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Moisture buffering capacity of highly absorbing materials
Energy and Buildings 41 (2009) 164–168
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Moisture buffering capacity of highly absorbing materials S. Cerolini a,*, M. D’Orazio a, C. Di Perna b, A. Stazi a a b
Department of Architecture, Construction and Structures (DACS), Faculty of Engineering, Polytechnic University of Marche, Via Brecce Bianche, 60100 Ancona, Italy Department of Energetics, Faculty of Engineering, Polytechnic University of Marche, Via Brecce Bianche, 60100 Ancona, Italy
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 May 2008 Received in revised form 1 August 2008 Accepted 14 August 2008
This research investigates the possibility to use highly absorbing materials to dampen indoor RH% variations. The practical MBV of sodium polyacrylate, cellulose-based material, perlite and gypsum is evaluated for a daily cyclic exposure that alternates high (75%) and low (33%) RH% levels for 8 h and 16 h, respectively. The adjustment velocity to RH% variations and the presence of hysteretic phenomena are also presented. The cellulose-based material proves to be the most suitable for moisture buffering applications. Starting from this material’s properties, the effect of thickness, vapour resistance factor (m) and mass surface exchange coefficient (Zv) on sorption capacity is evaluated by the use of a numerical model. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Moisture buffering capacity Cellulose Effective thickness Vapour resistance Mass surface exchange coefficient
1. Introduction The demand for controlling energetic consumptions and gas emissions led several European states to adopt regulations requesting low values of transmittance for the building envelope. For this reason designers and building constructors introduce thick layers of insulating material in walls and roofs even in mild climates and the producers of building components are bringing out new highly performing products with regard to thermal performance and air permeability. This fact causes a general reduction of the envelope’s air permeability and the increase of indoor RH% levels in occupied buildings. The importance of indoor RH% on respiratory comfort [1], skin humidity [2] and perceived indoor air quality [3] is well known. Besides, high levels of relative humidity may cause the deterioration of building materials [4] and, in combination with a sufficient nutritive capacity of the substratum, they play a crucial role on mould growth and biological organisms proliferation [5,6]. On the other hand the introduction of HVAC systems providing an adequate mechanical ventilation in order to discharge high moisture loads seems not to be a solution, particularly in small, low-density interiors, such as houses, where it would be a source of noise for people who live into, besides causing a further rise in energetic consumptions.
* Corresponding author. Tel.: +39 071 2204587; fax: +39 071 2204783. E-mail addresses: [email protected], [email protected] (S. Cerolini). 0378-7788/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2008.08.006
A promising strategy in this sense is related to the use of hygroscopic materials to dampen indoor humidity variations. By means of laboratory and field measurements and numerical models, researchers have shown that several materials used in the building construction (cellular concrete, bricks, wood and woodbased materials [7–10] and cellulose insulation [11]) or in furniture and furnishing [12,13] (textiles, wood and paper) interact dynamically with the indoor air they are exposed to [14], helping to improve indoor climate, in terms of hygienic conditions, comfort and air quality [15,16], and contributing to reduce energy consumption for heating and cooling [17]. The necessity of a standardized quantity to characterize the moisture buffering capacity of materials led [18] to define the Moisture Buffer Value (MBV) and to propose an experimental method for practical categorization of materials. The practical MBV (MBVpractical (kg/(m2 %RH))) is defined as ‘‘the amount of water that is transported in or out of a material per open surface area, during a certain period of time, when it is subjected to variations in relative humidity of the surrounding air’’. The hygroscopic materials tested by the NORDTEST Project [18] are concrete-based, wood, gypsum and brick. Starting from these results and considering the recent innovations introduced in material industry, this research tries to extend the dynamic characterization to highly absorbing materials coming from industrial sector. The possibility to use highly performing materials to dampen indoor RH% variations could actually improve the system’s efficiency, allowing to reduce the moisture buffering exposed area.
