U.S. Cooler uses both Expanded Polystyrene and Extruded Polystyrene as their insulation in their walk-in coolers and freezers. Polystyrene has many characteristics that prove it to be the best insulation material for walk-ins. The three main insulations found in walk-ins are Extruded Polystyrene, Expanded Polystyrene and Polyurethane. Each insulation has differentiating characteristics and should be optimized for the specific application.http://tinyurl.com/2x4wzj

Thanks for the mention ergodesk! As of 2009, due to EISA requirements, U.S. Cooler no longer uses Expanded Polystyrene in walk-in coolers shipped domestically. Extruded Polystyrene has a slightly lower fresh R-value than Polyurethane, however a U.S. Army Corps of Engineers study has shown that Extruded Polystyrene will retain 75% of its R-value in a freezer over 5 years while a Polyurethane box will only keep 25%.
Our sister company Enviro Buildings uses Expanded Polystyrene for the construction of communication shelters, in-plant offices and various other buildings.