Insulation works by "trapping" air 

Heat is lost by convection and conduction through the walls and roof of a building. Glasswool insulation ‘traps' air and slows down convection thereby, reducing heat loss. Knauf Insulation's advanced technology produces very soft and long strands of wool which "trap" air more effectively to save even more energy.

In a building, the larger the temperature difference between the inside and the outside, the thicker the layer of insulation needs to be to reduce heat flow.