Mostly air, held in a shape
Why Is Foam So Light?
Foam is a solid or liquid with gas bubbles trapped all through it, so most of its volume is nothing at all.
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Step 1 of 5
Foam is a material with holes on purpose.
Blow gas through something while it is still soft, then let it set. The bubbles stay where they are, and now most of the solid is gaps.
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Step 2 of 5
Most of it is nothing.
A block of packaging foam is often 95 to 98 percent air. You are carrying a very small amount of plastic arranged into a big shape.
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Step 3 of 5
Trapped air makes it warm.
Soak it and the insulation is gone — water fills the bubbles and carries heat well Air carries heat badly, and inside a bubble it cannot circulate. That is why foam cups, camping mats and cavity wall insulation all work the same way.
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Step 4 of 5
Squashy or stiff depends on whether the bubbles connect.
Open bubbles that join each other let air move, so it squashes — a sponge or a cushion. Closed, separate bubbles cannot, so it holds firm and floats.
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Step 5 of 5
Bubbles are also how it protects.
In a knock, thousands of bubble walls buckle one after another. Spreading the blow over that whole crumple is what saves what is inside.
Polystyrene foam is about 98% air. A cubic metre of it weighs roughly 15 kg, while the same volume of solid polystyrene would be over a tonne.
The short version
Packaging foam is 95 to 98% air. Trapped air cannot circulate, so it insulates until water fills the bubbles. Bubbles that join up squash like a sponge, closed ones hold firm and float, and thousands of walls buckling absorb a knock.
Try it yourself
Push a foam cup and a plastic one under water. The foam fights back hard — you are pushing all that trapped air down with it.