How does adding air to plastic foam reduce heat transfer?

Adding air to foamed plastic reduces heat transfer because the resulting structure significantly lowers the material's thermal conductivity compared to that of the solid polymer.1

By incorporating a blowing agent or an inert gas into the resin, the plastic is converted into a spongelike mass containing trapped gas bubbles. These bubbles create either a closed-cell or open-cell structure that acts as an effective barrier to heat flow. Because gases are generally poor conductors of heat compared to solid plastics, the presence of these air-filled cells within the polymer matrix makes foamed plastics highly efficient for thermal insulation.21