During certain launches of the space shuttle, thermal protection tiles on the bottom of the shuttle were damaged by impacts from fragments of insulating foam material that broke away from the external liquid fuel tank. Under contract to NASA Johnson Space Center, SwRI researchers conducted a study to assess the effects of these impacts on the tiles so that the safety of the shuttle during reentry could be assessed. A small compressed-gas gun was modified to shoot pieces of the very low-density foam insulating material at the tile. Images of the impacts were recorded with a high-speed digital camera to better understand the damage process. -LINK from 1999.
It's a low density ceramic. The ceramic is full of voids. The voids are not gas filled, but are empty space, that provides for the least thermally conductive material. I don't know exactly what the major ceramic component is for the tiles, but I'd guess it was aluminum oxide, because it has the highest emissivity, very low thermal conduction and is extremely inert. The high emissivity means it radiates whatever heat it picks up fast. Radiation is the only way to get rid of the heat. Any carbon in a composite structure is used as a very high melting thermal conductor to conduct heat to other radiating surfaces where the aluminum oxide that protects the carbon, radiates the heat.
My local station from Orlando was broadcasting the landing live ... the reporter there had what he said was a small piece of the "foam". He easily stuck a pen into it ... it reminded me of packing foam.