Solved! What Are the Most Common Types of Window Alarm Sensors?

Window sensors can be the first warning that an intruder has breached the safety envelope of your home. Make sure you have the right types of window alarm sensors for your needs.
Meghan Wentland Avatar
Types of Window Alarm Sensors
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Q: I wanted to add window sensors to my home for extra peace of mind, and I thought it would be a simple task of searching for “window sensors for security” or “alarms for windows in house.” But it turns out there’s a huge range of types and styles, and it’s overwhelming. Which are the best types of window alarm sensors, and how do I know what I need?

A: There are quite a few different types of window alarm sensors. As home security systems have become more commonplace in recent years, manufacturers of alarm door and window sensors saw the demand and began producing different variations to meet the needs of their customers. Not all windows are alike (and neither are all security situations), so it stands to reason that the sensors would need to be different in order to make sure they’re available for as many types of windows as possible.

Window sensors are an integral part of the best home security systems (like those offered by Vivint, ADT, and SimpliSafe). In general, there are three main types of window sensors—contact sensors, glass break sensors, and motion sensors—although there are variations within those three categories. The best type of sensor for each customer will depend on the types of windows the home has and the customer’s security goals. Customers will want to learn more about each type to determine which are the best door and window sensors for their home.

Contact sensors are among the most common types of window alarm sensors. They can either be recessed or surface mounted.

Easy to install and simple to use, contact sensors are the most frequently used window alarm sensors. These devices are constructed of two parts: one is a magnet, and the other is a trigger for the alarm system to which the sensor is connected. One part is installed on the window sash, and the other part on the frame, with the magnet and the sensor aligned. When the window is opened, the magnet’s contact with the sensor breaks and triggers an electrical pulse and signals the alarm or notification to sound.

This type of sensor can be hardwired through the walls for a permanent installation, but many contemporary sensors are powered by small, long-lasting batteries and connect to the larger system with transmitters so there’s no need for unsightly wires lining the walls and windowsills. The simplicity of installation, connection, and usage make this type of sensor a go-to for DIY security system installations, but they’re also an easy add-on to a larger system.

Surface-mounted sensors are easy to attach; many even come with an adhesive on the back so there’s no need to risk putting screws into a window frame. Surface-mount sensors are available in several styles. Because they’re visible, users will want to minimize their appearance, so some are longer and slimmer, some are “mini,” and some are nearly flat to accommodate the different opening styles of different types of windows. Many are discreet enough that they look like an additional lock mechanism or aren’t noticeable at all. The adhesive backs and wireless options also make surface-mount wireless contact sensors an ideal element of the best apartment security systems, because the sensors can be easily removed and transferred to a new location with minimal damage to the surface.

There are some situations where a surface-mount sensor won’t work, either because of the mechanics of the door or window or because a visible sensor would spoil the aesthetic of the window. In this case, a recessed sensor may be the best answer. This type of sensor requires inserting the two parts of the sensor into the window sash and the frame so that there’s no visible component of the device. Most of these require that two holes be drilled into the window and sash, although there are some that use a magnet that is so low-profile and small that it can be surface mounted and nearly invisible. Drilling into window sashes comes at a significant risk. Many window manufacturers will invalidate their warranty if the frame is drilled or manually breached, and the likelihood that an inexperienced installer (such as a homeowner) might accidentally drill a bit too far in and break the seal between the panes is pretty high. Then the window will grow foggy in between the panes and lose some of its insulation value. If it’s done carefully and slowly, however, installing recessed sensors can streamline the appearance of the windows while providing excellent security.

Types of Window Alarm Sensors
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Glass break sensors send an alert if the window glass is shattered, signaling a break-in.

In some homes, the risk of the window being opened is less than the chance that an intruder will just smash the glass and climb in; this is a particular problem with basement window security, when it’s less likely that the residents of the home will hear the glass shatter. Unlike contact sensors, glass break sensors do not attach to the frame or the sash; they adhere directly to the glass. When the glass is impacted or broken, the sensor is triggered by the shift in the glass surface tension and the vibration, notifying the system to sound an alarm.

