Abstract: A thermal window insert includes at least two substantially parallel, substantially transparent sheets of flexible polymer material having a gap therebetween. The two sheets are bonded together at deformations formed in at least one of the sheets, and have a size approximately corresponding to the dimensions of a window opening in a building. A strip of resilient material is directly bonded to the sheets around the perimeter thereof, so as to seal the gap between the transparent sheets and to provide a seal between the window insert and the window opening. The invention is similar to storm windows, but is of a simpler and less expensive construction, and is easy to install, remove and store.