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These 2 statements are both true, but I'm having trouble reconciling them, please help:

  1. When you're sleeping in a sleeping bag, you want the sleeping bag to be form fit (hence the mummy shape). If it's not form fit, there's too much dead air for your body to effectively warm up, which may lead you to feeling cold

  2. When you're sleeping in a sleeping bag, you want the sleeping bag to be an optimal thickness. If it's too thick, you'll trap too much body heat, which will lead you to overheat

I don't understand statement #2 in the context of statement #1. To me, it would seem that too much thickness means that there is too much dead air. So then why is it that in #2 the excessive dead air causes you to overheat, whereas in #1, the excessive dead air causes you to feel cold?

If I'm conflating multiple phenomena, please help me disaggregate

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  • $\begingroup$ Dead air can also convect, i.e circulate, and this removes heat. Insulation is full of really dead air, i.e, air trapped in many tiny spaces. Air is the best insulator when it is trapped in material but if not trapped can convect and remove heat efficiently. $\endgroup$
    – PhysicsDave
    Aug 20, 2019 at 21:49

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Statement 1 is saying that the amount of dead air is greater and it takes heat from your body to heat this dead air. This may take a while due to the volume of the dead air.

Statement 2 says that you want the right thickness of the sleeping bag such that heat will leave the bag at the same rate as the dead air is heated so the dead air doesn't overheat and cause you to overheat as well.

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  • $\begingroup$ OH that makes much more sense thank you! $\endgroup$
    – james
    Aug 20, 2019 at 7:13

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