Abstract: A filter assembly for use in filtering air in an air conditioning system has a water resistant front grille through which air enters the assembly and a water resistant rear grille through which filtered air is exited. The ends of these grilles are not apertured and are bent over and joined together to form a container structure. An air permeable filter element in the form of a pleated or undulating sheet is installed in the container structure, the opposite ends of the pleated sheet being attached to opposing sides of the grilles. The pleated sheet runs between the front and rear grilles and extends from the top to the bottom of the grilles. The filter sheet has a filtering media which may be of fiberglass on the side thereof which faces the front grille and a stiffened backing providing structural support, on the side facing the rear grille. The fiberglass, while passing air resists the passage of liquid therethrough.