S. Cerolini et al. / Energy and Buildings 41 (2009) 164–168
165
By means of an experimental activity, the dynamic behaviour of four different materials is evaluated, comparing their MBV and estimating the velocity of adjustment to RH% variations. In order to take the moisture buffering effect of the materials into account in a real building, scientific data showing the ability of such materials to maintain their sorption properties are needed. The possible presence of hysteretic phenomena in the tested materials is presented in this paper. Furthermore, the effect of thickness, vapour resistance factor m and mass surface exchange coefficient Zv (s/m) on the maximum and final water content of a system exposed to indoor RH% fluctuations is investigated by the use of a numerical model. 2. Materials In this study the MBV of two industrial products with high sorption capacity is determined. Sodium polyacrylate is a super adsorbent polymer able to adsorb water up to 100 times its weight. It is currently used in several industrial sectors ranging from personal hygiene, medical field and cosmetics to the packaging of perishables and nursery-gardening. Cellulose-based material is a combination of cellulose and superadsorbent polymers, yet used in personal hygiene field. In order to understand the potential of these industrial products compared to the dynamic behaviour of porous building materials and components, even perlite and gypsum board are tested. 3. Methodologies 3.1. Experimental facility The moisture buffer performance of the mentioned materials is evaluated according to the NORDTEST Project [18]. For this test, the materials, except for the gypsum specimen, are wound in a nonwoven fabric and then placed in a non absorbing plastic container to facilitate weighing procedure. The container is closed above with a perforated sheet metal to simulate the presence of a surface closure and sealed with aluminium adhesive tape. The edges and the back-sides of the gypsum specimen are sealed with a sheet of polyethylene and aluminium adhesive tape in order to obtain one-dimensional moisture flow (Figs. 1 and 2). An empty container is also tested to assure the negligible hygroscopicity of the closer and sealing systems.
Fig. 2. Picture showing the closing and sealing systems used for the loose materials.
The samples are pre-conditioned at 23 0.3 8C and 50 3% RH in the climatic chamber Angelantoni CH250 and the weight of the specimens at equilibrium is determined. Table 1 summarizes the exposure area and the weight at equilibrium for each specimen. According to the NORDTEST method [18], the materials are exposed to cyclic step-changes that alternate high levels (75% for 8 h) and low levels (33% for 16 h) of relative humidity. The duration of the entire cycle is 24 h, that is a daily exposure. During the test the temperature is held constant at 23 0.3 8C. The amount of water absorbed by the materials for each step is determined monitoring the change in weight of the specimen and the practical MBV is calculated according to the NORDTEST method [18]. 3.2. Numerical investigations The selection of the tested materials is carried out depending on the exposed criteria and the characteristics of the specimens are determined according to the NORDTEST Project [18]. Actually, there is a great variety of porous materials and different exposure conditions, that may not be all reproduced by means of experimental activities. Therefore, a numerical model may be used to quickly investigate the effect of material’s thickness, vapour resistance factor m and surface exchange coefficient Zv (s/m) on the moisture buffering capacity of a material, enabling to define an optimum functioning range for these quantities and to detect the classes of materials that are more suitable to dampen RH% variations. For this purpose the software Delphin4 is used. This program, developed by the Institute of Building Climatology of the Technical University of Dresden [19], is able to model the coupled heat, air, salt and moisture transport that occurs in porous building materials, allowing to investigate the thermal and hygric behaviour of constructive building details for arbitrary standard and natural climatic boundary conditions. In this case the problem is described by a single layer of homogeneous material exposed to a one-dimensional flux. The material has a surface area of 1 m2 and a variable thickness Table 1 Specimen’s exposure area and weight at equilibrium after the pre-conditioning at 23 0.3 8C and 50 3% RH in the climatic chamber
Fig. 1. Picture showing the closing and sealing systems used for gypsum board.
Material
Exposure area (m2)
Weight at equilibrium (g)
Perlite Cellulose Gypsum Sodium polyacrylate
0.025 0.025 0.028 0.025
369.25 69.60 368.70 2041.30
S. Cerolini et al. / Energy and Buildings 41 (2009) 164–168
166 Table 2 Summary of simulation conditions Simulation
Thickness
Vapour resistance factor
Surface exchange coefficient (s/m)
Case 1: effect of thickness (boundary and test conditions: T = 20 8C, initial RH = 50%, simulation period = 30 gg and daily vapour sources = 3) 1 20 1.5a 5.88 10 2 30 1.5a 5.88 10 a 3 40 1.5 5.88 10 4 50 1.5a 5.88 10 5 60 1.5a 5.88 10 6 70 1.5a 5.88 10 7 100 1.5a 5.88 10 a 8 120 1.5 5.88 10 a 9 140 1.5 5.88 10 Case 2: effect of vapour permeability 10 11 12 13 14
50 50 50 50 50
Case 3: effect of surface exchange coefficient 15 50 16 50 17 50 18 50 19 50 20 50 a
8a 8a 8a 8a 8a 8a 8a 8a 8a
1 3 4 5 10
5.88 10 5.88 10 5.88 10 5.88 10 5.88 10
8a
5 5 5 10 10 10
5.88 10 5.88 10 5.88 10 5.88 10 5.88 10 5.88 10
6
8a 8a 8a 8a
7 10 6 7 10
Value for cellulose insulation.
