Abstract: A double-hung replacement window unit comprises an outer window and an inner window joined together with mortised joints by plastic thermo-break members on all four sides. The outer and inner windows have an identical construction and each comprises a balanced lower sash and a latched upper sash. The thickness of the window unit may thus limit to about three to five inches and is compatible with existing window openings in buildings particularly older dwelling buildings such that it may be easily used to replace an existing window unit without having to make major modifications to the window opening.