Abstract: A thermally insulative covering device is provided which can be removably associated with the interior side of windows or sliding doors of a building. The device is fabricated as a frame which tautly holds four sheets of a transparent film in a manner to form three successive stagnant air spaces. A continuous border strip of a resilient material causes the device to make a snug friction fit with a window or door and causes the space between the device and the window or door to be a fourth stagnant air space.