I notice that plans for cold frames almost always have the north wall, or usually three walls, made of an insulator like wood.
If almost all the solar rays come from the south, the other walls probably lose more heat through the day and night than they can absorb during the brief period when the sun shines on them.
Maybe a greenhouse would benefit from heat blankets on North, East and West walls during the winter.
Some book about winter gardening for markets and restaurants in Maine spoke well of "layers" for keeping winter crops warm enough.
The outer greenhouse walls (or outer hoop tunnel) were one layer.
small hoop tunnel inside the main greenhouse was a second layer.
Spun-woven row covers were a third layer.
Cloches under the row cover were a fourth layer (for example, for germination or hardening off).
I recall they got something like 5-15 degrees temperature difference from each layer. The differences were cumulative, and the inner layers protected the plants against drafts.
(Any draft or air leak was a huge heat loss.)
