. Howeer, if the surface resistances from the measurements are compared with the estimated surface resistances, it is found that the measured alues are higher than predicted. This could be a sign of that the equations slightly underestimate the surface resistances. For comparison the results of Bednar & Dreyer (003) showed that the moisture transfer coefficient, β for drying is around p for a room with still air. h This number can be conerted to a surface resistance alue of This number seems quite small compared to Figure 7 where both the estimated and the measured surface resistances for elocities less than 0. m/s are higher. Howeer, it was a drying experiment where the sample was wet and since the liquid mass transfer within the sample is faster than eaporation this can explain the lower alue. In the present study the difference between the estimated alues by use of Lewis relation and the measured alues decreases as the airflow elocity is increased. Howeer, the normal airflow elocities in dwellings near construction surfaces are often quite small so there the underestimated alues could be a problem. This also indicates that there is a potential error when the water apour permeability is determined if Z s1 is found by Lewis relation. Still, this error will be almost negligible for materials with high water apour resistance (see Equation ). 5 Conclusion It has been found possible to determine the surface resistances of moisture transfer by measurements on samples with different thicknesses by using dry cup experiments as described in the Japanese standard JIS A :00 test method. The results show that the surface resistances decreases for increasing airflow elocity aboe the boundary layer of the material surface. This was expected since the surface resistances estimated by use of Lewis relation also shows that the resistance increases with decreased airflow elocity. The measured resistances are somewhat higher than the ones estimated by use of Lewis relation. 6 Acknowledgements The work was carried out with support of the Technical Research Council of Denmark. Thanks are due to Associate Professor, PhD Kurt Kielsgaard Hansen for experimental help with use of the cup facility. 7 References Bednar, T. & Dreyer, J Determination of moisture surface transfer coefficients under transient conditions. Proceedings of the nd International Conference on Building Physics, Leuen, Belgium, p ISBN EN ISO 157:001. Hygrothermal performance of building materials and products Determination of water apour transmission properties. European committee for standardization. Brussels Galbraith, G., Kelly, D.J. & McLean, R.C Alternatie methods for measuring moisture transfer coefficients of building materials. Proceedings of the nd International Conference on Building Physics, Leuen, Belgium, p ISBN Hagentoft, C.-E Introduction to Building Physics. Lund: Studentlitteratur. ISBN Hansen, K.K Equipment for and results of water apour transmission tests using cup methods. Proceedings from the ICHMT Symposium Heat and Mass Transfer in Building Materials and Structures. Dubronik, Yugoslaia. Hansen, K.K. & Lund, H.B Cup method for determination of water apour transmission properties of building materials. Sources of uncertainty in the method. Proceedings of the nd Nordic Symposium on Building Physics, Trondheim, Norway. Hens, H., 003, Whole building heat, air and moisture response (MOIST-ENG). Proposal for a new annex; International Energy Agency, EXCO Energy Conseration in Buildings and Community Systems. Department of Ciil Engineering, KU- Leuen, Belgium. JIS A :00. method of adsorption/desorption efficiency for building materials to regulate an indoor humidity Part 1: Response method of humidity. Japanese Industrial Standard, translated and published by Japanese Standard Association. 8

9 Scheffler, G. & Plagge, R Defined drying behaiour measurement for building materials. Proceedings of the 7 th Symposium on Building Physics in the Nordic Countries, Reykjaík, Iceland, p ISBN Sennberg, K. & Wadsö, L A modified cupmethod for lightweight and highly permeable materials. Proceedings of the nd International Conference on Building Physics, Leuen, Belgium, p ISBN Worch, A The behaiour of apour transfer on building material surfaces: The apour transfer resistance. Journal of Thermal Enelope & Building Science. 8 ():