Microorganism on façades reasons, consequences and measures K. Lengsfeld, M. Krus Fraunhofer-Institute for Building Physics (IBP), Holzkirchen, Germany ABSTRACT: Humidity is the essential condition for biological growth on façades. Beneath wetting by wind driven rain, condensation occurs in consequence of long wave radiation in clear nights by reaching temperatures below the dew point temperature of the air. The importance of this wetting mechanism is obvious with regard to the occurrence of microorganisms mostly on the northern and western sides of buildings. To find out which nutrients and circumstances of temperature and moisture algae need for the growth several façades have been erected at the test field area in Holzkirchen. In this paper results of the field examinations of different construction types with different surface finishings will be shown. 1 INTRODUCTION In recent times more and more complaints about algae growth on façades have been made. Due to the higher insulation standards the temperature and moisture situations have been changed on façades. This growth occurs mostly in the first years after completion which leads to displeasure from the building owner. Up to now there are different opinions about the reasons of growth of microorganism because the physical causes are not yet clarified. For a detailed analysis different wall constructions have been erected at the test field area of the Fraunhofer- Institute for Building Physics (IBP) in Holzkirchen, Germany. 2 GROWTH CONDITIONS Algae are spread over the whole world and they are a major part of the ecosystem. They are reckoned as the biggest oxygen producer worldwide. A lot of research is done in the field of aquatic algae. But aerophytic algae have been recognised in the last years as important part in the microbiological growth on façades. The current opinion is that mainly blue and green algae grow on façades. Sporadically some red or gold algae are identified. There is a lack of knowledge about the growth conditions and the biotic influences. But some common demands can be specified. For photosynthesis sufficient light, water, temperature, carbon dioxide and some mineral nutrients must be present. Some algae need some trace elements (as Fe, Mn, Si, Zn, Cu, Co, Mo, B, V) for growth, which are normally available in our environment (rain, dust), so that the local micro climate is the determining factor for biological growth on façades. The most important climate conditions are humidity and temperature. Humidity is fundamental for algae growth as it is needed for photosynthesis. Because algae don t have any roots, the water uptake must occur directly through the cell wall by osmosis. The growth limit for green and for blue algae is 100 % RH (liquid water) (Scherer 1993). Wind driven rain and dew water are the main reason for wetting of façades with liquid water. Algae can survive dry periods without any harm and can restart their growth when enough humidity is available. Therefore a drying of façades during the day is not sufficient to prevent algae growth. Venzmer describes the optimal growth conditions on façades for green algae within a temperature range from 0 C to 40 C (Venzmer 2001). Under dry conditions algae can withstand extreme thermal conditions (heat or cold stress) much better than in humid conditions. Therefore the prediction of algae growth on façades is an unsolved probelm. This approach describes a first attempt to solve this problem.
3 HYGROTHERMAL BOUNDARY CONDITIONS The hygrothermal boundary conditions on the outdoor surface are very important for the growth of microorganism on the façades. These conditions are influenced by several and concurrent physical phenomena. The irradiation between the outdoor surface and the surrounding leads to a permanent loss of energy of the surface. The surface temperature increase by solar radiation during the day and therefore the relative humidity on the surface decreases and the wall gets drier. In the night there is no solar radiation and because of the thermal irradiation the surface temperature decreases enhancing the risk of condensation on the façade. As mentioned above suitable temperature and humidity conditions at the outer surface of walls are necessary for biological growth. Moulds need a relative humidity of approximately 80 % (depending on temperature) for a longer period of time for example (Sedlbauer 2001), whereas algae need higher humidity for their growth or even free water. Whereas an interim drying out do not harm them. The fact that microbiological growth mostly occurs on the northern oriented façades, where very low wind driven rain arise, shows that the surface condensation due to natural long wave irradiation is the main moisture source on the façades. Therefore the periods of surface condensation during the night time are taken as criterion to classify the results. If the surface temperature gets colder than the dew point temperature of the outdoor air then condensation of the surface occurs. In figure 1 the courses of surface temperatures of an old building (U-value = 1,1 W/(m²*K)) and a well insulated façade (U-value = 0,35 W/(m²*K)) are shown for a night in September. The effect of an additional insulation on the temperature of the outdoor surface is clearly visible. The surface temperature of the old building is higher than the dew point temperature during the whole night whereas the temperature of the well insulated façade decrease under the dew point temperature from 3 to 8 in the morning. The result is a higher amount of condensation on the surface of a well insulated façade. Figure 1: Courses of the surface temperature of an old building and a well insulated façade with ETICS, compared to the dew point temperature of the outside air. 4 EXPERIMENTAL INVESTIGATIONS The main construction of the test façades are made of concrete with an ETICS (20 cm concrete, heat conductivity 1,6 W/(m*K); 10 cm polystyrene slabs, heat conductivity 0,04 W/(m*K) and different plasters and coatings. The thermal transmittance of the test walls are about U = 0,35 W/(m²*K). Starting from this standard construction a series of tests with varying parameters were performed to find out the influences on algae growth. The list of variations is shown in table 1.
