There are currently two major types of exterior building envelope finishing systems: window wall systems, and curtain wall systems. Window wall systems use panels which fit between concrete floor slaps, and hence, are prone to leakage and are not aesthetically appealing. Curtain wall systems are installed proud of the concrete slabs, so have better performance, but are expensive to install. The system of the invention has the high performance of a curtain wall system, with the ease of installation of a window wall system. The invention uses rigid panels which cover the face of a building like a curtain wall system, providing a complete gasketed seal without the need for caulking as in the case of window wall systems. The vertical mullions are notched, so that the mullions can be easily anchored to the upper and lower surfaces of the concrete slabs.