I use a thermarest Xtherm and part of it warmth comes from a reflective coating that traps heat. I wonder if putting a closed cell foam pad on top would defeat this feature?
I put a foam pad under my Xtherm mostly to protect the Xtherm but I also think it is the warmest configuration. I've been cold a few nights but it was not caused by my pad and mattress.
I use this foam pad that weighs less than 3oz.
http://gossamergear.com/thinlight-backpacking-pads.html
