Building codes have for many years specified sound isolation using the STC rating. Strictly STC applies only to a building element, such as a wall or a floor, and not to the attenuation between two homes. The correct rating for that is the apparent sound transmission class - ASTC. Suggested criteria for buildings are discussed and presented in Construction Technology Update 50. The recommendation given there sound attenuation between normal rooms in homes is for minimum values of ASTC and FIIC of 55 and 50 respectively.