This article will discuss the different styles of replacement windows, without regard to the replacement window's frame material, hardware, or the window glass.
Hung - Replacement Windows:
Hung replacement windows, as shown in Figure 1, or single hung replacement windows as it is sometimes termed, are a vertical slider, whereby the inner window unit slides up and down in the frame. It gets its name because the window that moves is connected to weights via a rope.
This is the most common window in older homes.
Because of the way the window is designed there must be some clearance, even if it is small, to allow the window to slide in the vertical channels. This necessary clearance, makes this window one of the worst, when it comes to air ingress.
Some of the newer hung and double hung windows are not "Hung" at all. They work on a sliding mechanism. The use of the sliding mechanism does provide a lower level of air ingress.
Figure 1 - Hung or single hung window
Double hung window
Double Hung - Replacement Windows:
Double hung replacement windows, as shown in Figure 2, are very similar to the hung window with the exception that the upper window in the frame has the ability to move down. This allows for the escape of hot air through the upper window, while cool air enters the room through the raised lower window. Because the top window must have clearance to slide in the window frame channels as well as the lower window, air ingress is worse than a hung window.
Note: If you are purchasing a double hung window, make sure that it is actually double hung and will open from bottom to top and top to bottom. Many retailers will call a simple "hung" window a double hung window.
Slider - Replacement Windows:
Slider replacement windows, as shown in Figure 3, move horizontally within a frame. In some cases, both windows will slide in opposite directions. These windows have become very common for low end housing, as they are inexpensive to manufacturer. However, because clearance is required between the window and the frame in order for the window to be able to slide there is always some air ingress.
Figure 3 - Slider window
Figure 4 - Casement window
Casement - Replacement Windows:
Casement replacement widows, as shown in Figure 4, are usually hinged on one side and open in the same manner as a door. Most casement replacement windows use a crank mechanism for opening and closing which holds the window in almost any open position.
It should be noted that casement windows open to the outside, not the inside of the home.
Some casement windows use a pivot instead of a hinge and the window opens from a point approximately 25 % of the width of the window. Because of the mechanical cranking mechanism, the window can be closed very tightly against the elements. A well designed, casement window is the best defense against air ingress if you require a window that opens.
One negative for casement windows is that screens must be placed on the inside of the window frame.
Additional information on Replacement Windows - Materials