To keep a painted façade protected from mold and algae, anti-growth agents are mixed in the coating. These biocides are small molecules with a high diffusivity inside the soft polymeric coating matrix resulting in a premature loss of protection. A promising improvement can be achieved by encapsulation of the biocide in micro-sized containers and thereby reducing the release rate from the coating. In this study, the biocide 2-n-octyl-4-isothiazolin-3-one (OIT) has been encapsulated by various formulation routes and the release has been studied with regard to different conditions. It was found that an exceptionally low interfacial tension between OIT and water practically prevents any formulation of core-shell particles. However, polymeric monoliths of OIT and poly(methyl methacrylate) could be formulated by the internal phase separation method. OIT release studies from these microspheres revealed formulation-dependent microscopic porosity where the chosen evaporation path of volatile solvent significantly alters the diffusion coefficient of the biocide in the microsphere. Dense microstructure with low diffusivity was given when the poly(methyl methacrylate) was provided time or heat for complete polymer relaxation. The microscopic porosity was evaluated within a framework of applied diffusion models to the experimental data. In an applied perspective, microspheres could offer a value for sustained release of OIT from coatings. In this work, we found a considerable decrease in release rate from dry-film coatings with encapsulated OIT compared to freely dispersed OIT. In addition, macroscopic porosity in the coating, i.e. the porosity in the binder material, was proven to be crucial for the diffusivity. It was shown that macroscopic porosity heavily depends on the drying time of the coating where longer drying times of several weeks gave a substantial decrease in macroscopic porosity and release rate of OIT.