A window spacer for a window assembly comprising three sheets (110, 120, 130), the window spacer comprising: a first elongate strip defining: a first undulating shape, a registration structure for receiving an intermediary sheet of the window assembly, characterized in that the first elongate strip further defines: first and second planar regions on opposing sides of the registration structure, wherein the first and second planar regions define first and second sets of apertures, respectively; and in that the window spacer further comprises: a second elongate strip defining a second undulating shape; first and second support legs disposed between the first and second elongate strips (150, 160); and at least one filler arranged in an interior space defined by the first and second elongate strips (150, 160) and the first and second support legs, the filler including a desiccant. a first elongate strip defining: a first undulating shape, a registration structure for receiving an intermediary sheet of the window assembly, characterized in that the first elongate strip further defines: first and second planar regions on opposing sides of the registration structure, wherein the first and second planar regions define first and second sets of apertures, respectively; and in that the window spacer further comprises: a second elongate strip defining a second undulating shape; first and second support legs disposed between the first and second elongate strips (150, 160); and at least one filler arranged in an interior space defined by the first and second elongate strips (150, 160) and the first and second support legs, the filler including a desiccant. a first undulating shape, a registration structure for receiving an intermediary sheet of the window assembly, characterized in that the first elongate strip further defines: first and second planar regions on opposing sides of the registration structure, wherein the first and second planar regions define first and second sets of apertures, respectively; and in that the window spacer further comprises: a second elongate strip defining a second undulating shape; first and second support legs disposed between the first and second elongate strips (150, 160); and at least one filler arranged in an interior space defined by the first and second elongate strips (150, 160) and the first and second support legs, the filler including a desiccant. first and second planar regions on opposing sides of the registration structure, wherein the first and second planar regions define first and second sets of apertures, respectively; and in that the window spacer further comprises: a second elongate strip defining a second undulating shape; first and second support legs disposed between the first and second elongate strips (150, 160); and at least one filler arranged in an interior space defined by the first and second elongate strips (150, 160) and the first and second support legs, the filler including a desiccant. a second elongate strip defining a second undulating shape; first and second support legs disposed between the first and second elongate strips (150, 160); and at least one filler arranged in an interior space defined by the first and second elongate strips (150, 160) and the first and second support legs, the filler including a desiccant.