Two types of foam—which one is better? In this video, Matt Risinger, master builder, looks at two types of Dow foam, a polyiso foam sheeting and an extruded polystyrene STYROFOAM brand sheeting. The difference between them is in their chemistry. The polyiso Thermax-brand foam is made from the urethane family and the polystyrene is from the family of the same name. The differences are really in their performance. Polyiso ranges from R-5.5 to R-6.5 per inch really are in the R-Value. The R-Value on polystyrene is R-5 per inch. The application of where they are used is different. Moisture can hurt the R-Value of an insulation material like the polyurethane, so it should not be used where it can get wet, such as in cavity walls," or anywhere water can get to it. They are both very tough. The 1" thick XPS is very hard to compress. The polyiso is easier to compress so there is more need to be cautious at the job site so that it is not damaged. Pricing is virtually the same on both products, so the application that is most important.