Strategies to enhance the resistance of ETICS finishing coats to microbiological growth: A systematic study based on small- and large-scale laboratory tests

External thermal insulation composite systems (ETICS) are, in general, more likely to suffer from colonisation by fungi and algae than conventional walls, regardless of the nature of the biocide technology used. This results from the greater propensity of finishing coats applied on ETICS to develop surface condensation and, consequently, favourable conditions for microbiological growth. Most commonly, aesthetic degradation is the only implication. However, some severe cases can cause coating deterioration and loss of adhesion. In this work aiming to study resistance of ETICS finishing coats to fungi and algae growth, different improvement strategies based on thermal, hydrophobic, and hydrophilic approaches were exploited. In total, five different acrylic- and silicate-based formulations (without biocide) were prepared to screen the risk of condensation and biological resistance in separate small-scale laboratory tests. Based on the results obtained, selected finishing coats were applied on a large-scale ETICS wall and tested in a bioclimatic chamber capable of simulating conditions conducive to the early development of biological colonisation. The concomitant influence of biocides was also addressed at this stage. The results showed that a simple silicate-based formulation performs better than other tentative formulations tested, exhibiting a higher resistance to fungi and algae. They also suggested that incorporating glass microspheres or adding an external siloxane layer is not on its own effective in enhancing the resistance to the biological growth of acrylic- and silicate-based formulations.