The thermal conductivity of a material is a measure of its ability to transfer heat. The lower the thermal conductivity, the better the material is at insulating. The thermal conductivity of water is 0.58 W/m-K, while the thermal conductivity of polystyrene is 0.034 W/m-K. This means that polystyrene is about 17 times better at insulating than water.

In other words, polystyrene will prevent heat from transferring through it more effectively than water. This is why polystyrene is often used as an insulating material in buildings and other structures.