Fig. 4 summarizes the practical MBV of the tested materials. Sodium polyacrylate shows remarkable sorption capacities com-
pared to the other tested materials. Cellulose, gypsum board and perlite follow in this order. The MBVs of both sodium polyacrylate (MBV 9 g/(m2 %RH) at 8/16 h) and cellulose-based material (MBV 3 g/(m2 %RH) at 8/16 h) are higher than the MBV of any other common building material tested in the NORDTEST Project [18] and they exhibit an ‘‘excellent’’ moisture buffer performance according to the classification proposed by Rode [18]. The super absorbent polymer reacts more quickly than the other materials to RH% changes both in the adsorption and desorption phases. Cellulose-based material comes after (Fig. 5). The determination of the mass of moisture that remains in the specimen at the end of the desorption phase enables to evaluate the presence of hysteretic phenomena. In this sense the sodium polyacrylate is not able to discharge all the amount of water absorbed and therefore to maintain its sorption properties (Fig. 6). On the contrary the other tested materials do not show hysteresis. The comparison of the amount of water stored up during the sorption phase and the maximum water content at equilibrium, expressed by the sorption isotherms, shows that (under experimental conditions), the materials are far from their functioning boundaries and may potentially meet with extreme RH% conditions or long term exposure.
Fig. 3. 3D visualization of the effect of the intermittent vapour sources on the material RH.
Fig. 4. Practical MBV of the materials tested in the climatic chamber.
(Table 2) discretised with smaller volume elements at the boundaries. In order to show the dependence of the moisture buffering capacity on the panel’s thickness, this quantity is varied from 20 mm to 140 mm and the maximum and final (at the end of the simulation period) water content stored up in the material are considered. The vapour resistance factor m and the surface exchange coefficient Zv (s/m) are varied separately starting from the characteristics of an existing building material (Cellulose insulation) to investigate the incidence of the water vapour permeability and the effect of the presence of a surface coating on the moisture buffering capacity. For this simulation the indoor RH levels are defined assigning initial conditions (RH = 50%) and intermittent vapour sources that raise RH to 90% (Fig. 3), while keeping the temperature constant (T = 20 8C). 4. Results 4.1. Dynamic characterization
S. Cerolini et al. / Energy and Buildings 41 (2009) 164–168
Fig. 5. Velocity of adjustment of the materials tested in the climatic chamber to indoor air RH% variations (sorption and desorption phases).
4.2. Effect of thickness and material’s properties Fig. 7 shows the dependence of the maximum and final water content (kgw/kgs%) on the panel’s thickness. The maximum water quantity that is stored in the whole thickness in a certain moment is a measure of the sorption capacity of a material under simulation conditions. The maximum amount of water rises according to the panel thickening but the maximum water content, that is related to the weight of the material, decreases when the thickness increases from 20 mm to 50 mm and reaches a steady value above 50 mm.
Fig. 6. Volumetric water content of the materials during the periodical exposure of 8 h at 75% RH and 16 h at 33% in climatic chamber. For the sodium polyacrylate specimen, the minimum water content at the end of the desorption phase increases with the number of the cycles.
Fig. 7. Simulated maximum and final water content (kgw/kgs%) stored up in a cellulose panel with variable thickness subject to intermittent vapour sources.
167
Fig. 8. Simulated final water content (kgw/kgs%) for different surface exchange coefficients. Variations of 10, 100 and 10,000 times of Zv of cellulose are investigated.
