In a method of supplying an element for covering a non-vision area in a curtain wall on an architectural structure, a plurality of shadow boxes is prefabricated at a location remote from both a unitized curtain wall assembly shop and the architectural structure. The shadow boxes are prefabricated by sealing an interior spacer between a vision glass panel and a back structure. The back structure includes an insulating material surrounded by a rigid envelope. The vision glass panel, the back structure and the interior spacer define a hermetically sealed void therein. A structural seal is applied about the interior spacer and is affixed to the peripheral edge of the vision glass, the interior spacer and the back structure. The plurality of shadow boxes is then delivered to a selected one of the unitized curtain wall assembly shop and the architectural site.