What fabrics are bad insulators and what fabrics are good insulators?

This answer leaves out as much as does the question.
It seems that there is no definition of how the insulation
is needed. Good, and bad insulation for electrical components
is much different than if needed for home insulation, clothing
insulation, pipe insulation, etc.
There are many types of insulation materials. Some in the
fabric line are very stiff, or even hard, while others are very
flexible where some are even very soft.

In a material line that is to be used for clothing, or sleeping
bags there are none that are universally "good", or "bad".
Each type of material has different qualities of insulation.

If an insulating material used can get wet, it may not be
very good when wet. Others can still retain the insulating
quality when wet.

People over the years are used to hearing about down

filled sleeping bags as being the warmest. However,

today this is not true. When down gets wet, it loses

almost all of the insulating properties it had while it was dry.

The proper type of a polyester-filled bag can retain those

insulating properties even if it was completely soaked,

then merely wrung out to use.

Wool is an excellent insulation for just about anything.
It even has a natural fire retardency since it will not
maintain burning when flame is removed. With this fact,
wool also reflects heat either to keep it inside, or to
keep it outside. It holds in body heat while reflecting
away outside heat. (That is one reason why people
in dessert climates wear wool robes.)

With fire, the wool will char, but doesn't actually burn when

flame is removed. However, many can't stand to wear wool

next to their skin. For this, a silk, or polypropylene garment

will serve to wick the moisture away from the body and passes

it to the next layer to 'breathe' away. Wool retains the
heat while allowing the moisture to evaporate out, and
away from the body. It doesn't hold it in to saturate. All
the while, even if wet, it still retains the insulating quality.

Cotton is another good insulator for this use. However, it
will hold moisture without wicking it away very fast. Cotton
does block wind, however.

Polyester does hold in heat, but does not block wind. This is

not good to use for an outer garment for this reason unless

it has a wind, and moisture barrier inside. However, with such

a barrier it doesn't allow for moisture to be taken away from the body.

A good rain coat can block wind well, and serves to insulate

by holding in heat. It is best to wear a rain suit where the jacket can
be opened so as to allow for some air path for the moisture
to be removed from the inner garments. Leather can also
serve somewhat in the same capacity as rain wear. It isn't
quite as good, but can appear more stylish.

The whole key to proper insulation from the cold on the
body is the layering of clothing. A light weight synthetic
such as polypropylene next to the body, then a wool
garment, then cotton, and finally an outer garment of
Nomex, or Goretex. Goretex is the most expensive, but
will stop the incursion of water while allowing air, and
moisture to be expelled from the body while still holding
in the warmth. Nomex does not totally stop the rain,
but does hold in heat and allow for air to escape.