Application of fluorometric and numerical analysis for assessing the algal resistance of external thermal insulation composite systems

As a consequence of the reduced transmission heat loss, algal growth on external thermal insulation composite systems has given rise to a serious aesthetical problem over the last decade. Manufacturers of paints and rendering systems are competing to increase the algal resistance of their products. The high time investment of free-weathering tests and the lack of objective measures to quantify the growth, however, prevent a systematic and efficient product advancement. Within a multiannual study, the application of fluorometric and numerical analysis was evaluated for assessing the algal resistance of external thermal insulation composite systems. The efficiency of pulse-amplitude modulation fluorometry for directly quantifying the algal biomass on the facade surface was analysed within three weathering tests which comprised 33 different external thermal insulation composite system specimens. The results show that the IMAGING-PAM (imaging pulse-amplitude modulation) fluorometer of the company Walz allows to measure the algal resistance in the course of the weathering process objectively and efficiently. The measurements confirm the effectiveness of biocides and indicate a higher algal resistance of the mineral rendering systems compared to the organic systems. The options and limitations of using numerical simulation for the assessment of the algal resistance of external thermal insulation composite systems were evaluated using the software WUFI (R) Pro 5.0 developed by the Fraunhofer Institute of Building Physics. Within selected parameter studies, an appropriate evaluation criterion was identified and the impact of varying material data and exterior boundary conditions was assessed. The integrated results emphasize the need to combine experimental and numerical analysis. The missing correspondence between the calculated and measured algal resistance for selected specimens of the weathering test is attributed to the simplifications inherent to the approximation of the hygric material functions and therefore emphasizes the need for further research.