quite easily from the groove to free the sheet preferably the side portions 38 and 43 are each at an angle of about 15 u00b0 to a plane perpendicular to the side wall of the channel member and intersect the head opening at angles that are with respect to such a plane and the center of the head opening approximately 30 u00b0 for side portion 40 and 60 u00b0 for side portion 38 respectively preferably such a longitudinal groove is provided in both side walls of the channel member permitting either one or two flexible sheets to be carried by the channel member thus each panel of the building structure may carry only one sheet to seal the enclosed structure from wind and rain or two parallel sheets to also assist significantly in thermally insulating the enclosed space if desired further insulation may be achieved by blowing or otherwise depositing in the space between the two flexible sheets an insultating material 48 some of which is shown in fig2 such as shredded styrofoam this material may be left in place or may it be blown in to insulate the enclosed environment during periods of thermal stress such as at night then sucked out of this space the next morning to again render the panel transparent to sunlight intersecting channel members as shown in fig6 may be connected by an angle bracket 52 this angle bracket being.