Many years ago, another Scoutmaster and I appropriated the tent site about 10 feet behind the stone chimney of the picnic shelter site where we were camped. The boys had built their typical fire, around 3 pm, for cooking supper at 6. It was December or January, in Ohio, with temperatures around 20 at night. We pitched a green poly tarp (yep, WallyWorld) with 3 sides pegged to the ground, and the open side facing the chimney. For good measure, we piled leaves up about a foot around the edges of the side.

We were using the old Grey Fox (?) sleeping bags by Peak 1, rated, I believe, to 15 degrees (and weighing about 3 pounds.) We went to bed around 9pm. Around 11:30, we bolted out of our sleeping bags, sweating like crazy, got rid of the leaves, and left our bags unzipped, draped over us, until about 4am, when it finally cooled down enough to crawl back in them.

It's amazing how much heat a fire can generate, even when it's just radiating through the back side of a stone chimney!