Neither a house nor a car makes a really good Faraday cage. From experience, I can aver that holes to the outside world can be insidious and elusive. While repairing a broken Faraday Cage – whose walls were double thicknesses of copper mesh – I carried my cell phone and a radio into it and turned them on. They still worked perfectly, despite the fact that I was almost completely encased in apparently seamless mesh. Carrying them around I soon found out where tears and breaks in the mesh were, and was able to repair them. When I was finally finished, my radio didn’t work any more, and neither did the cell phone. But it wasn’t easy to find and repair all the holes – some of which were linear rips in the mesh.
By comparison, the windows in a metal-covered house or a car are enormous. If a tear so small you can’t see it can let signal leak in, any gap as big as a window you can easily see through is hopeless, and the “Faraday cage” with that big a gap will leak signal like a sieve.