Today, with a high of about 49 degrees, the temperature hit 100 inside the cold frame. That I understand, and I know I can vent it to keep it from getting that hot. But when I got home a couple of hours after sunset, the temperature inside was down to 32... exactly what it was outside. As I write this it's 29 outside and 29 in the cold frame.

It's been the same story since I got it a few days ago. It's exactly matching the outside temps at night -- and this is what I don't understand. Where's the benefit if I can't keep the "microclimate" inside at least a few degrees warmer at night?

Aside from trying to seal any cracks around the lid and throwing a blanket over it at night, I don't see an answer. And if I can't get home in time before the cold frame cools off... I can't blanket it in time to help.

Am I missing anything?

Thanks,

Rex

My coldframe is 2x6 sides and double-glazed top, one side opens with an auto vent opener. I can get down to ~28-26ºF and maintain ~40-45º overnight due to soil temps and water jugs. Nonetheless, here in Denver area I cover with an old sleeping bag with tarp over that if it drops below freezing. I have mustard, lettuce, radish and spinach coming on hard now.

Without any information, I presume you are leaking heat out the sides as well as the top, as in an all-polycarb 1-layer coldframe. You'll want to insulate the sides to slow down heat loss, then add another layer to the top to have a dead-air space to help. Cheap window film at Home Depot will last a season and perform reasonably well.

Good luck,

Dan