Could be an interesting idea for a regular window, thanks. Since this is supposed to be a greenhouse I need to keep the windows clear... I read about the black water-filled containers in a book on greenhouses, and figured that since the room's on the 2nd floor, we get a fair amount of light in there all day, much of which hits the lower half of one of the inside walls all afternoon -- so, the water heats up, then I'll lower the insulated shades at dark, and the heat is slowly emitted to the room during the night. Okay, I don't know if 'emitted' is the verb I wanted, but you know what I mean.