It makes nothing — it slows
Does a Coat Make You Warm?
A coat produces no heat at all; it just slows down the heat your own body is already making.
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Step 1 of 5
A coat is not a source of heat.
Leave one on a chair and it never becomes warm by itself. It only ever does anything once a warm body is inside it.
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Step 2 of 5
Your body is the actual heater.
You produce heat constantly just by being alive, roughly as much as a light bulb, and that warmth is always escaping to the cooler air.
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Step 3 of 5
Still air is a remarkably poor conductor.
Air hardly conducts heat at all, provided it stays still. Moving air is a different matter, because it carries warmth away bodily.
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Step 4 of 5
A coat's real job is trapping air.
The fibres, down or foam exist to hold thousands of tiny pockets of air still, and it is that trapped air doing the insulating.
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Step 5 of 5
Slowing the loss is what feels like warmth.
With escape slowed, your own heat builds up around you instead of vanishing. You feel warmer without any heat having been added.
This is why a wet coat is useless: water fills the air pockets and conducts heat roughly 25 times better than air.
The short version
A coat makes no heat at all; it traps still air that slows the escape of the warmth your own body is constantly producing.
Try it yourself
Wrap an ice cube in a coat and leave another out. The wrapped one lasts longer, because a coat slows heat moving in as well as out.
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