On the other hand, the capacity of the material to retain the absorbed water at the end of the simulation period improves with the increase of thickness up to 50 mm. Combining these acceptability ranges, we can notice that the variation of the material’s thickness affects the maximum and final water content of the system till the thickness reaches 50 mm. Beyond this value the increase of the material’s thickness does not significantly contribute to the variation of the examined quantities and the system shows a steady response to thickness changes. The final water content increases even with the vapour resistance factor m because the reduction of the water vapour permeability improves the capacity to retain water. Starting from these results, the surface exchange coefficient Zv (s/m) is varied between 5.88 10 6 s/m and 5.88 10 10 s/m, emphasizing the influence of a different surface coating on the moisture buffering capacity of the hygroscopic material. As Fig. 8 shows, the final water content slightly arises (about 0.1%) while Zv increases 100 times from 5.88 10 10 s/m to 5.88 10 8 s/m and it is steady over this value. These data allow to conclude that the moisture buffering capacity is slightly influenced by the surface exchange coefficient and therefore if a surface coating or a packaging system is needed it will have no considerable effects on the material’s sorption capacity. 5. Conclusions This paper presents the MBV of two highly absorbing materials coming from the industrial sector (sodium polyacrylate and cellulose-based material) and of two materials already used in building (gypsum board and perlite) as the results of a dynamic characterization in climatic chamber. As expected, sodium polyacrylate and cellulose-based material exhibit an ‘‘excellent’’ moisture buffer performance and a high velocity of adjustment to RH% variations, that exceed the capacity of more common building materials. Furthermore, all the tested materials, under experimental conditions, are far from their sorption limits. Nevertheless, sodium polyacrylate shows a hysteretic behaviour that in time may cause the loss of its sorption properties. The cellulose-based material seems therefore to be the most suitable for moisture buffer applications. Starting from the properties of a cellulose-based building material (Cellulose insulation) a numerical model is used to investigate the effect of thickness, vapour resistance factor and surface exchange coefficient on moisture buffer performance. The analysis of the numerical results enables to define optimum functioning ranges for these quantities, that may guide the
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selection of a material with a potentially high-performance behaviour. On this point, the numerical data confirm that cellulose is a good moisture buffering material. However, further numerical studies are necessary to investigate the effect of different ambient usage conditions and moisture production rates on the material performance. Field measurements are also needed in order to verify the moisture buffer capacity of cellulose-based material in real conditions. Acknowledgements We thank Eng. A. De Chirico for her practical contribution. This work was developed by S. Cerolini and M. D’Orazio and is a part of a more general research coordinated by A. Stazi. C. Di Perna takes part in the discussion on the possibility to use moisture buffering properties of materials in order to reduce energy consumptions into buildings. References [1] J. Toftum, A.S. Jorgensen, P.O. Fanger, Upper limits of air humidity for preventing warm respiratory discomfort, Energy and Buildings 28 (1998) 15–23. [2] J. Toftum, A.S. Jorgensen, P.O. Fanger, Upper limits for indoor air humidity to avoid uncomfortably humid skin, Energy and Buildings 28 (1998) 1–13. [3] L. Fang, G. Clausen, P.O. Fanger, Impact of temperature and humidity on the perception of indoor air quality, Indoor Air 8 (1998) 80–90. [4] F. Lucas, L. Adelard, F. Garde, H. Boyer, Study of moisture in buildings for hot humid climates, Energy and Buildings 34 (2002) 345–355. [5] K. Sedlbauer, Prediction of mould growth by hygrothermal calculation, Journal of Thermal Envelope and Building Science 25 (2002) 321–335. [6] M. Matilainen, J. Kurnitski, O. Seppa¨nen, Moisture conditions and energy consumption in heated crawl spaces in cold climates, Energy and Buildings 35 (2003) 203–216.
