Each of these words, based on a Latin word for "health," means "unhealthy," "likely to cause disease." Although unsanitary is heard more often than insanitary, the latter is the preferred listing in five leading dictionaries. If conditions are so unclean, filthy, or contaminated as to be a threat to health, they may be called unsanitary or insanitary, as one chooses.
Dictionary of problem words and expressions. Harry Shaw. 1975.