in an old house we used to live in i hung a fleece blanket over one of the upstairs windows in the winter- IT WORKED GREAT! for some reason it seemed to collect the sun and radiate the heat- when you walked past the stairs you could actually feel heat radiating down from that window- it cut our heating bill by alot!
then i got the bright idea to add a cotton (top) with a fleece back and actually made it a curtain- that was even better- seems the cotton on one side blocked the (heat) so it only went in one direction-
either way- i think it's a good idea- worked well for us :)