TomD -- I can't find anything here to disagree with...

And that's one reason I'm suspicious of the claim about bags being more efficient or having less surface area, etc., than that what is experienced when wearing insulated clothing in lieu of a bag...

You've made two winning points; 1)the head is a big radiator and, 2)the difference between bags (and, I believe, clothing) is not that big of a difference.

What one uses for a sleep system is an individual thing. The system is/can be made of many parts and the bag (or clothing) is just one. Beyond consideration of the multiple parts is consideration of the individual sleeper and the conditions faced (temp being only one factor of the conditions...there's humidity, precip, wind speed, hammock hanging or on the ground, etc.).

I certainly don't advocate everyone leaving a bag at home although I and others do it quite successfully. But the blithesome disregard for the idea of not using a bag has surprised me.

FB
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"...inalienable rights...include the right to a clean and healthful environment..." Montana Constitution