Abstract: A formation of interior covering material is furnished for a greenhouse or covered atrium enclosure which has an original formation of exterior covering material to achieve a thermal-like insulation structure for the greenhouse-enclosure. The enclosure includes vertically aligned mullions having central portions or standards and marginal flanges extending from a sill to a head. The mullions carry panels of exterior covering material which provide a single glazing. The interior covering material is formed of flexible panels of synthetic resins which are bowed and inserted between the mullion standards and rest on the flanges. Associated members are provided to seal the space formed between the interior and exterior panels and furnish dimensional stability to the interior panels.