panels to the open position will effect a cooperation between the rail 85 and the frame 57 so that the panels will telescope back into the respective pockets at the side edges of the window unit when it is desired to utilize my invention in association with buildings that have already been constructed it will not be conveniently possible to provide pockets for the insulated panels at the side edge of the window units however i have provided a modified construction for cooperation with window units such as is illustrated in fig1 to 16 in this particular embodiment i have illustrated a slightly different window construction 110 having a window frame 111 with glass panels 112 mounted in the window opening 113 the window frame 111 is mounted in the wall 114 conventional header beams 116 are provided over the top of the window opening 113 and conventional sheathing 117 and outside siding 118 and trim 119 are provided in the usual manner in addition decorative trim 121 is provided on the inside of the building encircling the window opening and the window has the usual stool 122 provided thereon i have developed a modified track frame 126 which includes upper and lower brackets 127 and 128 respectively the upper bracket being secured to the wall 114 by screws 129 and the lower bracket being secured to the inside edge for example of the stool 122 by screws 131 the bracket 127 includes an inverted u shaped trough 132 which has downwardly extending parallel legs 133 and 134 connected.