An air-to-air heat exchanger, is provided that comprises a folded fin core formed from a continuous sheet of thermally conductive material that has been folded into alternating flat ridges and troughs; an insert overlay having an opening including two sets of uniform fingers, wherein each finger has a portion protruding into and essentially filling each trough on one surface of the folded fin core; an inset region between each finger portion and the end edges of each trough; a sealant within each inset region sealably attaching the insert overlay to the folded fin core; and an air flow divider plate. Heat exchanger components are also provided.