Double-hung replacement window unit

A vertically operative double-hung metal replacement window unit comprising:

• an inner window and an outer window having identical frame members, an identical vertically slidable counter balanced lower sash and an identical vertically slidable non-counter balanced upper sash,
• said frame members comprising two mutually parallel spaced vertical side frame members secured at the upper end therein by an upper cross frame member and at the lower end therein by a lower cross frame member, each of said frame members having two mutually parallel spaced inverted L-shaped lateral longitudinal flanges,
• said non-counter balanced upper sash having two lateral latches at the upper corners therein operative from the inner side of said upper sash to engage associated openings in said vertical side frame members to lock said upper sash selectively in a closed position and selected vertically opened positions,
• thermo-break members made of low thermal conduction material and having a generally U-shaped cross section body portion with two spun-over flanges slidably engaged with said inverted L-shaped lateral longitudinal flanges of said frame members between the inner and outer windows to form mortised joints therewith and to secure said inner and outer windows together, said U-shaped body portion being sandwiched between said frame members of the inner and outer windows to serve as a thermal break therebetween.