Here’s the math for those interested:
Over on instructables, user kelseymh states:
Aluminum foil may not do it. The skin depth for aluminum at ~1 MHz (WiFi) is about 100 µm. Since that is also a typical thickness for retail foil, you’ll only get about 37% (1/e) attenuation or so.
For a decent Faraday cage, he’ll want to use perforated aluminum sheet, at least 500 µm thick, which will give five e-foldings of attenuation (0.0067).
On using multiple layers of aluminum foil:
You can treat each layer independently, and the attenuation simply multiplies.