Interlocking insulated wall sections (20-24), each of which contains a shell (30) having an outer panel (33) and a pair of end walls (35-36). A cover (31) is having an inner panel (38) and two end walls (39-40) is fitted over the shell. A first end wall (39) of the cover is mounted in contact with an end wall (35) of the shell and a second end wall (40) of the cover passes into the shell between the two end walls of the shell. An L-shaped flange (43) is connected to the second end wall of the cover and has a first leg of the flange (44) that rests in contact with the inner panel and a second leg of the flange (45) rests in contact with the other end wall of the shell. A space (50) is thus provided between the two panels of the wall section and is filled with an insulation (55). Each wall section is dimensioned so that it can be received in the opening between the second end wall of the cover and the end flange so that two wall sections can be interlocked together in assembly.