Abstract: According to some implementations, a multi-pane window assembly and a multi-pane window insert are described. The multi-pane window insert includes a first pane formed of polycarbonate resin thermoplastic material and a second pane formed of polycarbonate resin thermoplastic material that is separated from the first pane by a spacer. A planar surface of each the first pane and the second pane are oriented substantially parallel to one another. A framing structure is provided for housing the plural panes and a gasket is provided for sealing the multi-pane window to a frame on installation. The multi-pane window insert is configured to be installed on an interior surface of a pre-existing window frame to improve the thermal, acoustic, and light transmittance properties of a multi-pane window assembly.