A lot of people and websites will say that there are condensation patterns to consider and curtains moving when the wind blows, and that is the best way to determine when to gut the old window. There is no divining rod. When deciding on which type of replacement window you want, you need to know that there are two main concepts: Sash Window Replacement Kits and Full Replacement Windows. Both are almost equals when it comes to specifications and what not. I mean, let’s just examine what you are doing here: re-building the inside of the window from inside the house or really replacing the window.
Sash replacement kits are when you replace the guts of the window – just the sash, the sides and the parts that make the window go up and down. See this simple window diagram for an overview of a typical double hung window. Basically, the sash is the interior part of the window minus the frame. Most people consider sash kits to be a less expensive alternative, and that would be why they are chosen in most cases. Another reason to do just the sash area is that you would like to preserve the old frames, especially on the exterior.
Replacing just the sash is a good solution with some limitations. The reason why it can be a good solution is that it allows you to save some time, resources, and can offer a better return on investment. Another purpose of replacing the sash section of the window is because you preserve the original millwork. One limitation in this scenario is that the old frame stays in place. It is important to remember that most of the air leakage from a window comes from things other than the glass. So, if you have leaky frames, then you’re probably not going to save money on your heating bill by using a sash kit. Glass itself does let heat out of your house, yet the leading theorists believe that air loss is where heat loss comes from. Either way, sash kits can have double pane glass with argon or other insulating gas.
Another key determining factor of when to replace just the sash is the physical or aesthetic desires you have. If you want to change the window shape, size, or position, then obviously a sash kit is out because the original frame would probably need to be altered in order to accommodate such dramatic changes.
It is most people’s opinion that replacing the entire window, frame and all, is the best solution. This way, you get the best warranty, looks, and performance from your new windows.