The mechanisms of floor-to-floor fire spread at the exterior walls have been established by the work of fire researchers and fire engineers dating back to the 1960s. Testing efforts of product manufacturers and testing laboratories during the 1990s have indicated that flames emitting from an exterior window can extend higher than 16.5 ft. (5m) above the top of the window; therefore, fire-stops must be included in the design. In multi-story construction, prevention of fire spreading from floor to floor is achieved by including a fire-stop in the space between the floor slab and a curtain wall. If the void between the floor and the curtain wall is not properly sealed, the fire can spread from the lower to the upper floors. Therefore, perimeter fire barrier systems are used to provide fire resistance and prevent passage of fire from floor to floor within the building. This article discusses development of a new design concept for fire-safing in complex curtain walls, as well as testing and evaluation