Aluminum foil can significantly suppress high-frequency electromagnetic waves and even small amounts of radioactive particles. When installed correctly on a large surface, you can block >99% of electromagnetic waves.
But as we all know, high-frequency electromagnetic waves will reflect. So unless the source of the electromagnetic wave comes from a reasonable distance > (10m or 30ft) you most likely won't achieve 100% damping, so the reflection will be very weak, or you make a Faraday cage. Making a Faraday cage may seem like an easy task, but making a completely enclosed cage is actually quite difficult.
Low frequency electric fields can be reduced by grounding shielding materials. But care should be taken in rooms with unshielded wiring not to create larger electric fields between the environment and the shielding material. The best approach is to also make a Faraday cage or replace all wires with shielded cable to prevent electric fields from becoming a problem.
Low-frequency magnetic fields cannot be shielded with aluminum foil. You can use an expensive material called Mu-metal to dampen the magnetic field, but the attenuation is usually about 50-70%, which is quite low since 1000x damping is considered good, which is 99.9%.
Additionally, you should not use any wireless devices in a shielded room because the shielding material actually reflects the electromagnetic waves you generate yourself. And blocking is not selective. With most shielding materials, you can shield everything. So not only do you have your neighbor's WiFi, but you also have your cell phone signal, which most people don't want. But if you're not using wireless technology and want as few electromagnetic waves as possible, using aluminum foil can be a cheap and useful way to achieve reasonable results. Just be sure not to stick it on the exterior wall as this can cause black mold to grow behind the foil.
