Faraday bags usually stop working for one reason: gaps. The metallized fabric cracks along folds and seams, and the closure stops sealing completely, so signal leaks in and out. You won't see it, but a 60-second call test will tell you.
If you're new to these, start with what a Faraday bag is and whether you need one.
Why gaps matter so much
A Faraday bag works by surrounding the device with conductive material. The catch is that openings have to be smaller than the wavelength of the signal you're blocking. Per the physics of Faraday cages, the shorter the wavelength, the more easily it passes through a hole of a given size. That's why Wi-Fi is usually the first signal to get through a failing bag.
NIST's mobile forensics guidelines list three ways shielding fails: the material doesn't weaken the signal enough, there are leaks or seams in the shield, or the shield itself acts as an antenna. The first one is about how the bag was made. The second one is how most bags fail over time.
What wears out first
The lining
The shielding layer is usually a thin metallized fabric. Every fold, roll and squeeze flexes it. Thatcham Research, a UK vehicle security lab, told drivers that on cheaper pouches it's possible to damage the mesh just by constantly taking keys in and out. A buyer's guide from CAR magazine says a good pouch should last 1 to 3 years with daily use and that performance can drop over time, so test it regularly.
The seams
Stitching punches holes through the shield. Better bags cover or weld the seams. An OffGrid test of commercial and DIY bags credited the bag that blocked all six signal types partly to its thermo-welded seams and a closure that combined magnets and Velcro.
The closure
This is the weak spot in daily use. In a SANS Internet Storm Center test, Bluetooth audio from a phone in a sealed bag "cut in and out" and was "heavily impacted by proper sealing of the bag."
Fold-top vs. zip vs. Velcro closures
- Fold-top (roll-top): The opening folds over on itself one or more times, then fastens. When folded the way the maker says, the shielding layer overlaps itself, which helps. Folding it once when the instructions say twice is the most common user error. The folding also puts stress on one line of fabric, so check that crease for cracks.
- Zipper: Easy to use, but a zipper is a line of tiny gaps unless the bag has a shielded flap over it. Look for a flap that covers the zipper and fastens down.
- Velcro only: Velcro isn't conductive. It holds the flap shut, but the shielding has to overlap underneath it. Lint and hair in the hooks keep the flap from lying flat over time.
Whatever the closure, the rule from the SANS test applies: follow the instructions and test it.
How to test a Faraday bag at home
NIST tells forensic examiners to test their own shielding products before they rely on them. You should too. Do all three tests. Each catches different failures.
1. The call test (cellular)
- Make sure the phone has full bars and Wi-Fi calling is on (it's a harder test).
- Seal the phone in the bag exactly as you would when using it.
- Wait 30 seconds, then call it from another phone. It should go straight to voicemail without ringing.
- Send a text, wait a minute, and take the phone out. The text should arrive only after you open the bag.
- Repeat standing next to a window, where the signal is strongest.
2. The Wi-Fi and Bluetooth test
- Play music from the phone to a Bluetooth speaker.
- Seal the phone in the bag. The music should stop within seconds and stay stopped. If it stutters but comes back, there's a leak, usually at the closure.
- For Wi-Fi, turn on the phone's hotspot, seal it in the bag and check from a laptop whether the network still shows up. In the SANS test, a working bag made the hotspot disappear.
3. The key fob test
- Put a keyless-entry fob in the bag and seal it.
- Hold the bag right next to the driver's door and pull the handle. Then sit inside and try the start button.
- The car shouldn't unlock or start. This is the same test a CAR magazine pouch review used. Details and car-specific tips are in do Faraday bags work for car key fobs.
When to replace a Faraday bag
- Any test fails, even once, with the bag sealed correctly.
- You can see cracks, shiny bald spots or holes in the lining, especially along the fold line.
- The closure no longer lies flat or holds shut by itself.
- The bag got soaked or crushed, or the lining is torn at a seam.
Test it monthly if you use it every day, and before any trip where it matters. Write the date on a piece of tape inside. If you'd rather make a cheap backup, see how to make and test a DIY Faraday bag.
Questions people ask
How long do Faraday bags last?
It depends on build quality and how often you open it. One buyer's guide puts a good-quality pouch at 1 to 3 years with daily use. Test monthly instead of guessing.
How do I know if my Faraday bag is working?
Seal your phone inside and call it from another phone. It should go straight to voicemail. Then check that Bluetooth audio stops and a keyless fob inside can't unlock your car.
Why does my phone still ring in a Faraday bag?
Usually the closure isn't sealed the way the maker intended, or the lining has cracked or worn through. Reseal it carefully and retest. If it still rings, replace the bag.
Can a Faraday bag lose its effectiveness?
Yes. Folding and handling wear down the metallized fabric, and seams and closures develop gaps. Security testers recommend testing pouches periodically for this reason.
Is a fold-top or zipper Faraday bag better?
Either can work if the shielding overlaps fully when closed. A fold-top needs to be folded exactly as instructed; a zipper bag needs a shielded flap over the zipper. Test whichever you have.
Your phone is only one way you get tracked. License plate cameras log your car whether your phone is bagged or not. Grab the free Flock Camera Field Guide to learn how they work and how to find them, or get Ghost Protocol v4 for the full privacy playbook.
This article is general information, not legal advice.
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