Why is Cotton a Good Heat Insulator?

Cotton is a good heat insulator, provided that it is NOT IN VACUUM.

First of all, please understand the fact that air is a very poor heat conductor. Heat conduction  occurs when molecules ‘crash’ into one another and heat energy is spread form molecule to molecule, and because air is a gas with not many molecules in one unit volume, air itself becomes a poor heat conductor.

Now, why is cotton a good heat insulator?

Cotton is made of loosely intertwined fibers, and between them a lot of air is trapped. This explains why cotton is a good heat insulator.