Styrofoam is a better insulator than cardboard because it has a lower thermal conductivity. Thermal conductivity is a measure of how well a material conducts heat. The higher the thermal conductivity, the more heat a material will transfer. Styrofoam has a thermal conductivity of 0.033 W/m·K, while cardboard has a thermal conductivity of 0.12 W/m·K. This means that Styrofoam is more than three times better at insulating than cardboard.

Styrofoam's low thermal conductivity is due to its cellular structure. Styrofoam is made up of tiny, closed-cell bubbles that are filled with air. Air is a poor conductor of heat, so the air pockets in Styrofoam help to prevent heat from transferring.

Cardboard, on the other hand, is made up of cellulose fibers. Cellulose fibers are good conductors of heat, so cardboard does not insulate as well as Styrofoam.

In addition to its lower thermal conductivity, Styrofoam is also more durable than cardboard. This means that it can withstand more wear and tear, making it a better choice for long-term use.

For all of these reasons, Styrofoam is a better insulator than cardboard.