Defacement of ETICS Cladding Due to Hygrothermal Behaviour

Transcription

1 Defacement of ETICS Cladding Due to Hygrothermal Behaviour Eva Barreira 1 Vasco Peixoto de Freitas 2 T 24 ABSTRACT External thermal insulation systems for walls (expanded polystyrene insulation faced with a thin rendering) ETICS have been used in Portugal since the nineties. Despite the thermal advantages, Portuguese buildings covered with ETICS are faced with a huge and yet unsolved problem: the system s defacement caused by microbiological growth. In recent years, although several studies have been carried out to solve this problem, the practical conclusions are not very satisfactory: so far the only feasible solution is to use biocides. These chemical products are not environmentally friendly and are considerably expensive because of their short-term effectiveness. To obtain a less expensive solution to this problem, it is essential to first understand the hygrothermal behaviour of façades covered with ETICS, namely, the dynamic heat exchange between the ETICS exterior layer and the atmosphere. It is also very important to study the influence of the surface s orientation and the local climate. This paper presents the results of an experimental characterization of ETICS hygrothermal behaviour performed by the Building Physics Laboratory (LFC) of the Faculty of Engineering of Porto University (FEUP). In-situ tests are being carried out on a building covered with ETICS located in Portugal s northwest region. KEYWORDS Cladding defacement, Hygrothermal behaviour, ETICS, Microbiological growth 1 2 Building Physics Laboratory (LFC), Faculty of Engineering, University of Porto (FEUP), Portugal, barreira@fe.up.pt Building Physics Laboratory (LFC), Faculty of Engineering, University of Porto (FEUP), Portugal, vpfreita@fe.up.pt

2 1 INTRODUCTION External thermal insulation composite systems (ETICS) have been applied on a somewhat regular basis in European buildings since the 70 s. The systems main advantages are guaranteed continued thermal insulation, thinner exterior walls, greater thermal comfort due to the higher interior thermal inertia and ease of application. The latter is particularly important in refurbishment since it may be applied without disturbing the building s dwellers [Freitas 2002]. However, past applications of ETICS have revealed some problems, particularly low impact resistance and diminished long-term durability. The scientific community has performed various studies to fully characterise these systems, to measure the properties of the respective materials, to identify the main problems and, in some cases, to develop solutions [Hens & Carmeliet 2002]. One of the problems not yet solved is the cladding defacement due to accumulated dirt and, in particular, microbiological growth (Fig. 1). Studies already performed in this field were not very satisfactory since, although the phenomenon is known, the only feasible solution is to use biocides which imply economic and, in particular, environmental drawbacks. Figure 1. Building covered with ETICS showing microbiological growth. During the last decade, studies have been performed about the physical phenomenon that promotes microbiological growth on the surface of ETICS [Kunzel & Sedlbauer 2001; Becker 2003]. It is known that the longwave radiation exchange between the exterior surface and the atmosphere during the night causes the ETICS surface temperature to drop [Kunzel & Sedlbauer 2001; Zillig et al. 2003; Holm et al. 2004]. Condensation forms when the exterior surface temperature is lower than the dew point temperature. If the drying process is not sufficiently fast, the surface moisture content remains high for long periods and consequently increases the risk of microbiological growth [Krus et al. 2006]. Knowledge of this physical phenomenon made it possible to develop mathematical models simulating heat and moisture transfer between the system s exterior surface and the atmosphere [Kunzel et al. 2002; Becker 2003]. However, no simple process has yet been developed to foresee the degradation of exterior thermal insulation systems and which may be used by designers and by ETICS application companies. It is therefore essential to study the causes underlying the degradation of ETICS. This is the only way to develop rules that will guarantee its durability. 2 DEFACEMENT OF ETICS CLADDING ETICS cladding represent a substantial market share of external thermal insulation systems for façades. Its growing popularity was, however, undermined by some pathologies [Freitas 2002].

