Abstract: A conduit system composed of insulated jacketed conduit units each having an exposed male conduit end and an opposite integral female conduit end with an internal seal ring carrying groove slidably and sealingly receiving the male end of an adjacent unit thereby forming an expansion coupling between the units. A flexible insulation ring surrounds the coupled ends of the adjacent units and a sleeve surrounds the ring and the adjacent ends of the jackets of the units and is sealed to the jackets. The conduit cores of the units are thus surrounded with insulation and jackets or sleeves and are free to expand and contract withot opening up leakage paths or buckling the installed conduit system regardless of temperature variations normally encountered by the system.