Which type of building material causes the most attenuation of Wi-Fi signals?

Among common construction materials, fire-rated steel doors present the highest level of RF attenuation. Steel reflects and absorbs significant RF energy, often reducing signal strength by 10–20 dB or more.

From CWDP-305:

“Fire-rated metal doors and elevator shafts are among the most attenuating structures in a building. These materials reflect and absorb RF signals, severely limiting Wi-Fi propagation.”

— Reference: CWDP-305 Official Study and Reference Guide, Chapter on ProtocolandSpectrum Analysis