The Short Answer
No, a standard PIR sensor will not work reliably behind glass. Glass is opaque to the mid-infrared wavelengths (8-14 μm) that PIR sensors detect.
Why Glass Blocks Infrared
Ordinary glass (soda-lime glass) is transparent to visible light but strongly absorbs thermal infrared. This is why greenhouses stay warm—they trap infrared inside. For a PIR sensor behind glass, the infrared from a warm body outside is absorbed by the glass and never reaches the sensor.
What About Special Glass?
Some materials, like germanium, silicon, or certain plastics (polyethylene), are transparent to IR. However, standard window glass is not. Even “low-E” glass, designed to reflect IR, will block it.
Can I Use a PIR Through a Window If…
- …the window is open? Then yes, the sensor can see through the opening.
- …I remove the glass? Then it’s not behind glass anymore.
- …I use an IR-transparent window? If you replace the glass with a material like polyethylene film, it might work, but this is impractical for most.
Alternatives for Through-Window Detection
If you need to detect motion outside from inside:
- Microwave/Radar Sensor: These use radio waves that penetrate glass easily. Sensors like RCWL-0516 can detect motion through walls and windows.
- Dual-Technology Sensor: Some units combine PIR and microwave; the microwave part works through glass.
- External Sensor: Mount the PIR outside, in a weatherproof housing, and run wires inside.
- Acoustic/Ultrasonic: Not typically used for through-glass, but possible.
What About Car Windows?
Automotive glass often has coatings that further block IR. Even without coatings, the glass itself blocks IR. A PIR sensor inside a car will not detect someone outside unless the window is down.
Testing for Yourself
If you want to verify, place a PIR sensor behind a closed window and try to trigger it from outside. You’ll likely get no response, or very erratic triggering if the glass is thin and the sun is strong (the sensor may detect the sun’s heating of the glass itself).
Conclusion
For reliable motion detection, do not rely on a PIR sensor behind glass. Choose a different technology or mount the sensor outside.