Insulation works by slowing the transfer of heat which can move in three ways: conduction, convection, and radiation. Understanding the influence of these three methods allows one to make better and more informed choices.
Convection occurs when warmer areas of a liquid or gas move to cooler areas in the liquid or gas. Cooler liquid or gas then takes the place of the warmer areas. This results in a continuous circulation pattern. Water boiling in a pan is a good example of these convection currents.
Controlling convection is the most overlooked element in cold-chain shipping. Often taken for granted, closing the lid on an EPS cooler traps a certain amount of air within the container. By reducing the air space in a container you reduce convection and improve performance.
Conduction is the transfer of heat between substances that are in contact with each other. The better the insulator, the slower heat will be transferred. Metal, as a good conductor, quickly transfers heat from one side to the other. Foam, as a good insulator, slows the transfer of heat.
Slowing conduction is the primary role of insulation. Most thermal shipping materials (EPS, FPF, PET, denim, rock wool, etc) have comparable performance slowing conduction.
Radiation is a method of heat transfer that does not rely upon any contact between the heat source and the heated object as is the case with conduction and convection. Heat is transmitted through empty space by thermal radiation (infrared radiation), a type of electromagnetic radiation. No mass is exchanged and no medium is required in the process of radiation. Examples of radiation is the heat from the sun, or heat released from the filament of a light bulb.
Matte black surfaces absorb the most thermal radiation while mirrors absorb the least. If the mirror surface is dusty or in contact with a material with higher emissivity (ability to absorb) such as cardboard, that material will conduct the absorbed heat to the mirror surface, defeating all radiation insulation benefits.
Radiation is the least influential method of heat transfer when it comes to thermal shipping. Even in the most extreme environments, heat gain comes primarily from the convection currents of air around the package conducting heat onto the surface of the package.