Window wall systems have increased in popularity for highrise construction. They are more economical than conventional curtain wall, while emulating the aesthetic look of such systems. A window wall system sits atop the floor slab. There is not a traditional joint like that which exists with traditional curtain wall. However, there is a gap between the edge of the floor slab and the slab cover (i.e. the decorative closure that conceals the edge of slab). 
In fire situations, portions of the window wall system are damaged or consumed. Even a partially consumed slab cover can create a chimney through which fire and hot gases can be channeled to the floor above. Testing has shown that this can be a concern. Naturally, installing some sort of firestop system into this void can mitigate fire spread. However, the construction sequencing for window walls coupled with the gap being inaccessible after the walls are constructed makes it difficult to effectively seal off this void.
STI recently introduced the SpecSeal® Window Wall Gasket (WWG). This 
intumescent gasket is installed either prior to placement of the window wall or during installation. It drapes into the void area. In the event of fire, it intumesces (i.e. expands with heat) to form an insulative char that seals the void between the slab cover and edge of slab. It is Intertek Listed and tested for up to 2 hours to ASTM E2307. For more information, visit this webpage or contact your local STI Representative.