Abstract: This invention relates to a thermal window construction which comprises a fixed frame member and a movable frame or operating sash member. The window is characterized in that the operating sash member includes an outwardly facing glazing surface to which a pane sub-assembly comprising two or more glass lites are connected, the bond between the pane sub-assembly and operating sash being provided by a bead or quantity of silicone interposed between an inwardly directed surface of the sub-assembly and the glazing surface through a laterally open channel, whereby complete filling of the said space may be assured. The overlapping areas of glass and metal bonded by the silicone rubber material are coordinated with the glass expanse and weight in such manner that the stress applied to the bonded areas will not exceed about 20 p.s.i.