A blast-resistant window attachment, or retrofit window insulation system, wherein panes of polymer film, such as TPU or ETFE, are held in a roll-formed stainless steel frame to form a pane assembly. One or more pane assemblies are stacked to make a multilayer unit or are mounted in a robust polymer casing that is sized to fit the window frames of an existing building window or to attach to the wall of the building surrounding the window. The polymer film or films can absorb energy of a blast without breaking provided that the collective thickness of the film(s) is at least 20 mil, and preferably 24 mil or more. The casing is, preferably made from a high strength polymer, such as Acrylonitrile Butadiene Styrene, or a metal such as stainless steel. In experiments conducted by the Army Corp of Engineers, the retrofit window insulation system of the present invention, when securely bolted to the structural components of the building around the window, demonstrates a remarkable ability of the polymer film panes to absorb blast energy and mitigate secondary debris hazards.