A unitized curtain wall system includes pre-assembled, pre-glazed aluminum framed curtain wall units. Vertical mullion components and top and bottom horizontal rail components interconnect adjacent curtain wall panels. The curtain wall units are suspended from one or more upper anchors installed on an upper floor slab. The bottom horizontal rail components are engaged with top horizontal rail components of adjacent, lower curtain wall units forming a continuous beam scheme for vertical mullion components and configured to accept lateral loads. The gravity loads of curtain wall units are supported by one or more floor anchors. Organic shaped insulating forms are disposed laterally between top and bottom horizontal rail components of adjacent curtain wall panels. The forms are compressible to permit flexing vertical movement of the curtain wall units while reducing air movement in a horizontal cavity formed between the top and bottom horizontal rail components forming a stack joint.