?StrataDensity? - Double sided skin fiberglass panels are constructed of continuous filament fiberglass pads, with a skin on both the air entering and air leaving sides. No metal backing. High volume construction, consists of a 4-pieces design, using moisture resistant chipboard material that is pre-printed with graphics. Frames are bonded with hot melt adhesive to the media pad around the entire perimeter of both sides of the filter. Frames are kraft colored, with colored printing. UL Classified in accordance with UL 900.