You lose most of your body heat through your head
Bundle up in winter and you'll often hear that a bare head leaks away almost half your body heat, so a hat matters more than anything else you're wearing. The claim traces back to a mid-20th-century US Army manual. Is it actually true?
Myth or fact? Reveal
Myth. It's a myth. The head accounts for roughly 10% of the body's surface area and loses heat in rough proportion to that, not 40–45% as the old claim suggested. The figure likely came from cold-weather tests where soldiers wore full-body survival suits but left their heads bare, making the head the only exposed surface left to lose heat from.
— BBC Science Focus, The great cold head myth: where you really lose your body heat from