Abstract: Biological growth on ETICS results in an unappealing visual aspect, health problems and dissatisfaction of the owner. Taking into account the EU Directive on dangerous substances and the use of larger thicknesses of the insulation panels, this problem will become more relevant. During a research project founded by the Federal Public Service Economy, a test method has been developed to evaluate the resistance to algae growth of rendering systems used in ETICS. This paper presents the test set-up and the procedure in which specimens are subjected to cycles of light and dark and spraying of an algae-solution. The tests have resulted in the establishment of an index of resistance to algae growth. The type of finishing coat and the capillary water absorption of the rendering system are the principal factors of the materials determining the resistance to algae growth. Further validation is however still needed. The final objective of the study is to formulate prescriptions on the use of renders (based by their performance) in relation to the classes of exposure (‘green' environment). Finally, it should be mentioned that a regular maintenance of the ETICS can prevent biological growth.