A cryogenic storage tank is disclosed having a bottom, a sidewall and a domed roof supported thereby, and an insulated false ceiling mounted on the sidewall and dividing the tank into a lower storage space and a dome vapor space and supported at its center by a support column, the ceiling being imperforate except for a foraminous portion allowing passage of vapors therethrough. There also is disclosed a double-vaportight sidewall with insulation therebetween and a purge space above the insulation communicating with the dome vapor space by way of openings in the inner sidewall.