alt Hacker NewsA steel case has too little effect on cosmic radiation, but it is good to shield electrical noise.
The computers in plastic cases can sometimes be affected by appliances with electrical motors that are used nearby them.
The cosmic radiation is attenuated only inside a big building, e.g. one with many stories above you. A really great attenuation is obtained only underground.
Good point. Some googling suggests that plastic may be better than metal as hydrogen absorbs high energy protons without generating secondary radiation.