There are two things present in all shields, reflection and absorption, and when combine together produce the shield true effectiveness. When energy is reflected, then some of the energy is absorbed by the shield, and for this reason the shield needs to be grounded.
When the shield is very thin a third reflection factor occurs at the shield far side boundary.
Using a second shield (double shielding) can increase the effectiveness of the shielding by many times compared to single shielding, and grounding may not be necessary. To give an example, a single shielded enclosure using 1.5mm copper sheet could provide about 55dB of shielding, when a double shielded enclosure using the same material could provide more than 100dB shielding.
I don't think silver would make much difference compared to copper when is about shielding.