The still air inside insulation is effective as an insulator primarily because it eliminates convection, which is the transfer of heat through the motion of air. Insulation materials trap air within their structure, creating pockets of still air that significantly reduce the movement of air molecules. This confinement of air particles minimizes the convection currents that would otherwise carry heat away from the heated space during winter or allow unwanted heat to enter during summer.
Furthermore, stationary air has a lower thermal conductivity compared to solid materials or moisture. By reducing heat transfer through convection, insulation maintains interior temperatures more effectively, contributing to energy efficiency in buildings. This principle is fundamental to effective insulation design and is why materials such as fiberglass, foam, and cellulose, which trap air, are widely used in construction.
While factors like moisture prevention, soundproofing, and various environmental impacts are relevant in building contexts, they do not directly explain the primary mechanism by which still air enhances insulation effectiveness.