I used to head to the field with an ECWS mummy bag. In 20 degree weather, I slept with the bag unzipped and a poncho liner tucked in over me and slept well.

I still use it to cover our bags in cold weather.

That thing is a quarter inch thick piece of great insulation. it breathes well and doesn't collect moisture.
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What you know isn't as important as knowing what you don't know