At the test site in Holzkirchen west-facing façades are affected mostly by algae discoloration due to extreme wind driven rain. But for this study the walls are oriented to the west and north because these orientations are the most critical for growth of microorganisms. Table 1: Variation of the façades Case 1 3 mm plaster with a white coating (a = 0,2) Case 2 10 mm plaster with a white coating Case 3 3 mm plaster with a grey coating (a = 0,4) Case 4 3 mm plaster with a IR-active grey coating (e = 0,7) Case 5 8 mm plaster with phase changing material (PCM) and a grey coating Case 6 8 mm plaster with PCM and an IR-active grey coating (e = 0,7) 5 RESULTS The measurements run over three years and the results in the following are shown only for the autumn because this period is the most critical period for the growth of algae. The evaluation of the different constructions depends on the duration when the surface temperatures of the façades decrease below the dew point temperature of the outdoor air. The temperatures of the façades are measured with PT100 sensors which are installed circa 1-2 mm under the surface of the outside plaster. 5.1 Influence of the thickness of the plaster A possibility to reduce the time of wetness on the surface is to use a thicker plaster. Normally a system with 3 mm plaster is applied. The first comparison to the standard construction is with a test façade with a thick plaster (10 mm). For façades with ETICS the thickness of the outer plaster layer should have an influence on the course of surface temperature because of the correspondent heat capacity. In figure 2 the courses of the surface temperatures in a night of September are shown. 18 15 dew point temperature ETICS 10 cm, plaster = 3 mm ETICS 10 cm, plaster = 10 mm Surface temperature [ C] 12 9 6 3 0 18:00 20:00 22:00 00:00 02:00 04:00 06:00 08:00 10:00 time [h] Figure 2: Courses of the surface temperature of two western oriented façades with 10 cm ETICS and different thicknesses of plaster (3 mm and 10 mm) in a night in September.
The temperature of the façade with a thick plaster decreases circa one hour later below the dew point temperature than the temperature of the standard construction with 3 mm plaster. But in the morning when the dew point temperature and the surface temperatures increase the temperature of the façade with thick plaster needs more time to gets above the dew point temperature. The effect of a thicker plaster is not much more than a time shift of circa one hour in comparison to the standard construction. 5.2 Influence of a painting with low IR-emission The long-wave radiation properties of the surface affect the surface temperatures of façades. In the Cases 3 and 4 the difference between a grey coating and a grey coating with low IRemissivity are compared. The radiation coefficients of the coatings are for the short-wave absorptivity 0,4 and the long-wave emissivity 0,9 respectively 0,7 for the coating with low IR. The surface temperatures of these two façades are shown in the figures 3 and 4. 40 35 dew point temperature ETICS 10 cm ETICS 10 cm with IR 30 temperature [ C] 25 20 15 10 5 0 00:00 03:00 06:00 09:00 12:00 15:00 18:00 21:00 time [h] 00:00 Figure 3: Courses of the surface temperature of two western oriented façades with 10 cm ETICS and different grey coatings with and without IR-effect for one day in September. The increase of the surface temperature of the façade in the morning with an IR coating is faster and higher as the standard construction with a grey colored surface. A positive effect of the high surface temperatures of circa 40 C is the possible lethal impact on algae. A more detailed view of the effect during the night is made in figure 4.
Figure 4: Courses of the surface temperature of two western oriented façades with 10 cm ETICS and different grey coatings with and without IR-effect in a night in September. The surface temperature on a wall with an IR coating is clearly warmer than on a façade without an IR coating. Due to the reduction of the emissivity of the paint the exchange of long wave radiation during the day and night decreases and in following the surface temperature of this façade is higher and the time of wetness by condensation is lower in the night. 5.3 Influence of phase change materials in the plaster A new technology to reduce the growth of algae on façades is the possibility to insert phase change material (PCM) into the outside plaster. Due to the latent heat effects of PCM the surface temperatures could be positive influenced. The change of the surface temperature of a façade with PCM is shown in figure 5. For the comparison of the effect of PCM the surface temperatures of Case 3, 5 and 6 are added to the diagram. Case 6 is a combination of a façade with 8 mm thick plaster with PCM and a coating with IR-effect. temperature [ C] 15 10 5 0 dew point temperature 3 mm plaster with grey coating 8 mm plaster with PCM and grey coating 8 mm plaster with PCM and grey coating with IR-effect -5 16:00 18:00 20:00 22:00 00:00 02:00 04:00 06:00 08:00 10:00 time [h] Figure 5: Courses of the surface temperature of three northern oriented façades with 10 cm ETICS and different grey coatings with and without IR-effect and with PCM in a night in November.
