[Dan Evans]

Any wave penetrating 12" of concrete isn't simply going to refelect off of tinfoil.

I think it would. Some things are transparent to EM radiation and some things aren't. For example, you can see light through 12 inches of glass but not through a .001 inches of aluminum foil. It depends a lot on the wavelength.

[SampleMan]

Right. It depends on the wavelength. That's all I was saying. The shorter the wavelength, the more likely it is to reflect or absorb.