Why a cotton garment cannot keep us as warm as a woollen sweater in the winter season?

Cotton clothes are thin and soft. They do not have spaces in their fabrics due to which air can not be trapped in them, in order to keep us warm. So cotton clothes do not prevent heat from coming out of our body.

Woollen clothes create a barrier as they have spaces and the air is trapped in them. Air being a bad conductor of heat does not let the heat to pass through it. Thus, they allow the body heat to stay in closer to the skin.