[7] H.M. Kunzel, A. Holm, K. Sedlbauer, M. Ellinger, Moisture buffering effects of interior linings made from wood or wood based products, IBP Report HTB-04/ 2004/e, 2004. [8] O.F. Osanyintola, P. Talukdar, C.J. Simonson, Effect of initial conditions, boundary conditions and thickness on the moisture buffering capacity of spruce plywood, Energy and Buildings 38 (10) (2006) 1283–1292. [9] S. Hameury, T. Lundstrom, Contribution of indoor exposed massive wood to a good indoor climate: in situ measurement campaign, Energy and Buildings 36 (2004) 281–292. [10] S. Hameury, Moisture buffering capacity of heavy timber structures directly exposed to an indoor climate: a numerical study, Building and Environment 40 (2005) 1400–1412. [11] T. Padfield, Humidity Buffering of Interior Spaces by Porous, Absorbent Insulation, Danmarks Tekniske Universitet Kgs. Lyngby, Electronic Publication, www.bkm.dtu.dk. [12] K. Svennberg, L. Hedegaard, C. Rode, Moisture buffer performance of a fully furnished room, in: Proceedings (CD) of the Performance of Exterior Envelopes of Whole Buildings IX International Conference, Clearwater Beach, FL, 2004. [13] K. Svennberg, Moisture buffering in the indoor environment, Doctoral Thesis, Report TVBH-1016 Lund 2006, Building Physics LTH. [14] T. Padfield, The role of absorbent building materials in moderating changes of relative humidity, Ph.D Thesis, Department of Structural Engineering and Materials, The Technical University of Denmark, November 1998, http://www.natmus.dk/cons/tp/phd/phd-indx.htm. [15] C.J. Simonson, M. Salonvaara, T. Ojanen, Improving Indoor Climate and Comfort with Wooden Structures, VTT Building Technology, Espoo: VTT Publications 431, 2001, http://virtual.vtt.fi/inf/pdf/publications/2001/P431.pdf, VTT Building and Transport. [16] C.J. Simonson, M. Salonvaara, T. Ojanen, The effect of structures on indoor humidity—possibility to improve comfort and perceived air quality, Indoor Air 12 (2002) 243–251. [17] O.F. Osanyintola, C.J. Simonson, Moisture buffering capacity of hygroscopic building materials: experimental facilities and energy impact, Energy and Buildings 38 (10) (2006) 1270–1282. [18] C. Rode, Moisture Buffering of Building Materials, BYG DTU Report, Department of Civil Engineering, Technical University of Denmark, 2005, http://www.byg. dtu.dk/upload/institutter/byg/publications/rapporter/byg-r126.pdf. [19] J. Grunewald, Documentation of the Numerical Simulation Program DIM3.1 Theoretical Fundamentals, Institute of Building Climatology, Technical University of Dresden, Germany, 2000 .
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Moisture buffering capacity of highly absorbing materials S. Cerolini a,*, M. D’Orazio a, C. Di Perna b, A. Stazi a a b
Department of Architecture, Construction and Structures (DACS), Faculty of Engineering, Polytechnic University of Marche, Via Brecce Bianche, 60100 Ancona, Italy Department of Energetics, Faculty of Engineering, Polytechnic University of Marche, Via Brecce Bianche, 60100 Ancona, Italy
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 May 2008 Received in revised form 1 August 2008 Accepted 14 August 2008
This research investigates the possibility to use highly absorbing materials to dampen indoor RH% variations. The practical MBV of sodium polyacrylate, cellulose-based material, perlite and gypsum is evaluated for a daily cyclic exposure that alternates high (75%) and low (33%) RH% levels for 8 h and 16 h, respectively. The adjustment velocity to RH% variations and the presence of hysteretic phenomena are also presented. The cellulose-based material proves to be the most suitable for moisture buffering applications. Starting from this material’s properties, the effect of thickness, vapour resistance factor (m) and mass surface exchange coefficient (Zv) on sorption capacity is evaluated by the use of a numerical model. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Moisture buffering capacity Cellulose Effective thickness Vapour resistance Mass surface exchange coefficient
1. Introduction The demand for controlling energetic consumptions and gas emissions led several European states to adopt regulations requesting low values of transmittance for the building envelope. For this reason designers and building constructors introduce thick layers of insulating material in walls and roofs even in mild climates and the producers of building components are bringing out new highly performing products with regard to thermal performance and air permeability. This fact causes a general reduction of the envelope’s air permeability and the increase of indoor RH% levels in occupied buildings. The importance of indoor RH% on respiratory comfort [1], skin humidity [2] and perceived indoor air quality [3] is well known. Besides, high levels of relative humidity may cause the deterioration of building materials [4] and, in combination with a sufficient nutritive capacity of the substratum, they play a crucial role on mould growth and biological organisms proliferation [5,6]. On the other hand the introduction of HVAC systems providing an adequate mechanical ventilation in order to discharge high moisture loads seems not to be a solution, particularly in small, low-density interiors, such as houses, where it would be a source of noise for people who live into, besides causing a further rise in energetic consumptions.
