The leaching of biocides contained in renders occurs through the contact of the surfaces of the renders with water. In the majority of cases such contact occurs during rainfall, and therefore it is intermittent in nature. The work presents a mathematical model of the process of the transfer of mass (biocides) in a porous material (a render). The process consists of two stages which follow each other in a cyclical manner: the leaching stage, in which the surface of the render is wet, and the stage in which the surface of the render is dry. In the latter stage homogenization of the concentrations of the biocides in the render occurs. The coefficients of the diffusion of biocides in renders, which were determined in a previous work, were utilized in the simulation-related calculations, in which the dependencies of the average concentrations of biocides in the render since the time during the leachings performed in the subsequent cycles were determined. The consistence of the results of simulation-based calculations with the results of experimental research that was obtained confirms the correctness of the model that is presented. The biocides which were applied in the research: diuron, cybutryn and isoproturon are applied in practice as supplements to building materials. The purpose of these biocides is to ensure protection against the growth of microorganisms. The biocides which were mentioned constituted a supplement to the acrylic render and to the silicone render. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.