Bay windows are a unique window design which was hugely popular during the 1870s to 1930s, and which is still very much in evidence today as there is so much housing stock remaining from that period. They are obviously designed with aesthetics rather than practicality in mind, but there is no reason why they cannot be made to be hard wearing with the right modern glass and materials. There are inevitably some limitations as to what you can do with this design of window, but they can still be made to fit in with a modern lifestyle and to give your home the insulation it needs.
If you have bought into a home from this era, you are obviously not totally averse to having these large outwardly protruding windows as part of your home frontage. To replace the windows with a different type would be a major renovation job, not impossible but certainly expensive and difficult. It is only worthwhile if you have a specific use for the property in mind, which needs a different type of window and for which you cannot find an alternative property. In the vast majority of cases, you will be better off working with your old windows to produce the best result from them.
No matter what the design of your windows may be, you can make them far more effective by using the best materials on the market, or at least by improving on what you have. Windows built during this period are typically single glazed, and the frames are usually wooden. By using a double or triple glazing solution, you can greatly increase the insulating potential of the windows. You can also change the glass to a more efficient one, such as the Low-E Energy Star glass which is now used nutrients all modern constructions. This will continually save you money on your heating bills.
The frames used in bay windows will also have a profound influence on the amount of protection they give you against the elements, and against the loss of the heat you generate internally. Traditional bay window frames are wood, and this natural material has been the most popular for window frames throughout most of the history of construction. It is effective to a degree as long as it is properly cared for and maintained, which will inevitably take some time. The modern plastic alternatives take a lot less maintenance, but they have a cheap appearance which cannot be altered.
There are also new material for frames which are showing great promise, such as fiberglass and other plastic polymers. Fiberglass is exceptionally strong and versatile, as it will not bend or warp in extreme weather conditions and it can be painted to fit in with any color scheme. It will make an ideal replacement material for older windows of this type, because of the large expanse and long edges which are inevitably present, and the fact that south facing windows will protrude directly into the strong sunlight. In hot climates, fitting fiberglass windows can greatly increase the longevity of your window fittings by reducing warping.
If you choose to completely rebuild and restructure your bay windows with new materials, it will be a long and difficult job. You may even need to keep the property under close surveillance for a long time as you will not be able to secure it properly. Using a team of dedicated professionals is likely to get the job done far more quickly than if you attempt it yourself, even if you have the necessary skills and can get permission. Because of the huge stock of housing which was built during the bay window era, there are many professionals used to working with bay windows.