Abstract: An insulated casing assembly for use in injecting steam into wells, comprising a plurality of interconnected casings, each casing having outer and inner tubular sections, an annular spacing between the two sections containing multilayered thermal insulation enveloped in a low conductivity gas, and tubular bellows connecting and allowing relative movement between the corresponding ends of the two inner sections. A leak-proof connection between adjacent casings includes a diffuser sleeve fitted tightly over the ends of adjacent inner tubular sections, a filler ring fitted on the diffuser sleeve and disposed between adjacent outer tubular sections, and a threaded coupling screwed onto threaded ends of outer tubular sections of adjacent casings.