Zeolite/nanoparticle composites are synthesized and used as additive for waterborne formulations in replacement of traditional isothiazolinone-based biocides. Silver nanoparticles dispersed onto micrometer-sized crystals of A-type zeolite were prepared by thermal treatment or chemical reduction methods applied to the Ag-exchanged zeolite. The antifungal efficiency against Trichoderma sp. of waterborne outdoor coatings containing the supported silver nanoparticles, measured in terms of inhibition halo development, shows similar results to those obtained by using traditional organic biocides, the type, size and size distribution of the nanoparticles being the main factors affecting the biocidal action of the prepared coatings.