3 Although no statistical studies are available, we may presume that there are a relatively high percentage of cases where algae and fungi have grown on the façades of Portuguese buildings coated with ETICS, particularly when located in coastal zones. Portugal s climate aggravates the problem, particularly in the west coast, which has relatively mild temperatures and very high relative humidity year-round (Fig. 2). Figure 2. Annual outdoor temperature and relative humidity variations in Lisbon (TRY) Degradation due to microbiological growth occurs, in particular, on façades facing north and west, since their coating s high surface moisture content is maintained for longer periods. 3 LONGWAVE RADIATION EXCHANGE Superficial condensation, which takes place predominantly during the night, is caused by a longwave radiation exchange between the exterior surface and the atmosphere. Although radiant exchange occurs on all exterior surfaces, it has more serious consequences on components with substantial thermal insulation which reduces the flow of heat through the construction element practically to zero and on components whose thin exterior layer has a very low thermal inertia, as in the case of ETICS [Kunzel & Sedlbauer 2001]. The radiant balance of the building s façade is affected by three parameters: the building s radiation, the sky s radiation and radiation emitted by terrestrial surfaces located near the building (Fig. 3). A building emits longwave radiation with a total intensity, E b, calculated according to the Stefan- -Boltzmann Law: E b 4 = ε b σ Tb (1) Tb Surface temperature 8 σ = 5, W m K Stefan-Boltzmann constant ε Surface emissivity b On the other hand, the façade absorbs part of the longwave radiation emitted by surrounding objects (terrestrial radiation) and by the sky (atmospheric radiation) [Holm et al. 2004]. Terrestrial radiation is caused by the sum of longwave radiation emitted by the different terrestrial surfaces whose temperature is similar to the building's temperature. Since the temperatures are identical, the radiation exchanges between the building s surface and the surrounding surfaces are somewhat balanced.

4 Figure 3. Radiant balance of a building s façade. Atmospheric radiation depends mostly on what comprises the atmosphere and may behave in two distinct manners. If the sky is cloudy, the atmosphere behaves like a grey body whose temperature is the same as the building's, and emits radiation in a continuous spectrum at an intensity similar to that of terrestrial surfaces. If the sky is clear, the atmosphere stops emitting continuously for all wavelengths. In wavelengths between 8 and 13 μm, the atmosphere s emitted radiation is decreased considerably mostly due to the influence of molecules of water, carbon dioxide and ozone. Since the maximum intensity emitted by all terrestrial surfaces occurs for 10 μm, there is a loss of radiation to the sky which is not compensated by atmospheric radiation. This negative balance that is not compensated by solar radiation during the night causes the building's superficial temperature to decrease, which is maintained until heat transport by convection and by conduction compensate for the loss by radiation [Holm et al. 2004]. Condensation takes place whenever the superficial temperature is lower than the dew point temperature. The accumulation of condensed water creates favourable conditions for algae and fungi growth. Algae is able to grow when the relative humidity on the façade s surface is greater than 70%-80%, within a temperature range between 0º and 40º C. Algae can withstand long dry periods and then begin to flourish again when enough humidity is available. Consequently, a dry surface during the day is not sufficient to prevent algae growth [Zillig et al. 2003]. Fungi require a minimum relative humidity of 80% to grow. However, and unlike algae, fungi need nutrients to grow, a factor which depends on the finishing layer s materials [Zillig et al. 2003]. 4 EXPERIMENTAL STUDY 4.1 Setting Up the Test To evaluate exterior superficial temperature variations on the ETICS, instruments were set up on the façades of a low building in the Northwest of Portugal whose walls face the four cardinal directions (Figs. 4 and 5). While the system was being applied, in April 2007, T-type thermocouples (copper constantan) were placed inside the system on the four façades, at about 1 m from the ground, as shown in Fig. 5. Three thermocouples were placed on each façade wall (on the interior side of the thermal insulation, between the thermal insulation and the rendering and on the rendering's exterior surface). A second thermocouple was placed on the North and West façades, on the exterior surface of the rendering next to the North/East and West/South corners, respectively. Two temperature and relative humidity sensors were also installed, coupled to a data acquisition system, to monitor the hygrothermal conditions (temperature and relative humidity) inside and outside the building.

5 Figure 4. Building where the measurements are being taken. 4.2 Results Figure 5. Layout showing thermocouple locations. The temperature and relative humidity measurements inside and outside the building (Fig. 6), between 17/05/2007 and 14//06/2007, reveal that: The exterior temperature varies from 11º C to 31º C; The exterior relative humidity is relatively high, on average about 76%; The interior temperature is somewhat constant and of about 24º C; The interior relative humidity is, on average, of about 64%. In the same period, the temperature variation in the ETICS of the West façade s wall is indicated in Fig. 7 and 8. These graphs also indicate the dew point temperature (Tdp), witch is dependant on the exterior temperature and relative humidity. An analysis of the data measured in the façade wall reveals that: The temperatures measured by the T1 and T2 thermocouples are very similar: remain somewhat constant during the night, increase progressively at dawn and morning and reach their maximum at about 18:00 h; The maximum temperatures measured by the T1 and T2 thermocouples varied from 23º C to 52º C, and the temperatures measured during the night varied from 10º C to 18º C; The temperatures measured by thermocouple T3 ranged from 17º C to 29º C, and the maximum temperature occurred at about 19:00 h. Note that the temperature measured by the T3 thermocouple differed, on average, by about 2º C from the interior temperature.

