Disclosed is a window sash assembly for use on an exterior surface at a window opening. The assembly comprises a rectangular framework formed by an upper horizontal frame member, a lower horizontal frame member and a pair of vertical frame members. The framework includes a window covering portion facing the window opening and at least one window panel housing portion on the exterior wall surface alongside the window opening. The sash assembly includes at least a pairof window panels which are retractable into the window panel housing portion to fully open the window opening.