Abstract: A building system is disclosed for constructing non-load-bearing walls in a building having preexisting ceiling and floor structures, using substantially dimensionally identical, lightweight wall panels. The wall panels are rectangular and each has a pair of facing sheets adhesively bonded to a plurality of elongate spacers. The facing sheets of each wall panel overlie each other and have aligned side and end surfaces which define sides and ends of the wall panel. Outer ones of the spacers are inset from the sides of their wall panel, whereby the back surfaces of the facing sheets and the outer spacers cooperate to define channels along the sides of the panels. Preformed elongate ceiling and floor runners are secured to the ceiling and floor structures, and the wall panels are positioned, one at a time, in mating engagement with the floor and ceiling runners to form continuous walls.