No, extruded polystyrene is not waterproof. It has a low moisture absorption rate but can be prone to water damage over time. Below are some key points about the material:
-It does not absorb liquid, but it will swell when in contact with water for long periods of time.
-It has poor resistance to humidity and vapor permeability which makes it vulnerable to water damage.
-It is best used in indoor applications where it will not be exposed to extreme weather conditions or direct contact with liquids.