The impact of the PCM and the IR-effect of the materials is shown in figure 5. In the afternoon the temperatures of the three façades decrease in a different way depending on the properties of the surface and the material. The standard construction (blue line) with a grey coating reaches the lowest surface temperature. The temperature is below the dew point temperature during the whole night from 5 p.m. to 5 a.m.. The influence of the PCM (black line) is reflected by the higher temperature as well as the façade with PCM and IR-effect (green line). The temperature of the façade with PCM is only for short periods between 6 to 8 p.m. and 2:30 to 5 a.m. below the dew point temperature. At this point the stored latent heat is exhausted. In the morning the temperature of the standard construction increases faster because the façades with PCM need more energy to warm up. In contrast to this the façade with PCM and IR-painting has nearly all the night no problems with condensation. 5.4 Results of the measurements A lot of different material properties are considered in this project. Due to the fact that the periods of growth of microorganism on façades are mostly in the autumn, the analysis is for October. The results are compared in figure 6. It shows the accumulated hours of the surface temperatures below the dew point temperature. Figure 6: The different constructions with various coatings, colours and thickness of plaster are compared in relation to the time below the dew point temperature. The left side of figure 6 shows the results of four western oriented and on the right four northern oriented test façades. It is clearly to see the influence of different changes on the system. On the west oriented façades a reduction of the time of wetness is reached with a thicker plaster of 12 %, a grey coating of 30 % and with a coating with IR-effect of 46 % in comparison to a standard construction with a thin white plaster. The reduction on the time of condensation on the surface on the north oriented façades is based on the thin plaster with a white coating. The application of a grey paint reduces the time of wetness about 13 %, with PCM in a plaster with 8 mm 34 % and additional with a coating with IR-effect about 55 %.
6 CONCLUSION To assess the risk of microbiological growth the amount of condensation on the surfaces due to long wave irradiation is a good criterion because microbiological growth can be found often on northern oriented façades. Only in regions with an extreme wind driven rain load, like on the test side in Holzkirchen, additionally on the western oriented façades microbiological growth can be found. In direct comparison of monolithic walls with constructions with ETICS the monolithic walls have less problems with microbiological growth, because of their higher heat capacity near the outside plaster. But for the energetic improvement of existing buildings in most cases only the use of ETICS is applicable. Therefore solutions have to be found for these systems, too. For these questions numerous measurements on the outdoor testing side have been conducted. It can be shown that the use of a thick plaster brings only a slight improvement, but the most advancement of this system lies in the capability of absorbing the condensate due to the material. More effective is the case of IR-paints, which reduce the time of condensation about 35 %. With PCM added to the plaster applicable only for plasters with a thickness of 8 mm minimal a comparable reduction is possible strongly depending on the choice of the PCM. The highest effect with about 50 % the reduction could be achieved with a combination of all three measures (thick plaster with PCM and an IR-paint). Up to now unfortunately IR-paints which are available on market are not stable enough for long-term weathering, but the results which are shown in this paper demonstrate that these IRpaints are worth to do necessary developments. For the assessment of the presented results you have to keep in mind that nevertheless most of buildings with ETICS are working well. Only a few percent show greater problems with microbiological growth. For this reason the authors assume that a reduction of the condensation period by about 25 % are enough to solve most of the problems. But in special cases, for example with shady façades near a forest and a river or alike, the only applicable measure is the use of biocides. 7 REFERENCES Scherer, S.: Sedlbauer, K.: Venzmer, H.: 1993. Anpassungen von Cyanobakterien in Wüsten. In: Hausmann, K. & Kremer, B. P.: Extremophile: Mikroorganismen in ausgefallenen Lebensräumen. VCH. Weinheim; New York; Basel; Cambridge; Tokyo, S. 179-193. 2001. Vorhersage von Schimmelpilzbildung auf und in Bauteilen (Prediction of Mould Growth on Top of and Inside Building Parts). Thesis, University of Stuttgart. 2001. Grüne Fassaden nach der Instandsetzung durch WDVS? Nicht bestellt und dennoch frei Haus. 3. Dahlberg-Kolloquium