I'd forgotten to mention that only via covering the glazing with some serious insulation- like a blanket of fiberglass insulation wrapped in waterproofing such as plastic tarps can you expect much winter performance. Double wall plastic is only R2, triple R3. This is too much loss without throwing energy at heating in most cold winter climates.
A roll down insulating blanket is one solution illustrated by the China greenhouse style. It becomes more practical for smaller cold frames. Polyester fiberfill, such as bargain sleeping bags come to mind. One of the China greenhouse variants is widely spaced double glazed (film) with insulating blanket rolled down between the protective film layers. This makes good sense in climates with snow and freezing rain issues in winter. Exterior (steel roof panel covered) foam insulation panels are used by one of my local greenhouse owners; they dramatically improve winter performance.
It is tempting to ignore hard design issues. Insulating shutters for greenhouses have been largely avoided due to design difficulty, complexity. The era of just throwing lots of energy at heating and cooling is coming to a close. We can and must do better. A proven, exceptionally good design for a specific climate is future treasure for all the gardeners in that climate and beyond. A forum like this can now spread a new, exceptional design, as well as share the equally valuable mistakes.
Cold frame type growing boxes maximize growing area with the lowest amount of construction materials and cost; all the access space around the bed is no longer enclosed, heated or ventilated. They need more design attention to go beyond the modest season extension similar to a row cover.
Best wishes for your design.
Bruce
