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Use of moisture-buffering tiles for indoor climate stability under different climatic requirements.

Introduction

Indoor humidity in residential buildings is a recurrent topic of building physics, since it is an important factor for human health and well-being. Even in buildings with high-quality insulation, excessive indoor humidity conditions may occur because the occupants produce too much moisture that is not compensated by adequate ventilation. In addition, the change of temperature in rooms can result in temporary high relative humidity (RH) of the air.

Beside furniture, indoor lining materials have the potential of a humidity-buffering effect. Appropriate materials absorb water vapor if indoor RH increases and release it if indoor RH decreases. The result is the damping of fluctuations in air humidity.

This so-called moisture-buffering effect was studied within the ECBCS ANNEX 41 (Holm 2008). The advantages mentioned are more stable climate conditions, which result in a higher comfort level. Furthermore, the energy use for humidification and dehumidification is expected to decrease, as short-time humidity extremes are buffered in the enclosing surfaces, and action from the mechanical system is not required.

Hygroscopic interior tiles

The new developed material is a ceramic interior tile that is optimized with its pore structure to show a good performance in terms of moisture buffering. The surface of the tiles is unsealed and the Brunauer-Emmett-Teller (BeT) surface area is 43 [m.sup.2]/g (209-944 [ft.sup.2]/lb). The meso-pore radius of alumina-based functional pores with a desired pore size of 4-5 nm (1.57E-7-1.97E-7 in) allows the material to adsorb or desorb water molecules depending on the difference in water vapor pressure between material and surrounding air. The structure pores (in the 30-50-nm [1.18E-6-1.97E-6 in] and 200-300-nm [7.87E-6-1.18E-5 in] ranges) need to be controlled during the firing of the tiles to allow the transfer of water molecules into the nano pore material inside. The approximate material composition is listed in Table 1.

The ceramic interior tiles were tested for their hygrothermal material properties at the certified laboratory of Fraunhofer IBP in Holzkirchen, Germany, with common and standardized test methods. The results of the material property tests are shown in Table 2 and Figure 1.

Climate chamber moisture-buffering test

A VCE 1000 test chamber was used for the experiments. This chamber allows conditioning the interior space from 20[degrees]C-130[degrees]C(68[degrees]F-266[degrees]F) with an accuracy of [+ or -]0.1[degrees]C([+ or -]0.18[degrees]F) and a temperature change rate of 0.3 K/min (0.54[degrees]F/min). The RH can be controlled by the dew point temperature in a range between 5[degrees]C and 60[degrees]C(41[degrees]F and 140[degrees]F) supplied by an air change rate from 0 1/h and 1 1/h. The useful chamber volume is 0.916 [m.sup.3] (33.937 [ft.sup.3]), the inner surface area 6.015 [m.sup.2] (64.745 [ft.sup.2]) resulting from dimensions 0.75 m (2.46 ft) width to 0.75 m (2.46 ft) height to 1.63 m (5.348 ft) depth.

[FIGURE 1 OMITTED]

Twenty-seven tiles with 0.3 m by 0.3 m (0.948 ft by 0.948 ft), which results in a total surface area of 2.43 [m.sup.2] (26.156 [ft.sup.2])(~40% of the space surface area), were stored inside the test chamber to allow airflow between the tiles. Sensors for temperature and RH were installed at 0.25 m (0.82 ft) and 1.00 m (3.28 ft) depths from the front opening in 0.25 m (0.82 ft) and 0.5 m (1.64 ft) heights. The temperature was measured with in-house calibrated ThermoSensor PT100 sensors with an accuracy of [+ or -]0.1[degrees]C([+ or -]0.18[degrees]F) and the RH with Rotronic SC05 sensors with an accuracy of [+ or -]2% RH.

The boundary conditions for the test are shown in Figure 2. For the summer case, a constant absolute humidity inside the zone is assumed. This means that the test chamber is kept airtight after the initialization, i.e., the air change rate is close to zero. The temperature for this case changes over the day between 20[degrees]C and 35[degrees]C(68[degrees]F and 95[degrees]F). For the first 10 h of the day, the temperature is kept at constant 20[degrees]C(68[degrees]F). In 2 h, the linear temperature increase result is 35[degrees]C(95[degrees]F), which is maintained for the next 10 h followed by a linear decrease back to 20[degrees]C(68[degrees]F). This is similar to an experiment in an airtight chamber described by Maeda and Ishida (2008), who found excellent humidity-controlling properties for hydrothermally solidified materials.