* Corresponding author. Tel.: +39 071 2204587; fax: +39 071 2204783. E-mail addresses: [email protected], [email protected] (S. Cerolini). 0378-7788/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.enbuild.2008.08.006
A promising strategy in this sense is related to the use of hygroscopic materials to dampen indoor humidity variations. By means of laboratory and field measurements and numerical models, researchers have shown that several materials used in the building construction (cellular concrete, bricks, wood and woodbased materials [7–10] and cellulose insulation [11]) or in furniture and furnishing [12,13] (textiles, wood and paper) interact dynamically with the indoor air they are exposed to [14], helping to improve indoor climate, in terms of hygienic conditions, comfort and air quality [15,16], and contributing to reduce energy consumption for heating and cooling [17]. The necessity of a standardized quantity to characterize the moisture buffering capacity of materials led [18] to define the Moisture Buffer Value (MBV) and to propose an experimental method for practical categorization of materials. The practical MBV (MBVpractical (kg/(m2 %RH))) is defined as ‘‘the amount of water that is transported in or out of a material per open surface area, during a certain period of time, when it is subjected to variations in relative humidity of the surrounding air’’. The hygroscopic materials tested by the NORDTEST Project [18] are concrete-based, wood, gypsum and brick. Starting from these results and considering the recent innovations introduced in material industry, this research tries to extend the dynamic characterization to highly absorbing materials coming from industrial sector. The possibility to use highly performing materials to dampen indoor RH% variations could actually improve the system’s efficiency, allowing to reduce the moisture buffering exposed area.
S. Cerolini et al. / Energy and Buildings 41 (2009) 164–168
165
By means of an experimental activity, the dynamic behaviour of four different materials is evaluated, comparing their MBV and estimating the velocity of adjustment to RH% variations. In order to take the moisture buffering effect of the materials into account in a real building, scientific data showing the ability of such materials to maintain their sorption properties are needed. The possible presence of hysteretic phenomena in the tested materials is presented in this paper. Furthermore, the effect of thickness, vapour resistance factor m and mass surface exchange coefficient Zv (s/m) on the maximum and final water content of a system exposed to indoor RH% fluctuations is investigated by the use of a numerical model. 2. Materials In this study the MBV of two industrial products with high sorption capacity is determined. Sodium polyacrylate is a super adsorbent polymer able to adsorb water up to 100 times its weight. It is currently used in several industrial sectors ranging from personal hygiene, medical field and cosmetics to the packaging of perishables and nursery-gardening. Cellulose-based material is a combination of cellulose and superadsorbent polymers, yet used in personal hygiene field. In order to understand the potential of these industrial products compared to the dynamic behaviour of porous building materials and components, even perlite and gypsum board are tested. 3. Methodologies 3.1. Experimental facility The moisture buffer performance of the mentioned materials is evaluated according to the NORDTEST Project [18]. For this test, the materials, except for the gypsum specimen, are wound in a nonwoven fabric and then placed in a non absorbing plastic container to facilitate weighing procedure. The container is closed above with a perforated sheet metal to simulate the presence of a surface closure and sealed with aluminium adhesive tape. The edges and the back-sides of the gypsum specimen are sealed with a sheet of polyethylene and aluminium adhesive tape in order to obtain one-dimensional moisture flow (Figs. 1 and 2). An empty container is also tested to assure the negligible hygroscopicity of the closer and sealing systems.
Fig. 2. Picture showing the closing and sealing systems used for the loose materials.
The samples are pre-conditioned at 23 0.3 8C and 50 3% RH in the climatic chamber Angelantoni CH250 and the weight of the specimens at equilibrium is determined. Table 1 summarizes the exposure area and the weight at equilibrium for each specimen. According to the NORDTEST method [18], the materials are exposed to cyclic step-changes that alternate high levels (75% for 8 h) and low levels (33% for 16 h) of relative humidity. The duration of the entire cycle is 24 h, that is a daily exposure. During the test the temperature is held constant at 23 0.3 8C. The amount of water absorbed by the materials for each step is determined monitoring the change in weight of the specimen and the practical MBV is calculated according to the NORDTEST method [18]. 3.2. Numerical investigations The selection of the tested materials is carried out depending on the exposed criteria and the characteristics of the specimens are determined according to the NORDTEST Project [18]. Actually, there is a great variety of porous materials and different exposure conditions, that may not be all reproduced by means of experimental activities. Therefore, a numerical model may be used to quickly investigate the effect of material’s thickness, vapour resistance factor m and surface exchange coefficient Zv (s/m) on the moisture buffering capacity of a material, enabling to define an optimum functioning range for these quantities and to detect the classes of materials that are more suitable to dampen RH% variations. For this purpose the software Delphin4 is used. This program, developed by the Institute of Building Climatology of the Technical University of Dresden [19], is able to model the coupled heat, air, salt and moisture transport that occurs in porous building materials, allowing to investigate the thermal and hygric behaviour of constructive building details for arbitrary standard and natural climatic boundary conditions. In this case the problem is described by a single layer of homogeneous material exposed to a one-dimensional flux. The material has a surface area of 1 m2 and a variable thickness Table 1 Specimen’s exposure area and weight at equilibrium after the pre-conditioning at 23 0.3 8C and 50 3% RH in the climatic chamber
Fig. 1. Picture showing the closing and sealing systems used for gypsum board.
