Sources such as Garden State Alarm Company and Cabling Plus, an electronic security systems distributor, recommend two primary types of contacts for double-hung windows: recessed and surface mounted. These contacts work in the exact same way, both using magnets. The contact, placed on or in the window frame, forms a circuit with the magnet placed on the window. When the window opens, the circuit breaks, sounding the alarm. Surface mounted contacts mount on window frames, while recessed contacts mount within frames -- you must drill holes to install them. Recessed contacts only work properly with wood window frames, because you can easily drill into wood.
Double-hung windows exhibit a special feature that affects alarm contact installation. Unlike most windows, double-hung windows can open from both ends. For instance, a single-hung window contains a single sash, usually located on the bottom of the window, that slides up and down. Double-hung windows, on the other hand, possess two sashes, one on the top and one on the bottom, both of which open. Because of this, double-hung windows present more options to potential home invaders -- they can open the window from both directions.
The placement of a contact on double-hung windows makes a greater impact on its efficiency than the type of contact you use, since recessed and surface mounted contacts work in more or less the same way. On a single-hung window, you simply install the contact at the bottom of the window, placing the magnet on the bottom pane of glass. When the window opens, the circuit breaks. For a double-hung window, you must install contacts at the top and bottom of the window, since it can open in either direction. Or, you can place one contact with two magnets, one on each sash, in the center of the window.
In his book “Security Now: A Guide to Electronic Security,” Ed Morawski writes that potential home invaders can easily outwit simply security contacts with decoy magnets. For instance, if a burglar opens your window while holding a decoy magnet even with the contact, the magnet can confuse the contact into thinking the circuit remains unbroken. According to Morawski, biased magnets represent the best alternative -- and optimum security option -- to standard contacts and magnets. Contacts used with biased magnets only respond to signals emitted by a special type of magnet, making it impossible to confuse the alarm system with a second, decoy magnet.