The invention concerns a thermally insulating curtain (1) for windows (2) and other rooms (4) provided with glass (3) or another transparent material. According to the present invention, said curtain (1) has an outer black solar energy absorbing surface (8) outside a thermally insulating layer (13). A space (11) is formed between said black surface (8) and a transparent layer situated outside the same and/or between said black solar energy absorbing surface (8) and the thermally insulating layer (13) of said curtain (1) situated inside the same. Furthermore, there is means that allows transportation of air (14, 15, 16) through said space (11) of the curtain (1), the curtain (1) having fastening members (17) to make possible curtain (1) arrangement on the inside of the window (2) in question, so that absorbed solar energy is supplied to the room, etc. (4) behind the curtain (1).