Abstract: An inexpensive detachable insulating cover assembly for greenhouses and similar heated structures, using plastic cover sheets for the creation of an enclosed insulating layer, the sheets being held in place by means of rows of manually clampable and releasable fastener assemblies. A typical fastener assembly consists of a fastener base, preferably glued to a structural member of the greenhouse, and a fastener cap which is insertable over and lockable against an upstanding central stem of the fastener base, thereby clamping the cover sheet between cooperating clamping flanges. The locking action between the fastener base and fastener cap may be obtained with the aid of screw-type cam surfaces or friction surfaces. The edges of the cover sheets are folded back to form sealing lips.