One of the key characteristics of XPS that has not yet been discussed is its chemical resistance. If used as a skin on a boat you would need to be very carful about complete encapsulation; never damaging the encapsulation and avoid the use of hydrocarbons in the boat as there will be inevitable spills that will find their way into any imperfection and eat out the core.
Every marina I have seen has some oil film on it somewhere. So any external surface scratching or through-hull fitting is a potential source for ingress of hydrocarbon and chemical attack of the core.
By comparison PVC foams have greater chemical resistance to common chemicals. When building there is no need to be cautious about the use of acetone for example.
The greatest consideration for a boat building material in terms of ultimate safety is flammability. I give this aspect great weight. Any material that is not self-extinguishing has to be used with care. Again I would not like to have a boat skinned with a material that is highly flammable. The significance of the linked table should be taken into consideration;
You can get fire retardants in XPS but it adds cost and the effectiveness is not well tested - or at least up to a few years ago when it was not given any value by insurers. Some insurers will not insure buildings that contain XPS or EPS panels. This video might influence where you will store the XPS sheets and the location of the area for building a vessel comprising large amounts of XPS.
When heated the XPS gives off flammable vapour that will inevitably flashover and cause an intense fire.
What are the Characteristics of Polystyrene?
Now that we know what it is used for, let’s examine some of the key properties of Polystyrene. Polystyrene is typically (but not always) a homopolymer meaning that it is composed only of the monomer styrene in combination with itself. Depending on the type of PS it could be classified as a “thermoplastic” or a “thermoset” material. The name has to do with the way the plastic responds to heat. Thermoplastic materials become fully liquid at their melting point (210-249 degrees Celsius in the case of Polystyrene), but they begin to flow at their glass transition point (100 degress Celsius for PS). A major useful attribute about thermoplastics is that they can be heated to their melting point, cooled, and reheated again without significant degradation. Instead of burning, thermoplastics liquefy, which allows them to be easily injection molded and then subsequently recycled. Thermoset plastics, by contrast, will not reliquify once they are “set” in solid form.