6 Figure 6. Interior and exterior temperature and relative humidity (17/05/2007 to 14/06/2007). Figure 7. Temperature variation in the ETICS and of the dew point temperature (17/05/2007 to 14/06/2007). By comparing the exterior superficial temperatures of the ETICS, obtained by the T1 thermocouple, with the dew point temperature (Tdp), we were able to determine the differences between the two temperatures. The graph in Fig. 9 shows only the negative temperature differences (superficial temperature lower than the dew point temperature), whereby the positive temperature differences were taken as equal to zero. The graph in Fig. 10 defines the total daily time during which superficial condensation takes place in the west façade's wall. 4.3 Review of Results An analysis of the results obtained until now reveals that: The thermal resistance of the ETICS exterior layer is practically nil, since the temperatures on the two sides are very similar. This result was foreseeable because of the thin exterior layer. The temperature on the inner side of the thermal insulation varied about 5º C during the day and is relatively similar to the temperature inside the building. Condensation takes place whenever the surface temperature is lower than the dew point temperature. In the period under analysis, surface condensation took place during less than 10% of the time.

7 Figure 8. Temperature variation in the ETICS and of the dew point temperature on 06/06/2007. Figure 9. Difference between the superficial temperature of the ETICS and the dew point temperature (17/05/2007 to 14/06/2007). Positive temperature differences were taken as equal to zero. Figure 10. Period during which superficial condensation took place (17/05/2007 to 14/06/2007).

8 4 CONCLUSIONS ETICS have been applied regularly in Portuguese buildings. However, despite their advantages, the cladding defacement is a very worrisome pathology. Increased suface moisture content, caused by nighttime condensation, is one of the factors that influence this aeshetical degradation. Superficial condensation is caused by radiative cooling of the surface, which is more detrimental during the night due to the lack of incident solar radiation. ETICS are highly susceptible to superficial condensation. Instruments were placed on a small building in the Ovar zone to evaluate superficial temperature variations in ETICS. Measuredata is being collect since May 2007, making it possible to evaluate the period during which superficial condensation takes place and the difference between the superficial temperature and the dew point temperature. In the future, the authors wish to maintain this experimental program by continuing to collect data during autumn and winter. Superficial temperature variations according to the orientation will also be evaluated. ACKNOWLEDGMENTS The authors would like to thank Iberfibran Poliestireno Extrudido, SA for having created the necessary conditions to carry out this experimental campaign. REFERENCES Becker, R. 2003, Patterned staining of rendered facades: hygrothermal analysis as a means for diagnosis, Journal of Thermal Envelope and Building Science, 26[4], Freitas, V. P. 2002, Isolamento Térmico de Fachadas pelo Exterior Reboco Delgado Armado Sobre Poliestireno Expandido (ETICS), HT 191A/02, Prof. Engº Vasco P. de Freitas, Lda, Porto, Dezembro. Hens, H. & Carmeliet, J. 2002, Performance prediction for masonry walls with EIFS using calculation procedures and laboratory testing, Journal of Thermal Env. & Bldg Sci, 25, Holm, A., Zillig, W. & Kunzel, H. 2004, Exterior surface temperature and humidity of walls Comparison of experiment and numerical simulation, Proc. Performance of Exterior Envelopes of Whole Buildings IX, ASHRAE, Florida, USA, 5 10 December Kunzel, H. & Sedlbauer, K. 2001, Biological growth on stucco, Proc. Perform. of Exterior Envelopes of Whole Buildings VIII: Integration of Building Envelopes, ASHRAE, Florida, 2 7 Dec Kunzel, H., Schmidt, Th. & Holm, A. 2002, Exterior surface temperature of different wall constructions Comparison of numerical simulation and experiment, Proc. 11th Symposium of Building Physics, TUD, Dresden, Germany, September 2002, Vol. 1, pp Krus, M., Rosler, D., Sedlbauer, K. 2006, New model for the hygrothermal calculation of condensate on the external building surface, Proc. Third International Building Physics Conference Research in Building Physics and Building Engineering, Montreal, 2006, pp Zillig, W., Lenz, K. & Krus, M. 2003, Condensation on façades influence of construction type and orientation, Proc. Research in Building Physics, Leuven, September 2003, pp