A thermally insulated access door installation for controlling an access opening to a refrigerated or cooled storage space, said installation large enough for allowing passage of a vehicle, said thermally insulated access door installation comprising: a frame comprising a hood and a pair of horizontally spaced upright frame members which define opposed sides of the opening, wherein the upright frame members comprise ducts or passages and guide means (16, 17); at least one door mounted in the frame which defines the opening, said at least one door comprises two horizontal support rollers (21,22) overlying an upper horizontal edge of the opening, said door configured to be raised and lowered by the two support rollers (21,22) and is formed by a double-walled flexible curtain of low thermal capacity formed from a single strip of flexible material with front and rear walls (13, 14) that hang downwardly and terminate in a common bottom edge thereby defining a space between the front and rear walls (13,14), wherein the guide means (16, 17) engage the side edge of the front and rear walls (13, 14); and a closed loop blower system comprising a blower wherein the ducts or passages of the frame communicate with the space defined between the front and rear walls (13, 14) of the flexible curtain; characterised in that the closed loop blower system further comprises a drying and warming unit, wherein said blower is operative with the drying and warming unit to generate a continuous supply of dry or at least low humidity air at a temperature greater than the temperature prevailing in the storage space; and the blower and the hood is configured to deliver the continuous supply of dry or at least low humidity air to the space from above the access opening, such that the dry or at least low humidity air is then routed through the space when the flexible curtain is in the closed position, in order also to receive said supply of air when the door is in the closed, lowered position and then routed internally of the ducts or passages of the upright frame members before being re-circulated to the closed loop blower system. a frame comprising a hood and a pair of horizontally spaced upright frame members which define opposed sides of the opening, wherein the upright frame members comprise ducts or passages and guide means (16, 17); at least one door mounted in the frame which defines the opening, said at least one door comprises two horizontal support rollers (21,22) overlying an upper horizontal edge of the opening, said door configured to be raised and lowered by the two support rollers (21,22) and is formed by a double-walled flexible curtain of low thermal capacity formed from a single strip of flexible material with front and rear walls (13, 14) that hang downwardly and terminate in a common bottom edge thereby defining a space between the front and rear walls (13,14), wherein the guide means (16, 17) engage the side edge of the front and rear walls (13, 14); and a closed loop blower system comprising a blower wherein the ducts or passages of the frame communicate with the space defined between the front and rear walls (13, 14) of the flexible curtain; characterised in that the closed loop blower system further comprises a drying and warming unit, wherein said blower is operative with the drying and warming unit to generate a continuous supply of dry or at least low humidity air at a temperature greater than the temperature prevailing in the storage space; and the blower and the hood is configured to deliver the continuous supply of dry or at least low humidity air to the space from above the access opening, such that the dry or at least low humidity air is then routed through the space when the flexible curtain is in the closed position, in order also to receive said supply of air when the door is in the closed, lowered position and then routed internally of the ducts or passages of the upright frame members before being re-circulated to the closed loop blower system.