Window and Door Sensors Explained: The Foundation of Every Home Security System

Every home security system starts with contact sensors on doors and windows. They're the simplest, most reliable, and most important component - yet they're also the most commonly under-deployed. In our homeowner surveys, the average system has 3-4 contact sensors protecting a home with 8-12 entry points. That means half or more of the entry points are completely unmonitored. Here's how to get the coverage right.

How Contact Sensors Work

A contact sensor consists of two pieces: a sensor unit (with a radio transmitter) and a magnet. The sensor mounts on the door or window frame; the magnet mounts on the door or window itself. When the door is closed, the magnet holds a reed switch inside the sensor in the "closed" position. When the door opens, the magnet moves away, the reed switch opens, and the sensor transmits an "open" signal to your base station.

This technology is elegantly simple and virtually failproof. There are no moving parts, no cameras to obscure, no software to crash. Reed switches have been used in security systems since the 1960s and the failure rate is extremely low - most sensors last 3-5 years on a single battery with zero maintenance. When the battery gets low, your system sends an alert weeks before it actually dies.

How Many Sensors Do You Actually Need?

The answer depends on your home, but here's a framework based on FBI entry point data:

Priority 1 - Every exterior door (non-negotiable). Front door, back door, side doors, and the door from your garage to the house. These four entry points account for 65% of all break-in entries. If you do nothing else, sensor every exterior door. Cost: 4 sensors at $15-$30 each = $60-$120.

Priority 2 - First-floor windows (high priority). First-floor windows account for 23% of entries. Prioritize windows that aren't visible from the street - side and rear windows that provide cover for a burglar. You don't necessarily need a sensor on every window; a glass-break sensor ($20-$35) covers all windows in a single room acoustically. For a typical 3-bedroom home, 2-3 glass-break sensors or 4-6 window contact sensors provide solid coverage. Cost: $60-$180.

Priority 3 - Second floor and basement (moderate priority). Second-floor windows are only 2% of entries, but accessible ones (near flat roofs, balconies, large trees, or fire escapes) should be sensored. Basement windows are 4% of entries and are often forgotten. For most homes, 2-4 additional sensors cover these. Cost: $30-$120.

Total for comprehensive coverage: A typical 3-bedroom, 2-story home needs 8-14 contact sensors plus 2-3 glass-break sensors. Budget: $200-$450 for complete entry-point coverage. That's a one-time cost that protects every way into your home.

Contact Sensors vs. Glass-Break Sensors vs. Motion Sensors

Contact sensors detect when a door or window opens. They're specific and reliable - zero false alarms. The limitation: they don't detect if someone breaks a window and climbs through without opening it. They also can't tell the difference between you opening a door and an intruder opening it - that's what arming modes handle.

Glass-break sensors detect the specific sound frequency of breaking glass within a 15-25 foot radius. One sensor covers all windows in a standard room. They complement contact sensors by catching the scenario contact sensors miss: a burglar breaking glass rather than opening the window. The limitation: they can occasionally false-alarm on certain sharp sounds (dishes breaking, loud TV audio), though modern sensors with dual-technology detection (acoustic + vibration) have largely solved this.

Motion sensors detect movement within a room. They serve as a backup layer - if an intruder somehow bypasses all door and window sensors, motion detection catches them moving through the house. The limitation: pets, and they only work when armed in away mode (otherwise your own movement triggers them). They're important but not a substitute for entry point sensors.

The ideal layered approach: Contact sensors on every exterior door + glass-break sensors in rooms with accessible windows + motion sensors in main hallways as a backup layer. This creates three independent detection systems that would all need to fail for an intruder to go undetected.

Installation Tips From a Professional

Sensor placement on the door: Mount the sensor on the door frame (the non-moving part) and the magnet on the door itself (the moving part). Place them at the top of the door, not the middle or bottom - sensors at the top are harder for an intruder to spot and defeat, and they're less likely to be damaged by daily use.

Gap tolerance: The sensor and magnet should be within 1/2 inch of each other when the door is closed. If your door has a wider gap, most sensors include spacer shims. If the gap exceeds 1 inch, consider a recessed sensor (mounts inside the door frame, invisible from outside) or choose a sensor model designed for wide gaps.

Window sensors: For double-hung windows (slide up and down), mount the sensor on the bottom of the window frame and the magnet on the moving sash. For sliding windows, mount on the side of the frame and magnet on the sliding panel. For casement windows (crank open), mount on the frame near the latch side.

Temperature matters: Some adhesive weakens in extreme heat (garage doors in summer, sunroom windows). If a sensor falls off, the adhesive likely failed - clean both surfaces, let them dry completely, and reattach with fresh adhesive strips. If sensors are in high-heat areas, consider screw-mounting for permanent reliability.

Sensor Comparison by Provider

SimpliSafe: Clean, compact design. Pre-paired to base station. $15-$20 per entry sensor. Reliable with good battery life (3-5 years). The standard choice for most DIY installs.

Ring: Slightly larger than SimpliSafe but very affordable at $20 per sensor. Integrates directly with Ring Alarm base station and appears in the Ring app alongside cameras. Good battery life.

ADT: Professional-grade sensors in the self-setup kit. $25-$30 per sensor. Build quality feels more substantial than SimpliSafe or Ring. The Google Nest integration means door open/close events can trigger Nest cameras and routines.