 Nearly a century old, the interior walls had been progressively covered with various forms of insulating materials, including cork impregnated with tar, polystyrene foam, and polyurethane foam, to a thickness of 18 inches.Once ignited, the large amount of fuel, fed initially by the large volume of air in the building, became virtually inextinguishable.The six-story building's exterior walls were constructed of approximately 18 inches of brick and mortar, with no windows above the second floor.