Making the choice of styles for replacement windows is a balancing act among cost, quality, convenience and energy efficiency. In some of those areas, the difference between single-hung and double-hung windows is subtle; in others, the difference is clear.
The "hung" part of the terms "single-hung" and "double-hung" refers to the window sashes that are "hung" or suspended in the window frame so that they are able to move up and down. In a single-hung window, only one sash, usually the lower one, is operable, so only the lower part of the window will open; the upper sash is immovable. In a double-hung window, both sashes are movable; the lower sash can slide upward, and the upper sash can slide downward.
Single-hung windows have fewer moving parts, and their design overall is simpler than that of a double-hung window, so they are cheaper to manufacture. This means that the retail price of single-hung windows is as much as 10 percent to 20 percent cheaper than comparable double-hung windows. The difference isn't great in terms of a single window, but it can add up to a significant difference when you replace all the windows in your home.
Ease of Cleaning
In most modern double-hung windows, the sashes not only slide up and down, but they also tilt inward so that you can access the outside of the glass from inside the house. That makes it much easier to clean the exterior of the windows, especially on upper floors where it's difficult to get to the windows from the outside. Many single-hung windows have the same tilt-in feature on the lower sash, but cleaning the upper sash requires reaching out through the lower opening, essentially cleaning the window from the outside while you're still indoors. This inconvenience may not be a problem if all your windows are on the ground floor, but if your house has multiple stories or you live in a particularly dusty or dirty location, the advantage of double-hung windows in this respect could be significant.
Ventilation and Energy Efficiency
Because both sashes of a double-hung window can open, these windows give you more flexibility when you need ventilation; you can open the lower part of the window, the upper part or both. However, the operable sashes also introduce the possibility of air filtration through the window when you don't want it. The fixed upper sash in a single-hung window is unlikely to suffer from air leakage that decreases the window's energy efficiency. The spaces around the movable sashes of a double-hung window are more vulnerable to leakage, although modern energy-efficient sashes are designed to fit tightly and minimize air infiltration.