One site says paint cold frame inside and out with white paint, to reflect sunlight.
Another site says white diffuses the sunlight and keeps plants from leaning south. And that to paint the inside of the cold frame black would turn it into an oven.
It looks like insulation is definitely a possibility. Maybe just scavenged or cheap rigid foam insulation, or even styrofoam sheets.
Another possibility for building the frames rather than use heavy 2x6s or 2x4s would be to build a skeleton frame out of 2x2s or 2x3s, and then sheet it in thinner plywood. I’m liking this idea. More design and woodwork than the previous plan, but lighter weight and less material. If I did that, maybe line the inside of the cold frame with very thin plywood painted white, or even foam core or white cardboard, to reflect the heat in. Rigid foam insulation could be cut to fit in-between the framing.
