I just realized that even if the moisture escapes the mat retains all the heat. It will be about as warm as your back. That might be the reason.
The engineer in me would want to try how much you can modify the insulation properties of the mat so it would still insulate during the nighttime. One easy method would be to get some water inside it, but that would kill the insulation for nighttime too.
Maybe a wet blanket below the mat so it would constantly tie energy in the evaporation process.
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