I just looked at Suntex at Home Depot and it looks pretty nice, for blocking out the sun.
But, if you stuck it on the inside of the window during the winter,
it could absorb the solar heat and re-radiate into the room..
While, adding a little R-factor to your windows.. If it's air-tight enough..
I'm thinking about my Den.. When a dark room is what I want to watch the big-screen TeeVee..