An interlocking curtain wall insulation system for a building having a backer bar spaced from a floor slab defining a perimeter void between the floor slab and an outer wall, a safing insulation disposed above the backer bar and compression fit within the perimeter void between the floor slab and the backer bar, a first curtain wall insulation extending from an upper surface of the safing insulation adjacent the backer bar, a second curtain wall insulation depending from the backer bar.