Window units in buildings, particularly older dwelling houses are usually made of an wooden frame having two window panels, namely an inner window and an outer window. The double window construction provides high insulation to the building. One type of such double window is commonly referred to as a double-hung window unit in which the sashes of the inner and outer windows are vertically slidable along their frames for opening or closing the windows. In some buildings, particularly older dwelling buildings such window units are made of wooden material which is not durable and readily subject to rot and breakage. Furthermore, it requires re-painting from time to time due to weathering and it is difficult to produce in a mass production process.
The drawbacks of wooden window units may be overcome by replacing them with window units made of a metal frame. Such metal window units lend themselves to mass prefabrication process, provide high durability and require virtually no repainting since the colour may be permanently provided in the metal material in the well known manners. However, in order to provide convenient operability, metal window units are normally provided with fully balanced sashes wherein each sash is provided with a counter balancing tension means such as a counter weight or a counter-biassing spring. The counter balancing tension enables the window to be opened with little force and can maintain it at any selected open position. But, due to the space required in housing the counter balancing mechanism, heretofore, it has been unable to provide a double-hung metal window unit having a thickness comparable or compatible with the existing wooden window units. Window openings in buildings particularly those having wooden window units, are usually provided with a jamb of only three to four inches wide, whereas known metal replacement window units, due to their constructional requirement for housing four counter balancing mechanisms, have a thickness of at least six inches, so that in order to install a metal replacement window unit in the window opening, major alteration must be made in the latter such as by removing the jamb or repositioning it so as to accommodate the window unit. Such major alteration of the window opening is time-consuming and costly to carry out. Attempts have been made to obviate the problems by making the inner window with balanced sashes while the sashes of the outer window are unbalanced and are provided with side latches. The elimination of the counter balancing mechanisms in the outer window in such window unit reduces its thickness to about four inches. However, it is difficult and awkward to operate. In order to operate the latches in the outer window so as to open or close it, one of the balanced sashes in the inner window must be moved downward or upward until the latches in the outer window are accessible for operation for moving the selected sash up or down half way and then the same balanced sash in the inner window must be moved back to its original position and to move the other balanced sash up or down until the previously moved latched sash can be reached to operate its latches so as to move it further or to open the window fully. It can be appreciated that the operation of such window unit is thus very awkward to carry out. Moreover, since the inner and the outer windows are different in construction, namely one has a counter balancing mechanism for both its upper and lower sashes while the other has a latching mechanism, the frame members of the outer window are thus different in configuration and construction from those of the inner window, so that they can not be produced with the same fabrication process.