Abstract: A gasket for use on ventilation louvers of buildings. The gasket fits snugly over the sidewall of the ventilation louver and has a circumferential radially inwardly facing concave side surface terminating in a narrow lip which presses against the louver sidewall. The gasket also has a circumferential concave rear surface which presses against siding or the like which abuts against the louver sidewall, to provide some degree of seal. The concave surfaces also help maintain the gasket's orientation. The gasket position can be manually adjusted on the louver sidewall to suit different siding thicknesses, or the gasket can be removed and discarded.