HelpfulGardener.com

OK, so I came across this https://www.gardenersworld.com/how-to/projects/cold-frame-insulate/

which got me thinking, how cold can it be at night for this to work and leave the seedlings out?

I would like to know the answer to this too. Wouldnl't you need a heat sink like a jug of water or some souce of heat for it to make any difference or is it just to hold in the heat gathered from the sun during the day. That wouldn't last all night on a below freezing night.

We are still getting a few below freezing nights here but I would like to put my seedlings in the cold frame!