A thermally insulating curtain for windows and rooms provided with glass or transparent material. The curtain has an outer solar-energy-absorbing surface outside a thermally insulating layer. A space is disposed between the solar-energy-absorbing surface and a transparent layer situated outside the solar-energy-absorbing layer and/or between the solar-energy-absorbing surface and the thermally insulating layer. The curtain includes a structure that transports air through the space, and a fastener for arranging the curtain on the inside of a window, so that absorbed solar energy is supplied to the room behind the curtain.