If you want r-value, make all but the south-facing wall out of other building materials such as styrofoam, or even 2x4 framed walls with fiberglass. The south and top can be clear, which will minimize the area you have to cover up at night. I have seen pictures of a Chinese design for a heated greenhouse built like that. I think they rolled a big blanket down over the clear part each night and then removed it at dawn. I am still working on my design of a blanket, and how it handles elements such as rain. It could also be insulated shutters instead of a blanket.
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