A frame assembly for a window or patio door includes an integrally moulded unitary master frame having upper and lower horizontal members, opposed first and second vertical jamb members extending between the horizontal members, and a vertical mullion midway between the first and second vertical members. An integrally moulded unitary sash frame is slidably mounted within the master frame. The sash frame can be lifted within the master frame to facilitate installation and removal of the sash frame. The mullion can include a weather buffering chamber to reduce fluid penetration from an exterior to an interior side of the frame assembly.