A friend mentioned to me that his windows appear to leak a lot of heat and cooling and asked what he could do about them.
Space heating and cooling likely consume about 40% of your home's energy, and, if the home is well sealed and insulated, a substantial part of that escapes through your windows. This is especially so if windows are exposed to solar radiation and you live in an area with a substantial air conditioning season. So, the possibilities are worth some more discussion.
Homes which are located in the south and are +/- 30 years old were often built with single pane, builder's grade windows which are outdated with today's energy prices. Ideally today, they would be built using multipane, low E glass with Argon between the panes and insulation in the frames. These windows would have an effective R value of approximately 4, cutting heat transfer by a factor of 4 over the old single pane windows. Unfortunately, retrofitting windows is an expensive proposition which is difficult to justify unless the windows need to be replaced for other reasons, so let's look at other possibilities.
To understand window alternatives, it is necessary to understand the main factors in window effectiveness. The energy efficiencies of windows are affected by three different heat transfer mechanisms: 1. Conduction, or heat passing directly through the materials. 2. Infiltration, or air passing through gaps between the materials, and 3. Radiation, primarily solar rays passing through the glass. Different transfer mechanisms often require different solutions, so let's look at each separately.
Conduction: Conduction is a function of the conductivity (inverse of insulation value) of the materials, the thickness of the materials, air gaps, and area exposed. One way to significantly reduce heat gain or loss by conduction is to add insulating material over the window, in the form of heavy drapes or shutters. These can be very effective, especially if they cover the entire window and seal tightly. Unfortunately, they also typically make the room dark and eliminate the view. It is often possible to open and close them when light and view are not an issue, but this can be time intensive and may reduce their effectiveness if not carefully managed. Another possibility is to add storm windows over the existing windows. This can take the form of either true storm windows, or installation of a glass pane which sits in the position of a full window screen. These additions are nearly invisible, but do keep you from opening the window are ventilation during the shoulder seasons unless they are removed and stored during the time when ventilation is beneficial. Or, for a more affordable, shorter term solution, you can add plastic films which are attached with tape to the window frame.
Infiltration: Infiltration can be either around the perimeter of the window, through the weather stripping of the window, or around individual panes. Leakage around the perimeter of the window can best be reduced by caulking all gaps around the window, both inside and outside. In the worst cases, it might be worthwhile to remove the molding around the inside of the windows and foam around the windows. For leakage of the weather stripping, it may be possible to reposition the existing stripping to improve the seal. Also, on many windows, aftermarket weather stripping can be installed. For sealing around panes, clear caulking or putty can improve the seal. And, storm windows or plastic films mentioned under the conduction section can also reduce infiltration.
Radiation: Radiation is generally minor, or even helpful during the heating season, but during the air conditioning season it can be one of the biggest factors affecting the load of your air conditioner. This is particularly true of unshaded windows on the east or west sides. Windows on the north side generally admit minimal radiation and those on the south side will also be minimized in the peak of the air conditioning season if the roof overhang is adequate. And, windows on the south side may offset air conditioning losses with solar gain in the winter. Drapes, blinds or other interior window treatments can limit solar gain by either trapping the heat next to the window or reflecting it back out the window, but are limited by the fact that they try to deal with the heat after it has entered the house. Reflective films applied to the glass can also be effective in minimizing heat gain from solar radiation. However, the best way to minimize solar gain is by stopping it before it enters the house. This is done by shading the windows, perhaps with solar screens, awnings, increased overhangs, or landscaping. Landscaping, though longer term, is likely the best way to manage solar gain. It stops the radiation prior to entering the house and can be managed to stop the radiation in summer and admit it in winter. And, water evaporation from plants tends to cool the area and cut down on wind, reducing conduction and infiltration losses as well, while adding value via improved beauty.
So, there you have it. Lots of opportunities to improve the efficiency of your windows, each with its own applications, issues and budgets. Clearly, you are limited only by your own situation and preferences.