A constant temperature at 23[degrees]C (73.4[degrees]F) defines the winter case. Moisture production inside the chamber is reproduced by a certain scheme of dew point temperature and air change rates. Both cases are initialized with the shown initialization conditions to reach consistent initial conditions for all test cases. The test procedure scheduled in each case is an initialization phase of one to three days and a test period of three days.

The first test case was the summer case with changing temperature conditions. In advance, the chamber was initialized to 20[degrees]C(68[degrees]F) air temperature and 18[degrees]C (64.4[degrees]F) dew point temperature. As this dew point temperature inside the chamber cannot be reached within a reasonable period of time (especially with the installed moisture-buffering tiles, which leads to a slow asymptotic convergence up to the desired chamber RH), the initialization process was stopped after three and a half days of initialization at 75% RH inside the test chambers, and the summer test case was started.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

Realistic humidity production cycles inside the zone are assumed for the winter case. This means a basic level of moisture production all throughout the day caused by humans, animals, plants, and so on. There are two cycles. one with high moisture production that reflects the morning time with showering people or people preparing their food for the day; the second reflects the afternoon time when people arrive back home from work and prepare food or take a bath. The first cycle is assumed to be 3 h from 7:00 a.m. to 10:00 a.m., and the second is 4 h from 4:00 p.m. to 8:00 p.m. The total moisture production as integral over the moisture production rate over the day is downscaled from 12 kg/d (26.455 lb/d) for a regular four-person family in a living space of 300 [m.sup.3] (10,594.4 [ft.sup.3]) using the volume ratio. This results in a moisture production of 40 g/d (0.088 lb/d) for the chamber experiment, which is provided by the increased RH of the supply air.

The results of the experiment are shown in Figure 3 for one day of the experiment. Temperature and RH inside the test chambers on one representative measurement point are compared in the graphs for the summer and winter test case, with both empty test chambers and test chambers with moisture-buffering tiles installed.

The winter test with moisture production rates inside the zone and a constant temperature shows a very significant reduction of daily RH fluctuations. The first moisture production cycle with the highest moisture production rates results in a higher peak than the second moisture production cycle in the afternoon, which lasts 1 h longer than the first moisture production cycle.

In the summer case, the empty chamber shows increasing temperature with decreasing RH, just as one would expect. With moisture-buffering tiles installed, the decrease in RH is much lower. The peaks at the beginning and end of every test cycle are caused by the control of the chamber.

The maximum daily range of RH without tiles is around 45%. With moisture-buffering tiles installed on the surfaces, the daily variation in RH decreases to 11% RH in the summer case and 7% RH in the winter case.

The measurement results show a significant effect of the moisture-buffering tiles on the RH inside the room. The tiles are able to buffer moisture in times with high RH and release the moisture again in times of low RH. This results in a damped RH fluctuation inside the room. The resulting more stable climate provides longer periods with good comfort conditions. similar results were found by using other types of moisture-buffering materials, such as wood-based materials (Kunzel et al. 2006). The performance in terms of moisture-buffer capacity is still higher for the tiles presented within this article.

Verification of simulation model

The advanced hygrothermal whole-building simulation tool WUFI[R]Plus is used for simulation (Holm et al. 2003). This software couples whole-building energy modeling with hygrothermal component modeling. it calculates the coupled heat and mass transfer, i.e., mass transfer by diffusion and liquid water transport, for every enclosing assembly and couples heat and moisture fluxes from the inner surfaces with the zone temperature and RH. This model allows the combined assessment of hygrothermal conditions of the building envelope, indoor climate, and energy demand of the building. it generally requires hourly exterior climate data, which is usually provided by the climate database that comes with the software. Materials are selected from a material database or specified by providing thermal and hygric properties so that assemblies can be built from one or more layers of different materials. Defining building geometry and orientation allows the assignment of an assembly to all components. As the indoor climate is a result of the simulation, appropriate assumptions for inner sources and set-points need to be made. The set-points can be met by specifying an ideal HVAC system.

The simulation model has to be verified by simulation of the experiments in the chamber. The used air chamber is not 100% airtight; therefore, an air change rate of 0.003 ACH is used for simulation.

