This paper presents the development of a risk assessment methodology applicable to the analysis of exterior surfaces defacement from algae growth. The specific case of external thermal insulation composite systems (ETICS) is frequently associated with a higher perceived risk if compared with other façade solutions. The design of the proposed methodology includes: the scope of the risk analysis definition, the determination of the parameters involved using a qualitative analysis of the problem, the collection of relevant data, and a quantitative probabilistic risk analysis based in Monte Carlo simulations. The detailed implementation of the methodology involved the preparation of a meta-model, response surface methodology, that was developed and compared with a hygrothermal simulation tool. The meta-model allowed a faster application of the Monte Carlo method, providing results of the methodology application to the use of ETICS in selected locations and a sensitivity analysis of the stochastic parameters.