Material
Exposure area (m2)
Weight at equilibrium (g)
Perlite Cellulose Gypsum Sodium polyacrylate
0.025 0.025 0.028 0.025
369.25 69.60 368.70 2041.30
S. Cerolini et al. / Energy and Buildings 41 (2009) 164–168
166 Table 2 Summary of simulation conditions Simulation
Thickness
Vapour resistance factor
Surface exchange coefficient (s/m)
Case 1: effect of thickness (boundary and test conditions: T = 20 8C, initial RH = 50%, simulation period = 30 gg and daily vapour sources = 3) 1 20 1.5a 5.88 10 2 30 1.5a 5.88 10 a 3 40 1.5 5.88 10 4 50 1.5a 5.88 10 5 60 1.5a 5.88 10 6 70 1.5a 5.88 10 7 100 1.5a 5.88 10 a 8 120 1.5 5.88 10 a 9 140 1.5 5.88 10 Case 2: effect of vapour permeability 10 11 12 13 14
50 50 50 50 50
Case 3: effect of surface exchange coefficient 15 50 16 50 17 50 18 50 19 50 20 50 a
8a 8a 8a 8a 8a 8a 8a 8a 8a
1 3 4 5 10
5.88 10 5.88 10 5.88 10 5.88 10 5.88 10
8a
5 5 5 10 10 10
5.88 10 5.88 10 5.88 10 5.88 10 5.88 10 5.88 10
6
8a 8a 8a 8a
7 10 6 7 10
Value for cellulose insulation.
Fig. 4 summarizes the practical MBV of the tested materials. Sodium polyacrylate shows remarkable sorption capacities com-
pared to the other tested materials. Cellulose, gypsum board and perlite follow in this order. The MBVs of both sodium polyacrylate (MBV 9 g/(m2 %RH) at 8/16 h) and cellulose-based material (MBV 3 g/(m2 %RH) at 8/16 h) are higher than the MBV of any other common building material tested in the NORDTEST Project [18] and they exhibit an ‘‘excellent’’ moisture buffer performance according to the classification proposed by Rode [18]. The super absorbent polymer reacts more quickly than the other materials to RH% changes both in the adsorption and desorption phases. Cellulose-based material comes after (Fig. 5). The determination of the mass of moisture that remains in the specimen at the end of the desorption phase enables to evaluate the presence of hysteretic phenomena. In this sense the sodium polyacrylate is not able to discharge all the amount of water absorbed and therefore to maintain its sorption properties (Fig. 6). On the contrary the other tested materials do not show hysteresis. The comparison of the amount of water stored up during the sorption phase and the maximum water content at equilibrium, expressed by the sorption isotherms, shows that (under experimental conditions), the materials are far from their functioning boundaries and may potentially meet with extreme RH% conditions or long term exposure.
Fig. 3. 3D visualization of the effect of the intermittent vapour sources on the material RH.
Fig. 4. Practical MBV of the materials tested in the climatic chamber.
(Table 2) discretised with smaller volume elements at the boundaries. In order to show the dependence of the moisture buffering capacity on the panel’s thickness, this quantity is varied from 20 mm to 140 mm and the maximum and final (at the end of the simulation period) water content stored up in the material are considered. The vapour resistance factor m and the surface exchange coefficient Zv (s/m) are varied separately starting from the characteristics of an existing building material (Cellulose insulation) to investigate the incidence of the water vapour permeability and the effect of the presence of a surface coating on the moisture buffering capacity. For this simulation the indoor RH levels are defined assigning initial conditions (RH = 50%) and intermittent vapour sources that raise RH to 90% (Fig. 3), while keeping the temperature constant (T = 20 8C). 4. Results 4.1. Dynamic characterization
S. Cerolini et al. / Energy and Buildings 41 (2009) 164–168
Fig. 5. Velocity of adjustment of the materials tested in the climatic chamber to indoor air RH% variations (sorption and desorption phases).
4.2. Effect of thickness and material’s properties Fig. 7 shows the dependence of the maximum and final water content (kgw/kgs%) on the panel’s thickness. The maximum water quantity that is stored in the whole thickness in a certain moment is a measure of the sorption capacity of a material under simulation conditions. The maximum amount of water rises according to the panel thickening but the maximum water content, that is related to the weight of the material, decreases when the thickness increases from 20 mm to 50 mm and reaches a steady value above 50 mm.
