Windows can reduce energy use in one respect in that they allow for natural lighting that may reduce the use of electric lights. However, they reduce the efficiency of the building envelope for two reasons:
-Convection heat transfer through the glass.
-Gaps at the window assembly and installation that allow air leaks.
Some gaps can be repaired with caulk or weather stripping, but other times it is difficult to both seal gaps and maintain functionality (the ability to open and close). Convection losses are the more significant efficiency issue for homes with single pane glass windows that are still in good condition. The most effective ways to reduce convection loss is either window replacement or the installation of storm windows.
The Replacement Decision
Well installed double or triple glazed windows are a major improvement in energy efficiency compared to single glazed windows. Unfortunately, window replacements are expensive and have long pay back periods. The incentives offered on new windows are usually a small portion of the project cost. Generally this inspector recommends that other efficiency investments with faster pay back periods, such as insulation and sealing, or high efficiency furnaces or water heaters be completed prior to window replacements.
There are other factors other in addition to energy savings that may influence the decision of whether to replace windows or not:
-New windows open and close smoothly.
-New windows reduce sounds from outside.
-New windows may eliminate condensation on the interior of the window.
-New windows usually include screens for summer use.
-Some new glass has coatings that reduce ultra violet rays that can fade carpets and fabrics.
-New windows may be configured to provide safe emergency egress (or access for rescue personnel).
Storm Windows
Storm windows are a less expensive option for improving window efficiency. Storm windows are not as efficient as new windows and they do not offer some of the other benefits listed above. However, they can be a good choice in some circumstances, especially when the original windows have architectural details that will be lost, or prohibitively expensive to recreate, if the windows are replaced.
Many older homes in the Portland area have large, non-opening picture windows facing the street. Many of these windows have attractive, multiple pane construction, especially along the top. These big windows are a significant heat drain that leaves the front rooms feeling cool and drafty. Storm windows can be a good solution for these cases where replacement windows may cost in the thousands per window. New window dollars can be saved for the other 3 sides of the house where operable windows will enjoy more of the multiple benefits of replacement units.
Try a Test of New Windows
Homeowners can test out the improvement offered by double glazing, whether it be by new windows or storm windows, by installing temporary plastic storm windows. These are inexpensive and easy to install and can give a good approximation of the improved thermal performance offered by double glazing. Pick a room with multiple windows that is typically cool and drafty in the wintertime and install plastic storm windows at the start of the heating season (if it's a room in a sleeping area make sure that any occupants know to tear through the plastic in the need of an emergency escape). Test out the improvement offered by this temporary measure. If your film is well installed, the thermal improvement with the plastic film will be very close to that offered by replacement windows or storm windows. Use this information to help decide whether to replace windows.
Doors
Like windows, replacement doors can be expensive in relation to their energy savings. Unless an exterior door is damaged, hollow, or a very thin panel it may not be worth the expense of replacing. The most important issue with doors is to seal any gaps when the door is in the closed position.
If a door is replaced, the new unit should generally be insulated with double paned glass and good weatherstripping at the jamb.
Related Links:
ResFen Software. That's "residential fenestration" which is a fancy way of saying "household windows". If you've got the time this software will give a detailed analysis of your particular window energy equation based on inputs such as your location, orientation to the sun, shading, and building type.
Department of Energy, window review. A good overview of window issues and options.