Interior designers, manufacturers and the U.S. Department of Energy agree that shades placed close to a window are the best single covering for blocking the flow of heated air to the outdoors. Blinds have slats that allow heat loss, but a shade's solid structure gives more protection.
Shades are the first line of defense, says Michelle Olson, project designer at Expo Design Center in Dallas. Cellular shades, also known as honeycomb shades, trap air in the cells and block heat loss, say interior designers and shade manufacturers.
Another good choice is a fabric shade, such as a Roman shade, lined with insulating fabric. Insulated shades can be found ready-made or can be custom made. The more opaque the shade's fabric, the more insulating value the shade will provide, advise the folks at Calico Corners.