Resources link: http://lightspread.org
Another window treatment option is installing shutters. Shutters are particularly good insulators because they are made of thick material such as wood or plastics and are typically installed on the inside wall of a house rather than the exterior. If they are well-installed and well-made, shutters can cut heat transferal of a window by half. Because of this, they are particularly good choices for home insulation.
Yet another cheap and easily installed window treatment option is buying window films. These are essentially large stickers that adhere directly to the glass, blocking heat transfer as well as helping to manage light levels inside a house.