Fig. 6. Volumetric water content of the materials during the periodical exposure of 8 h at 75% RH and 16 h at 33% in climatic chamber. For the sodium polyacrylate specimen, the minimum water content at the end of the desorption phase increases with the number of the cycles.
Fig. 7. Simulated maximum and final water content (kgw/kgs%) stored up in a cellulose panel with variable thickness subject to intermittent vapour sources.
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Fig. 8. Simulated final water content (kgw/kgs%) for different surface exchange coefficients. Variations of 10, 100 and 10,000 times of Zv of cellulose are investigated.
On the other hand, the capacity of the material to retain the absorbed water at the end of the simulation period improves with the increase of thickness up to 50 mm. Combining these acceptability ranges, we can notice that the variation of the material’s thickness affects the maximum and final water content of the system till the thickness reaches 50 mm. Beyond this value the increase of the material’s thickness does not significantly contribute to the variation of the examined quantities and the system shows a steady response to thickness changes. The final water content increases even with the vapour resistance factor m because the reduction of the water vapour permeability improves the capacity to retain water. Starting from these results, the surface exchange coefficient Zv (s/m) is varied between 5.88 10 6 s/m and 5.88 10 10 s/m, emphasizing the influence of a different surface coating on the moisture buffering capacity of the hygroscopic material. As Fig. 8 shows, the final water content slightly arises (about 0.1%) while Zv increases 100 times from 5.88 10 10 s/m to 5.88 10 8 s/m and it is steady over this value. These data allow to conclude that the moisture buffering capacity is slightly influenced by the surface exchange coefficient and therefore if a surface coating or a packaging system is needed it will have no considerable effects on the material’s sorption capacity. 5. Conclusions This paper presents the MBV of two highly absorbing materials coming from the industrial sector (sodium polyacrylate and cellulose-based material) and of two materials already used in building (gypsum board and perlite) as the results of a dynamic characterization in climatic chamber. As expected, sodium polyacrylate and cellulose-based material exhibit an ‘‘excellent’’ moisture buffer performance and a high velocity of adjustment to RH% variations, that exceed the capacity of more common building materials. Furthermore, all the tested materials, under experimental conditions, are far from their sorption limits. Nevertheless, sodium polyacrylate shows a hysteretic behaviour that in time may cause the loss of its sorption properties. The cellulose-based material seems therefore to be the most suitable for moisture buffer applications. Starting from the properties of a cellulose-based building material (Cellulose insulation) a numerical model is used to investigate the effect of thickness, vapour resistance factor and surface exchange coefficient on moisture buffer performance. The analysis of the numerical results enables to define optimum functioning ranges for these quantities, that may guide the
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selection of a material with a potentially high-performance behaviour. On this point, the numerical data confirm that cellulose is a good moisture buffering material. However, further numerical studies are necessary to investigate the effect of different ambient usage conditions and moisture production rates on the material performance. Field measurements are also needed in order to verify the moisture buffer capacity of cellulose-based material in real conditions. Acknowledgements We thank Eng. A. De Chirico for her practical contribution. This work was developed by S. Cerolini and M. D’Orazio and is a part of a more general research coordinated by A. Stazi. C. Di Perna takes part in the discussion on the possibility to use moisture buffering properties of materials in order to reduce energy consumptions into buildings. References [1] J. Toftum, A.S. Jorgensen, P.O. Fanger, Upper limits of air humidity for preventing warm respiratory discomfort, Energy and Buildings 28 (1998) 15–23. [2] J. Toftum, A.S. Jorgensen, P.O. Fanger, Upper limits for indoor air humidity to avoid uncomfortably humid skin, Energy and Buildings 28 (1998) 1–13. [3] L. Fang, G. Clausen, P.O. Fanger, Impact of temperature and humidity on the perception of indoor air quality, Indoor Air 8 (1998) 80–90. [4] F. Lucas, L. Adelard, F. Garde, H. Boyer, Study of moisture in buildings for hot humid climates, Energy and Buildings 34 (2002) 345–355. [5] K. Sedlbauer, Prediction of mould growth by hygrothermal calculation, Journal of Thermal Envelope and Building Science 25 (2002) 321–335. [6] M. Matilainen, J. Kurnitski, O. Seppa¨nen, Moisture conditions and energy consumption in heated crawl spaces in cold climates, Energy and Buildings 35 (2003) 203–216.
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