Double-hung windows are windows that operate vertically and allow both the top and bottom window to open. A single-hung window duplicates the design of a double-hung window, but the top frame remains stationary in the frame. The double-hung window allows for the window to let in and release air over half of the window surface area. The frame of the double-hung window is the sash; there is both an upper and lower sash.
One major benefit of double-hung windows is the ability to vent air from either the top or the bottom of the window. With proper opening of double hung windows throughout a house, a person can greatly reduce the use of an air conditioning system. Using the scientific principles of thermodynamics, a person can regulate the airflow throughout a home by opening and closing certain vents in the windows.
The principles of thermodynamics state that hot air rises and cold air sinks. The design of double-hung windows allow the hot air in a home to rise and leave the home via the open top vent. The colder air from the outside environment will then enter through the open lower vent. This convection-like process allows for the continuous release of hot air and the continuous entry of cooler air.
Homeowners can also alternate which vents are open on which windows. If one window has the lower vent open and another window has the top vent open, cool air flow will enter in one room and heated air will exit from another room. The separation of ventilation allows for the cooler air to push the warmer air out of the room. Double-hung windows also work with vent locks for improved protection. Vent lock installation allows the vents to open a few inches but can help prevent forced entry. Some locks also work with alarm systems so that the windows can remain vented a few inches without setting off an alarm.