How did the Vikings avoid Hypothermia

 An excellent answer to an excellent question.  I will add that we are no longer acclimatized at all to cold weather exposure and so we shy away from it.  The fact is that we can accommodate severe conditions and acclimatize.  Think soldiers in Flanders where it was cold and continuously wet..

Fabrics block rapid heat loss, even when damp.  After all most of the contact is with air, due to the weave.  Circulation can handle the difference.  It may take a bit, but you must simply get used to it.

One good reason to expose yourself to foul conditions is to stimulate your natural protections.  However it is also wise to follow that with a hot soak.  It is our high body temperature that suppresses viruses and the like.

During WWII the Finns would have their soldiers spend time in a sauna before going on patrol or taking their post.  This made them fully effective against Russian soldiers made almost comatose by the cold.

How did the Vikings avoid hypothermia when out in the rough seas?

John Bartram, I learn history through the archaeology I study and places I visit.

I grew up sailing in the same seas also, before modern, artificial fabrics, so how I sailed was pretty-much as did the Vikings and other Norse of ancient times. Even my boats were wooden, with natural-fibre cordage and sails. They sure would get heavy when wet and I really appreciated the modern materials when they became available.

Wet-weather gear were called oilskins and I remember my father standing in the face of storms, wearing his (expensive) oil-soaked silk jacket. The point here is the oil, so an oiled skin would resist water and provide warmth.

We wore the heaviest, wollen gear possible, the sort made for fishermen up in the isles off the north of Scotland. Mind you, when a wave crashes over you, I never did find a means of stopping the cold water running down my chest. I wore masses, from undergarments, to woollens, to oilskins. You might think I’d drown if overboard, but I could swim in any weather, weighed down by any clothes except boots (which you kick off). Really, I was tested in winter and in waters riven by cross-currents.

Faroese postage stamp with a picture of a Viking helmsman in a wadmal tunic.

Wadmal: “(Old Norse: vaðmál; Norwegian: vadmål, "cloth measure") is a coarse, dense, usually undyed wool fabric woven in Iceland, Norway, Sweden, Greenland, and the Orkney, Faroe and Shetland Islands from the Middle Ages into the 18th century.”

Hypothermia: I’d never heard of it, so it couldn’t have been much of a problem. Even now, I love feeling the cold air outside.