For an affordable alternative to increasing the R-value of your windows, consider insulating window treatments, including shutters, shades, draperies and blinds.
Single-glaze windows are poor insulators having an R-value of 0.9. Double glazing raises window insulation value to R-1.84. Energy-efficient glazing raises R-values significantly higher.
Window insulating products, often called movable insulation, add considerable energy efficiency to any window. Insulating shutters, for example, can raise the R-value to as high as R-10 in a home located in the northern section of the United States, resulting in a decrease in fuel consumption by as much as 25%.
Insulating shades operate in almost the same way as ordinary window shades. They are wrapped on spring-loaded rollers. They are heavier, however, than paper shades and may slide in tracks rather than swing free. Some models are electrically operated.
Insulating shutters usually are positioned inside the house. They lie back against the wall and swing forward or slide over the window. Some are designed as a panel that must be inserted and removed manually. Shutters are available as single or double fold panels. The installation of insulating shades and shutters is no more difficult than installing a Roman shade or louvered blind.
Insulation fixed within windows, such as blinds, comes built into the window itself and require window replacement. Insulating draperies are installed similar to ordinary draperies.
Cost-effective placement of movable insulation in your house is an important consideration. If your home has single-glaze windows, and where no door is protected by a storm sash, insulation over every opening would slash your fuel bills. But even in this extreme example, a combination of glazing replacement and adding less expensive window treatments to strategic windows usually is less costly than adding storms to all your windows.
To save money, limit window treatment to north-opening windows and possibly to west-openings or on the side where prevailing winds blow in the winter. Limiting the use of movable insulation makes good sense, especially when your home is already tightly sealed and protected at windows and glass doors with at least double glazing. Then heat loss through windows (minus normal heat gain on sunny winter days) accounts for less than 20% of the total heat loss.
Another saving can be realized by mixing types of movable insulation. Use the more expensive, high R-value treatments for the coldest side of the house. On the less vulnerable sides, lined draperies may be all you need if you remember to keep them pulled whenever the room is not in use.