[bborud]

And yet, this isn’t what what people in polar regions do. Nor is it how professionals are taught to dress in cold climates.

Reinhold Messner believes in yetis. Linus Pauling thought you could cure a cold with vitamin C and shot coffee up his ass. Clever people believe in dumb shit too sometimes.

Perhaps you should read the manual for how armies that operate polar regions dress? Or perhaps get some first hand experience before you insist?

[fingerlocks]

Physics is not on your side. You need more energy to heat up a larger thermal mass. Dense layers closer to the bodies core reduces the total volume.

[bborud]

I don't think you have chosen the correct physical model for insulation. Insulation is about thermal resistance, not minimizing mass.

Your body is continuously producing heat. What matters for staying warm is how fast you lose that heat, not how much “extra fabric” you have to warm up. The thermal mass of clothing is tiny compared to the thermal mass of your body so it isn't numerically relevant. The limiting factor is heat loss to the environment.

In clothing insulation comes from dead air space and preventing convection and conduction. Down jackets are warm because they trap a large amount of air in place and create a very gradual temperature gradient. That is, it spreads the temperature drop out over a thick, fluffy layer, so the inner surface stays close to skin temperature and the outer surface closer to ambient. The result is a much lower heat flux for the same inside–outside temperature difference. (A better mental model of this is that you have two boundary layers separated by a gradient that keeps ΔT low at each boundary layer. We know that heat flux is linearly proportional to ΔT)

Of course, clothing adds another dimension of complexity that is critical for comfort and survival: it has to deal with how your body actively regulates temperature: by perspiration. That is: you have to manage moisture too. And quite possibly a lot of it if you are active.

Wet fabric has higher thermal conductivity. Worse still, if it is dense and gets stuck to your skin you get very efficient direct heat transfer. The thing we want to avoid.

Think about why it is important to have a good thermal paste layer between CPU and heatsink. Now imagine you place the heatsink on a 0.1mm layer of aerogel. Do you think the latter configuration will cool the CPU efficiently?

When metabolic heat production drops that wet, conductive layer becomes a heat sink and you chill rapidly. In cold environments this can happen fast and be lethal.