warmth comes from several factors.
fiber (some fibers are actually hollow like a drinking straw!) and trap dead air (air is a good insulator!)
HOw the fiber is spun--other fibers trap air by how they are handled.. fluffy wool (loosely spun) traps more air than worsted spun (a tighter, strong fiber)
How the fiber(yarn) is fabricated--that is: how the stuff is knit or woven.
tightly woven or knit fabrics block air movement.
this BLOCKS air movement
or have multiple layers.. fabric is made from LOOSELY woven or KNIT LAYERS--the LAYERS trap air.
the Final factor is related to breathability.
that is: the ability to block liquid water, but to let water vapor pass
saran wrap traps air (and so do vinyl rain coats..)and block WATER.
but these fabrics also block moisture( water vapor). and very quickly, when wearing them, you get sweaty, then damp, then the damp gets cold (since water is not a good insulator!)
Wool is very breathable (it traps air, but lets moisture out.. so you stay dry and feel warmer!) Wool also is some what WATER PROOF (vs water vapor). so wool is slow to get wet.
Cotton is also pretty good a traping air, and letting moisture out.. but is only about half as good at trapping air, so it doesn't feel as warm! (cotton gets wet in heart beat..and wet (WATER) doesn't insulate, so cotton fails at the first bit of rain (or snow)
Acrylic is Warm,but like saran wrap or vinyl, not so good at letting moisture out.. (and it, too gets wet easily) so synthetic hats are warm, but quicky make you feel sweaty.
the best bet for NON wool fibers is the 'thermal blanket' trick--
LOTS of THIN OPEN LAYERS the openess lets water vapor out (and keeps you dry--and dry means that air can insulate! the many layers trap air, and trapped air is warm!
Some northern native peoples were Furs.. (and fur fibers are often hollow and trap air.)
but some were parka's made from whale intestines (think sausage skins) the 'fabric' is very thin,-it is transparent! (thinner than a sheet of paper) it has almost NO warmth! but they layer and layer and layer (20 or more layers) of the whale intestines..and each layer traps air.
IN addition, each layer, like wool, is WATER PROOF (to some degree) BUT breathable (water vapor readily passes through) so they are DRY and enveloped in layer of trapped air (and once again, air insulates!)
parka's like this can be seen in the Smithsonian's American Indian Musuem (at the Custom's house in lower manhattan)[/COLOR]
to make a warm non wool hat, lots of thin, thin layers.. trap the air.. and trapped air will make it warm.