Because they’re on the glass, these sensors (while small and unobtrusive) are visible from the outside and may act as a deterrent similarly to a visible alarm system. In addition, most home security systems and smart-home systems offer glass break sensors as part of their product line, but there are also stand-alone glass break sensors for residents who just need an alarm sounded if a far-off window is tampered with but don’t want a whole system installed.

Motion sensors can be aimed at windows to detect intruders attempting to enter through the window.

For customers loath to drill into their windows or attach visible sensors to the frames or the glass, there’s another option. Basic motion sensors are common components of nearly all security systems, whether DIY smart-home systems or more traditional professionally installed systems. Usually, motion sensors are used to detect movement in an area such as a hallway or entryway, but they can also be more focused and directed at windows to serve as alarms should someone approach the window or attempt to enter. Motion sensors used as window alarms can also be useful in situations where the residents might want to be able to leave windows open during the day while they’re occupied on another floor or outdoors, without leaving them fully unattended. Depending on the home security setup, these motion sensors can be set to trigger lights, sounds, or a notification on the resident’s phone or alarm panel.

Modern window alarms are typically wireless, though some security companies offer hardwired versions as well.

There’s a certain sense of security to hardwired systems. No chance the signal will drop, no worries about reconnecting after router or Bluetooth blips, no concerns about dead batteries—unless the power goes out, in which case all bets are off. For those who are more comfortable with wired systems, they are still offered by some security companies (often with a battery backup, just in case). A hardwired system requires wires to be run through the walls or run through conduit on the walls’ surfaces to connect each sensor to the main panel.

Nearly all of the best window alarm sensors are offered in a wireless version—even those companies who provide wired connections usually also have a wireless option, and sometimes the two versions can be combined in the same system where appropriate. The wireless versions are discreet and usually attach to the framework of the window with adhesives to preserve the warranties on the windows themselves. Those wireless devices that are recessed require some caution during installation, and a recognition that the window’s warranty is likely voided, but in some circumstances they’re the best—and possibly only—option, depending on the placement and installation of the window or door.

Window sensors for home security use small, long-life batteries, which means customers won’t need to worry about battery life. Unlike some types of sensors, these aren’t drawing power all the time; the sensor isn’t using power unless the window opens, because the circuit isn’t actually connected until the magnets separate and the switch is triggered. Each system will be a little different in terms of battery life expectancy (some claim a battery life of as long as 5 years on regular AAA batteries), but most of the best window alarms provide notification of low battery power in plenty of time to change them before losing coverage.

When connected to a smart-home system, a window alarm sensor can alert the customer of any unusual activity no matter where they are.

Certainly the most important time to be notified of an attempted break-in or unexpected activity at the windows is when residents are home, because the safety of a home’s residents is the most critical part of any home security system. The best door and window alarms can be connected to a smart-home system and provide immediate notification via smartphone when a sensor is triggered, giving the home’s residents the time they need to get to safety and call for law enforcement. These notifications are also helpful when residents are not home—they can provide peace of mind while residents are away on vacation (a notification gives residents the opportunity to ask a neighbor to take a quick look at the house or to call law enforcement directly). The real-time alerts also provide security and peace of mind for parents whose children are home alone or adults who are keeping an eye on older family members; an alert that a window has opened can prompt a parent to call the children and check to see what’s going on and trigger an appropriate response, whether that’s asking the child to close the window or getting prompt help if there’s trouble.

The vast majority of door and window sensors connected to smart-home systems do not automatically include an audible alarm. In most cases, the smartphone alerts are the primary form of notification, unless the system also includes an add-on siren or alarm to which the sensors are connected. This is important to bear in mind while building a home security system, and something that requires thought: Most people don’t want an audible alarm every time they open a window, but there may be some cases in which they do (for example, many people would prefer an audible alarm on first-floor windows overnight). Most smart-home systems will permit those conditional situations to be programmed into their optional settings, but again, for audible alarms most sensor systems require the separate purchase of the alarm itself.