The following list is ranked from worst to least. 1 - Metal. The ultimate signal blocking material. ... 2 - Concrete Walls. ... 3 - Plaster and Metal Lath. ... 4 - Ceramic Tile. ... 5 - Windows and Tinted Glass. ... 6 - Mirrors. ... 7 - Drywall. ... 8 - Devices Operating on the 2.4 GHz Frequency. More items...