Read this and you’ll become a supurhero yourself…pooooof!
To answer this question it is important to know that there are dozens of different types of PUR foam, each with its own specific characteristics. If a polyurethane [PUR] foam is waterproof is mainly related to the open or closed cell structure of the insulation foam. So it’s a chemical matter …
Generally, the type of foam you buy in your local retail shop is open-celled foam. They absorb moisture. These are mainly one component PU foams. On the other hand there is also two components PU foam, which often has a closed cell structure. The percentage of closed cells will determine whether the PUR foam is watertight. A PUR foam with more than 95% closed cells is a perfect water barrier. Waterproof testing can be done by the immersion test. A piece of PUR foam is weighed with a precision balance, and then completely submerged for 24 hours. Afterwards, the weight gives an indication on how watertight the polyurethane foam is. In the technical data sheet this is expressed as a percentage of the original mass weight.
So yes, there are PUR foam that are perfectly waterproof. For this you need to view the percentage of closed cells that is mentioned in the technical sheet of the PUR foam.