Storm windows can increase the efficiency of single-pane windows, which are the least energy-efficient type of glazing.

The simplest type of storm window is a plastic film (usually available in prepackaged kits) taped to the inside of the window frame. They are easy to install and remove, but are easily damaged and may reduce visibility.
More durable plexiglass, acrylic, polycarbonate, or fiber-reinforced polyester (also available in kits) can be fastened directly to the window frame or mounted in channels around the frame—usually on the outside of the building.