We live in an old house. Needless to say the windows in this house are hardly efficient thermal double paned sealed. In fact most of them leak, have cracks, or are otherwise conduits for the Wyoming cold. (When we get it, as today is in the mid 40's and all the precious snow is turning to not so pleasant slush.) The only sealant they have on them is the layers of paint keeping them from opening.
In order to reduce the amount of air/cold transfer A bought a thermal curtain for our home office. It works quite well. In the evening I can shut it, basically applying another (or in this case "a" ) layer of insulation to this window. The difference in temperature between the curtain and the window and the room is generally on the order of 10 - 15 degrees at night. It should work over the summer to keep the sunlight from heating up the house even more but we will see about that when the time comes. Plus I really like the sunlight.
The only issue with this curtain. It came in a heavy duty plastic wrap. The only way around this would have been to make our own I suppose. But until I get a sewing machine I will have to deal with the extra plastic.