Lee,
I am sure you could add another layer to insulate. I thought of building another set of ribs using shorter lengths and covering that inside. I didn't feel that the ground below was a heat sink. The grass I didn't trample stayed alive all winter. All the greenhouse plans I saw had a single door designed to limit heat loss. I put double doors at both ends because I wanted to be able to work there when the sun came out.
I still can't post the plans I have, but a search of agricultural/exetnsion service sites should find some.