A 3D-model of the chamber is built in the software with all parameters recorded in the experimental setup used as input conditions. "External" temperature and RH conditions as well as ventilation rates are used from the chambers target value logging. The simulation runs with a 2-min time step to accurately model all changes in temperature and RH. Outputs of the simulation are resulting temperature and RH inside the chamber, which are used as verification values.

The verification of the software model is performed in two steps. In a first step, the results of the measurements of the empty chambers in the summer and winter test cases are compared. This validates the zone model, as moisture transport in the building enclosure is not existent.

Figure 4 compares the simulation results with measurements of the RH. In both the summer and winter cases, very good agreement between verification simulation and verification measurement is found. In the winter case, the peak in RH during the first moisture production cycle is slightly higher for the measurement.

Verification measurement and verification simulation also show very good agreement for the winter case with moisture-buffering tiles installed, as shown in Figure 5. In the summer case, the starting level is not the same, resulting from differences during the initialization process between simulation and measurement, which was also modeled in the simulation. Still the fluctuations in measurement and simulation are found to be very close but slightly higher for the simulation case.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Modeling the experiment with the hygrothermal whole-building simulation allows validating the simulation tool. Very good verification results can be achieved by accurately applying all necessary boundary conditions in the model and using appropriate simulation time steps.

Calibration of the verification simulation required two main adjustments. Measurement results showed, especially for the summer case, that the assumption of a completely airtight chamber is not correct. As a result, the air change rate was adjusted to match the measurements. Furthermore, the experiments with differing air change rates in the test chamber by mechanical ventilation result in changing air velocities. The air velocity close to the surface is one of the parameters to define the surface resistances. The software uses fixed heat transfer resistances on the inner surface. The moisture transfer resistance is assumed to be linearly dependent on the heat transfer resistance. For the combined simulation of initialization and test phase, a separate file with varying resistance values per phase was provided to better match simulations with measurements.

Still, the real velocity for the summer case, with a close to airtight chamber and constant interior surface temperatures, is very low. Zero air velocity, i.e., a very low heat transfer coefficient, is not allowed within the building simulation software. The linearly dependent moisture transfer coefficient in that case is computed with a fixed heat transfer coefficient of 3.5 W/([m.sup.2]K) (0.616 BTU/(h[ft.sup.2][degrees]F)) (WUFI 2010). This results in the small differences for the summer case verification simulation.

Overall, it can be concluded that the application of hygrothermal whole-building simulation allows an assessment of the performance of moisture-buffering materials.

Real room application

After a successful verification, analysis and upscaling to real room scenarios can be performed with the simulation software. This allows energy demand computation and enables the assessment of thermal comfort conditions inside the balanced zone combined with the assessment of heat and mass transfer in the building assemblies.

Detailed assessments are undertaken by performing simulations in eight different climatic zones in the united states. The same model of a room--one equipped with moisture-buffering tiles on one wall and the ceiling, and the other equipped with painted gypsum boards on the inside--is simulated with climate conditions from Anchorage, Atlanta, Baltimore, Chicago, Fargo, Miami, Minneapolis, and Phoenix according to ASHRAE Standard 90.1 zone specification (ASHRAE 2007). The modeled room contains one zone with a volume of 229.8 [m.sup.3] (8,115.3 [ft.sup.3]) and a floor area of 96.7 [m.sup.2] (1,040.87 [ft.sup.2]). North-facing 5.4 [m.sup.2] (58.125 [ft.sup.2]) and south-facing 9.0 [m.sup.2] (96.875 [ft.sup.2]) windows are installed. A constant ventilation rate of 0.5 ACH with a total daily moisture production of 7.5 kg (16.534 lb) produced in a daily cycle to reproduce residential usage was assumed. The hours with RH in the range between 35% and 75% are assessed in the first step. In the second step, the annual humidification /dehumidification loads to maintain the RH range assessed in the first step are computed.

[FIGURE 6 OMITTED]

The effectiveness of the moisture-buffering tiles depends on the external climate and on the use of the room, as well as on the used HVAC. Figure 6 shows that an effect can be found in every one of the eight climate zones except zone 8. For every other climate zone, an improvement, i.e., a longer period of time with RH above 35% and below 75%, is found.

