I have a world I've been developing whereby the entire world is locked in a cycle of perpetual winter. The "seasons" still exist but are merely a spectrum of mild cold ranging toward horrific winters. More or less it is the Antarctic but across the entire surface of the planet.
Inhabitants have created societies by living in the vast network of caves throughout the rocky surface. Geothermal energy has been harnessed to power and heat these cities. Magic in this world is normalized and minimal. There are a few artifacts of vast power but mostly magic is confined to everyday practical use (minor cantrips for household chores, ward off diseases, etc). The individuals in these cities can and do travel overland to trade, make war, pass along news.
That said, I am curious if there are real world fabrics or processes of weaving fabrics that could be used with the technology available late medieval to a early renaissance society that would be effective at warding off such extreme temperatures. The fabrics ideally would be as light weight as possible.
As stated above the temperature range would be -18 F (-27.7 C) in the Summer down to -76 F (-60 C) in the Winter months.
The Climate will vary across the surface of the planet with some
areas experiencing wetter snowier climes and others drier
The fabric ideally will keep the wearer warm enough to survive
exposed during the day time temps and survive under cover during
The fabric will ideally be as thin as possible so that the wearers
are able to move, work, unencumbered by massive bulky clothing.
The flora and fauna of the world resembles a combination of that found in the Ice Age (Pleistocene) era and that found in the present day Permafrost climates.