I'm going winter camping in a couple of weeks. It will be at the limit of my sleeping bag comfort zone. We may be sleeping on snow. I plan to take my Big Agnes air Mattress and a closed cell foam pad like maybe my RidgeRest. The question is: do I put the RidgeRest on top of the air Mattress or the other way around. I am only interested in warmth, comfort either way will be acceptable to me.

The old theory was that air in an air mattress circulated cold air. But my new air mattress does have a small R rating and if it is on top of the CC Foam pad and covered by the sleeping bag where would the cold air come from? I have slept on snowand ice several times with just a 72" RidgeRest and was warm enough so I think it alone is pretty darn good insulation..

Put the CC foam pad on top. I've consistently heard anecdotal evidence that it is warmer that way. What I think happens is this: you fill up the air mattress and by the time you go to bed, it has cooled off quite a bit. As a result, when you go to bed your body has to warm up the air in the mattress. Not very much air in the CC foam so it cools your body off less.

About 10 years ago, I was out the last week of September when the nights went down to 15*F. Basically, a three-dog night with only one dog! I had a very inadequate air mattress and a 1/8" thick Gossamer Gear foam pad. While either way (foam on top or foam underneath) was cold, it was slightly less frigid with the foam on top.

What kind of air mattress? BA has both plain and insulated versions.

I would tend to put the foam on the bottom and the mattress atop it. My thinking is you will have a barrier at ground level and convection heat loss from the mattress would be mostly limited to the edges. If you flip the order then the mattress is free to conduct heat to the ground across its full area. I think it will also be easier to tweak the mattress volume for comfort when it's on top.

There could be an issue of them slipping against each other, but that you can experiment with to find out. Some mattresses have anti-slip bottoms.

My $0.02.