Abstract: A thermal insulation structure for windows including a flat, transparent or translucent plastic sheet and a frame structure adapted to clamp the edges of the sheet and adapted to be magnetically attached to an existing ferromagnetic window frame or to a ferromagnetic or magnetic strip adhered to a window, window frame or opening. The frame structure comprises a U-shaped member and a magnetic strip secured to one prong of the U-shaped member or a L-shaped member secured to one surface of the sheet while the magnetic strip is adhered to the opposing surface of the sheet. The insulation structure is transparent to outside light and provides a simple means for significantly reducing conducted and radiated heat loss through the window and for eliminating window air infiltration heat loss.