Why is polystyrene foam a good insulator?

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Expanded polystyrene, or EPS, is a familiar product. It is used extensively in packaging and for lightweight display materials. You may also find it containing very hot coffee -- that you are able to hold quite safely without scalding the skin from your fingers. This is because EPS is an effective insulator. It does not conduct, or move, heat from one surface to another. Hence your barely warmed coffee-holding hand.


EPS foam is made from small beads of plastic, itself derived from oil. The beads contain a harmless hydrocarbon agent that expands when the beads are exposed to steam. The beads are then about 40 times their original size and, after drying, form cell-like structures filled with gas.

Insulation properties

Heat transfer, or conduction, is the flow of heat from a hot area to a colder one. For example, if you were to hold an iron rod at one end while resting the other end in a fire, you would very quickly drop the rod as heat travels along it and into your hand. Iron is a good heat conductor because heat can flow quite quickly through its dense molecules. The cell structure of EPS with its tiny gas pockets prevents heat travelling easily from one surface of the polystyrene foam to the other. This makes EPS very efficient at keeping heat where you want it, i.e. in your coffee, not your hand. The principal is the same with larger applications, such as building insulation.

Uses as an insulator

EPS beads can be reheated and moulded into almost unlimited forms. Its shock absorbing, moisture resisting and insulating characteristics make it highly suitable for food packaging -- it can keep things cold by preventing heat coming into the package as well as keeping items warm. It is used in the building trade as cavity wall and loft insulation. It is placed under floors and in ceilings to reduce heat loss, thus saving energy and money. Additionally, it provides sound insulation as well as heat so is widely used in situations where noise reduction is required. EPS is used in appliance manufacture from refrigerators to cool boxes.


Expanded polystyrene was discovered in 1949 by chemist Fritz Stastny while researching materials to insulate telephone cables. It was quickly realised that this unusual material had many commercial applications and it was given the trade name “Styropor”.

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