If you assume any bag labelled “Faraday” will block signals, you could be in for a surprise. Law-enforcement agencies have seen phones reconnect to networks while sitting in bags that claimed to be RF-proof. Corporate security teams rely on pouches to protect trade secrets, only to learn that a poorly stitched seam lets Bluetooth leak out. At JEMIC, we test bags the same way we test gaskets: by examining fabrics, seams, closures, and layered construction. Real evidence, not marketing, reveals which bags truly isolate devices.
Why Fabric Choice Matters
At first glance, a shiny foil or metallic fabric seems like it should block any signal, yet different materials behave very differently across frequencies. A guide published by the Australian security firm SecurityBase notes that aluminized Mylar bags perform reasonably well for lower-frequency signals but suffer significant attenuation loss at Wi Fi and Bluetooth bands.
In contrast, nickel–copper woven fabric delivers more than 85 decibels of attenuation across a broad spectrum and retains performance after repeated bending. Premium silver-coated textiles achieve even higher attenuation but cost significantly more. Our own tests mirror these findings: mylar-based pouches may suffice for shielding documents or low-frequency RFID tags, but we recommend nickel–copper or silver-coated fabrics for mobile devices.
When evaluating fabrics, pay attention to weave density and surface resistance; a dense weave with surface resistance under 0.05 Ω is a good indicator of quality.
Seams and Closures: The Hidden Weak Points
Even the best fabric cannot compensate for a leaky seam. It should be emphasized that seams stitched with non-conductive thread act like slot antennas, letting RF energy slip through. Double-folded seams and conductive stitching help maintain a continuous metal edge. Independent researcher Matt Blaze tested several phone-sized pouches and observed that bags with only a single line of Velcro closure performed poorly below 3 GHz but leaked at higher frequencies due to small gaps. Conversely, pouches with a dual closure (Velcro and magnet) showed consistent attenuation across the entire tested range. In practice, that means a properly folded and secured bag can add 10–20 decibels of isolation at the opening compared with a simple flap. Users must follow the manufacturer’s folding instructions and inspect seams regularly; visible holes or a single row of stitching are red flags.
Single Versus Multi-Layer Construction
Layering matters because a single thin layer of aluminum or conductive fabric may not attenuate high-energy signals. Single-layer bags often fail with modern smartphones and two-way radios, whereas multiple layers separated by a dielectric improve attenuation. Blaze found that wrapping devices in heavy-duty foil could achieve up to 90 decibels of attenuation, but results were inconsistent because it is difficult to achieve a uniform seam. For critical evidence handling, we recommend dual-layer pouches made from nickel–copper fabric with conductive stitching. The outer layer provides mechanical protection and an additional barrier, while the inner layer delivers the primary shielding. Multiple layers also help when the fabric begins to wear; if one layer develops a pinhole, the other layer maintains isolation.
How EMI Gasket Research Explains Bag Behavior
What do EMI gaskets have to do with Faraday bags? A lot more than you might think. Articles on gasket design explain that rough surfaces, poor flatness, and inappropriate material pairing create leakage paths in enclosures. Bags behave the same way: a rough seam or misaligned closure acts like a slot antenna, and dissimilar metals in the fabric and stitching can set up galvanic corrosion. When engineering teams already specify compression ranges, surface finishes, and material compatibility for gaskets, they should apply the same rigor to bag seams. Understanding the physics that govern both enclosures and pouches helps organizations adopt a consistent approach to shielding.
Risk-Based Matrix for Bag Selection
Different scenarios call for varying levels of protection. Based on independent guides and research, JEMIC recommends the following simple matrix:
| Fabric | Typical cost | Attenuation at 2.4 GHz | Recommended use |
| Aluminized Mylar | Low | <10 dB | Low-risk RFID tags or documents |
| Nickel–copper | Medium | >90 dB | General evidence handling and corporate security |
| Silver-coated | High | >90 dB | Highest-stakes forensics and military use |
The key is to match construction to the threat environment. A law-enforcement agency seizing phones should not rely on a single-layer aluminum pouch, any more than a high-frequency telecom device should depend on an under-deflected gasket. Consider the cost of failure when selecting a bag; spending more up front may prevent the loss of critical evidence or intellectual property.
Choose the Right Bag, Protect Your Evidence.
Securing mobile devices in the field is not as simple as dropping them into a shiny pouch. Real test data and independent guides show that material, seam design, and closure mechanism determine whether a bag truly isolates a device. To make an informed choice, confirm the frequency bands that matter most for your cases, verify fabric type and weave density, and require documented seam and closure designs.
Reserve high-performance bags for high-value evidence and use lower-cost options only when appropriate; tests show that mylar bags are inadequate for phone isolation. Periodically test pouches with simple field tests, such as making a phone call or pinging a Bluetooth tracker, and replace any that show signs of degradation. By choosing the right bag and properly maintaining it, you protect your evidence and avoid unpleasant surprises.