for sealingly engaging outer walls 58 and 94 the vent sash top and bottom members 62 and 98 also have roller assembly guide flanges 258 260 functional only in the bottom member 98 for supporting roller assemblies 112 as the roller assemblies roll sideways on a raised roller path 262 in channel 90 in order to remove the two sashes 20 and 22 the vent sash 22 is moved to a partly open position raised slightly the bottom edge is swung inwardly and the sash 22 is lowered out of the upper channel 54 the screws 154 are removed from the intermediate interlock rail 24 and the interlock rail 24 is removed to the inside the screws not shown in the inwardly directed flange 214 of the fixed sash jamb 150 are removed and the fixed sash 20 is raised slightly the bottom edge tilted inwardly and the sash 20 is lowered out of the upper channel 52 in order for the fixed outer sash 20 and the slidable inner sash 22 to be able to fit between the sill member 14 and the head member 16 it is necessary to form notches at the top and bottom of the sash vertical members in portions which extend into the central portion occupied by the rigid strips 48 and 84 in order for the.