Answer:  

An anonymous professor, who is studying about clothing, kindly answered the question above.
  
  

The reason why we feel warm when we wear a scarf If we do not wear a scarf, air inside the clothing and air outside are exchanged. So the temperature inside the clothing changes. This is called “stack effect”.     If the air exchange does not occur, the inside of the clothing becomes warm because of “still air”, which has an insulation effect. I will explain “still air” in the next section.

Scarf and Sweater Courtesy of  Sarah Mackinnon

   
The reason why we feel warm when we wear a sweater
The air which does not move (“still air”) hardly conducts heat. Fiber conducts heat 10 times more than air. Fabric is composed of fibers and air (and water in some cases). Fabric conducts heat about 2 – 3 times more than air. Since fabric wraps around still air, it does not conduct heat very much. Therefore we feel warm when we are wearing clothing, especially a knitted sweater. 
 
Actually not only heat conductivity but also the thickness of clothing affects its warmness. It would be common sense that we feel warmer when we wear more clothing. A knitted sweater is usually thick, so we can say that this is also a warm material from the thickness point of view.
 
Therefore, in warm fabric (cloth, textile), there is:

• Still air surrounded by less fiber

• Thickness

However, it is a totally different story if air moves. When air moves, it moves with heat. Therefore, clothing which has a lot of space, such as a knitted sweater, loses its warmth retaining property, when the weather is windy.
 
About clothing which has less space
The amount of fibers is hardly related to the heat conductivity. The thickness would be the main reason why you feel warmer in a sweater than in clothing which has less space. I guess that the clothing which has less space may be made by fine strings. If the clothing is knitted using fine string, the clothing becomes thin. So the warmth retaining property becomes less.
  
  
Acknowledgement
We would like to thank anonymous professor for his kind answer.
  
 

This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp.  Please let us know if you find any errors.