Energy and Buildings 41 (2009) 164–168
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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
168
S. Cerolini et al. / Energy and Buildings 41 (2009) 164–168
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.
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