[FIGURE 7 OMITTED]

The same is found for humidification and dehumidification loads. The load can be decreased in every climate zone by using moisture-buffering materials as an inner lining. Figure 7 compares the necessary loads to keep the RH between 35% and 75%.

Conclusions

A successful experiment was conducted. It showed the influence of specially designed ceramic interior tiles on the moisture performance of rooms. The tiles proved to have a large effect on the dynamic changes of the interior humidity conditions.

The verification simulations showed that it is possible to use hygrothermal whole-building simulation tools for the assessment of the effect of moisture-buffering materials in buildings. This allowed the assumption that upscaling the experiments to real room scenarios via simulation is possible. As energy use for heating/cooling and dehumidification /humidification is significantly influenced in rooms with moisture-buffering surfaces, the use of modeling tools capable of modeling the hygrothermal interaction between room and surrounding surfaces is highly recommended.

Using moisture-buffering materials as an inner surface material allows maintaining a more stable climate without active measures. A higher comfort level can be achieved, and extremely high RHs can be avoided. However, if the RH is already high, it remains on a high level. The risk of mold growth should be further analyzed. Energy use for humidification and dehumidification can be reduced significantly in every climate zone by using moisture-buffering materials.

How much moisture-buffering material needs to be installed for different building types and usages applied in different climate zones must yet be assessed. This will allow an optimization of building energy use and comfort conditions under an economical point of view.

DOI: 10.1080/10789669.2012.645399

References

ASHRAE. 2007. ASHRAE Standard 90.1-2007, energy standard for buildings except low-rise residential buildings. Atlanta: American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc.

Holm A. 2008. ANNEX 41 Whole building heat, air and moisture response (MOIST-EN), Volume 4: Applications: Indoor environment, energy, durability. Final Report International Energy Agency.

Holm, A., H.M. Kunzel, and K. Sedlbauer. 2003. The hygrothermal behaviour of rooms: Combining thermal building simulation and hygrothermal envelope calculation. Eighth International IBPSA Conference, Eindhoven, Netherlands, August 11-14.

Kunzel, H., A. Holm, K. Sedlbaver, F. Antretter, M. Ellinger, and J. Vesely. 2006. Feuchtepufferwirkung von Innenraumbekleidungen aus Holz und Holzwerkstoffen. Fraunhofer IRB Verlage: Bauforschung fur die Praxis, Band 75.

Maeda, H., and E. Ishida. 2009. Water vapor adsorption and desorption of mesoporous materials derived from metakaolinite by hydrothermal treatment. Ceramics International 35(2009): 987-90.

WUFI[R]Pro. 2010. WUFI[R]Pro 5.1 Moisture design tool for architects and engineers Online Help. Holzkirchen, Germany: Fraunhofer-Institut fur Bauphysik.

Received October 31, 2011; accepted November 3, 2011

Florian Antretter is Group Manager Hygrothermal Building Analysis. Christoph Mitterer is Scientist. Seong-Moon Young, PhD, is General Manager (IMA Group Leader).

Florian Antretter, (1), * Christoph Mitterer, (1) and Seoung-Moon Young (2)

(1) Fraunkofer-Institut fur Bauphysik, Holzkirchen, Germany

(2) Hausys Research & Development.InC PJT, LG Hausys, Ltd, Daejeon, Korea

* Corresponding author e-mail: florian.antretter@ibp.fraunhofer.de
Table 1. Material composition of
humidity-buffering tiles.

Compound Percentage

Si[O.sub.2] 45.8
Al2[O.sub.3] 39.2
[Fe.sub.2][O.sub.3] 0.8
Ti[O.sub.2] 0.5
CaO 12.6
MgO 0.3
[K.sub.2]O 0.7
[Na.sub.2]O 0.1

Table 2. Material properties of ceramic interior
tiles.

 Humidity-
 buffering
 tile

Thickness, mm 7.2
Bulk density, kg/[m.sup.3] 1335
Porosity, [m.sup.3]/[m.sup.3] 0.51
Specific heat capacity dry, J/kgK 850
Thermal conductivity, W/mK 1.6
Water vapour diffusion factor (--) 9.8
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Author:Antretter, Florian; Mitterer, Christoph; Young, Seoung-Moon
Publication:HVAC & R Research
Date:Jan 1, 2012
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