My cold frame (planted with cold weather greens) is getting 10-20*F above outside temperature on sunny days, but falls to outside temperature without sun.
Last night was very chilly - 29*. We covered it with a plastic tarp that covers up some of the open hinges where air can flow in and gives it an extra layer of insulation. At 30* outside right now, its 46* inside! The plants look cozy.
I read that a cold frame is a good place for hardening off seedlings. I'll give this a try later for my tomatoes and other warm weather seedlings. Still too cold out there for them.